OAK GEOGRAPHY

Gander's Oak - Quercus x ganderi = Quercus agrifolia x kelloggii (x wislizenii)

Canyon Oak - Quercus chrysolepis

Quercus chrysolepis is widely distributed throughout the mountains of California, Oregon and the American Southwest. It is virtually impossible to find outside of its native habitat.

Quercus chrysolepis is rarely found growing below 2,000 feet in elevation in California. Trees planted in typical urban valley locations rarely survive. Stanford lost the one Q. chrysolepis ever recorded in their inventory (I personally confirmed its disappearance), one of the two Q. chrysolepis planted in the Shields Oak Grove in the 1960s is long dead, while the second looks terrible. Quercus tomentella is a very closely related native tree, with a vastly more restricted range. Q. tomentella can be successfully grown in areas where Q. chrysolepis does not thrive, or even survive. Q. chrysolepis is a great mountain tree best appreciated in the mountain habitats it already occupies, and is recommended for northward range expansion trials.

California Blue Oak - Quercus douglasii

Native to large areas of California, with primary range known as a "bathtub stain" around the Central Valley. Q. douglasii is found in Foothill locations, hundreds of feet above the valley floor, and giving way to other oaks and pines at mountain elevations. There are also "coastal" populations of Q. douglasii of limited size. For instance, Q. douglasii is native to the area around Stanford University, although restoration efforts have been largely unsuccessful for 35 years, due to an increase in summer humidities which is part of climate change in these areas.

Q. douglasii is highly susceptible to powdery mildew. Having personally collected and planted thousands of Q. douglasii on lands with plentiful mature trees, I can attest to the lack of acorn production, lack of regeneration, and heavy disease loads on seedlings and young trees. In the 1980s and 1990s, finding Q. douglasii acorns in the Stanford area was highly challenging, and by the 2000s had become impossible in some years. This is exactly the kind of local extirpation at the edges of a trees native range that is predicted by climate change, and is being document for a vast number of species worldwide as climate change accelerates.

Q. douglasii performs poorly in urban coastal California, and planting in these areas is discouraged. As an alternative, the California native Quercus engelmannii has proven to give excellent performance in urban coastal California, and, with its bluish coloration, is easily confused with Q. douglasii.

California Blue Oak - Quercus douglasii

Native to large areas of California, with primary range known as a "bathtub stain" around the Central Valley. Q. douglasii is found in Foothill locations, hundreds of feet above the valley floor, and giving way to other oaks and pines at mountain elevations. There are also "coastal" populations of Q. douglasii of limited size. For instance, Q. douglasii is native to the area around Stanford University, although restoration efforts have been largely unsuccessful for 35 years, due to an increase in summer humidities which is part of climate change in these areas.

Jolon Hybrid Oak - Quercus x jolonensis (Q. douglasii x lobata)

Jolon oak is the name of the hybrid that is almost inevitably found wherever the range of the California Blue Oak (Quercus douglasii) overlaps with the Valley Oak (Quercus lobata). Although they are variable as with any hybrid, the mix typically makes for a strong and beautiful deciduous oak, deeply characteristic of the California landscape. These trees are notoriously difficult to propagate from seed. This is an oak worth propagating vegetatively using hot callus grafting techniques.

Oregon White Oak - Quercus garryana

Q. garryana is a deciduous California native oak, mostly found in mountainous areas of the northern part of California. Q. garryana is most at home in Oregon, and points north, extending even across the international border into British Columbia. Q. garryana is not recommended for urban areas in the southern half of California. This is a species for far northern California, Oregon, Washington, and points north.

California Black Oak - Quercus kelloggii

Native to mountainous regions throughout California, and into southern Oregon.

Just like many other of the California native tree oaks, Q. kelloggii is a beautiful native tree which doesn't adapt well to planting in urban coastal California. The leaves of straight species Q. kelloggii are highly susceptible to several leaf diseases in areas of marine influence. Q. kelloggii only occurs in areas with excellent soil drainage, low summer humidities, and 25 inches of rainfall, or more, per year.

But because these are oaks, and California has a truly dizzying array of microclimates, there are subtle variations that can increase the adaptive range of Q. kelloggii hybrids, particularly the rare but wonderful Quercus x ganderi, the hybrid with Q. agrifolia. There are places in California where the range of these two trees overlap, and the Q. agrifolia parent, with its coastal adaptation, can help enormously with survivability in near-coastal environments.

Another hybrid of Q. kelloggii is much more famous than Q x ganderi, and this is the Oracle oak found in the Sierras, Q. kelloggii x Q. wislizenii. Neither parent species is adapted to California coastal urban conditions, so this is a tree best appreciated in the wild, where the tree get the Oracle name because the trees are semi-evergreen, and their leaves typically turn a brilliant yellow hue, but in the middle of winter, rather than the fall, making themselves very conspicuous on hillsides in the Foothills of the Sierras.

But it must be remembered that hybrid oaks are notoriously difficult to identify, and even more difficult to recreate/propagate. Natural hybrids in a natural setting typically make acorns that revert to one parent or the other, and rarely recreate the mother tree.

Valley Oak - Quercus lobata

Native to many locations in California, except the far north, the far south, and the far east. Q. lobata tends to also stay away from the ocean. Genetic testing of Q. lobata throughout California has shown relative genetic uniformity for Q. lobata in the Central Valley, while genetic diversity characterizes Q. lobata growing in the coastal valleys and foothills.

The world famous California Valley Oak is among the largest-growing oak trees in the world. To grow to maximum potential size, Valley Oak needs deep soils and access to groundwater. The utility of Valley Oak for plantings in urban coastal California is limited due to a variety of factors.

The first of these limits is encountered with the enormous trunk girth causing substantial infrastructure damage - remember, this is one of the largest oaks in the world. Pavement damage is associated with even young Valley Oaks on normal urban planting sites. The second of these limits for Valley Oak is found with soil compaction, and sites without naturally deep soils and underground water availability. Proper planting sites for Valley Oak can be hard to come by in our coastal cities.

Valley Oak displays an enormous variation in growth habit. On the Stanford University campus for instance, mature Valley Oak can be found with an incredibly pendulous growth habit, behaving like a giant weeping willow. But elsewhere on campus one can find Valley Oaks with a very upright growth habit, and no pendulousness at the branch tips. When planting in urban areas, pedestrian and vehicular clearances are typically of paramount importance, and pendulous trees have a major long term maintenance cost associated with near constant clearance pruning.

Powdery mildew is another issue within areas of marine influence. After 30 years of planting Valley Oak all over the Stanford campus, powdery mildew susceptibility has proven variable from individual to individual, just like growth habit. Planting a powdery mildew susceptible Valley Oak in the zones of marine influence is a very frustrating experience - just as the trees start to put on long and lovely new growth, all that new growth turns white, and the trees become substantially stunted and disfigured. There are strong indications that the zones of marine influence, and powdery mildew susceptibility, are increasing over time with climate change.

Island Live Oak - Quercus parvula var. parvula

Native to specific locations on the Channel Islands, and perhaps adjacent mainland areas. Shields Oak Grove, UC Davis.

Quercus parvula is split into two varieties, one entirely found on the mainland, and the other almost exclusively on the Channel Islands. Both are closely related to Quercus wislizenii, but are tolerant of high levels of rainfall and humid near coastal conditions. Q. parvula var. parvula is rarely planted outside habitat, but the small oaks have thrived with no irrigation at UC Davis, though it should be noted that growing conditions (soil and underground water) are exceptional.

An interesting experiment was carried out in 2007 and 2008 in East Palo Alto, where 150 Q. parvula var. parvula acorns from the Davis trees were direct seeded into the 101 Soundwall, along with nearly 1,000 other drought tolerant trees and shrubs. At the end of a year, 90% of those Q. parvula var. parvula were dead, unable to tolerate the watering necessary to establish the other trees in the planting.

But the interesting part is that 10% survived, and it's apparent that the survivors are hybrids of some kind. But who might the parent of these hybrids be? In the Shields Grove, there are only a few oaks that could be the male pollen parent of those hybrids. Q. agrifolia is the most likely second parent based in the number of trees related to Q. parvula var. parvula.

This is a great example of how hybrids really work - 80% to 90% turn out to be nothing worthwhile, while 10% to 20% are interesting and/or useful. In true species form, Q. parvula var. parvula are very sensitive to soils with anything less than lightning fast soil drainage. This was the lesson of the Soundwall planting.

The lesson of the successful Q. parvula var. parvula hybrids on the Soundwall is that hybridization of highly soil drainage sensitive oaks species with more species more tolerant of slow soil drainage can yield much more adaptable offspring.

Shreve Oak - Quercus parvula var. shreveii

Native to coastal California forests from Point Conception northward. Rarely if ever seen outside of native habitat.

Venturing into the coastal California mountains, a dizzying array of related native oaks. In the Santa Cruz Mountains, for instance, once can find Q. agrifolia, Q. wislizenii, Q. kelloggii, Q. parvula var. shreveii, plus an utterly bewildering array of hybrids between all of these, sometimes showing traits of up to three of these primary species. This is a classic oak "swarm", and the variation tests even the most practiced oak horticulturist.

Quercus parvula var. shreveii is unusual among oaks as it can grow alongside redwoods in native habitat. It is perhaps the most fog tolerant of our native oaks. In fact, in near ocean areas like urban Santa Cruz and surrounding cities and rural areas, there are currently expanding and widespread death of Quercus agrifolia that eclipses the current fixation on the so-called sudden oak death driven by Phythophthera ramorum.

And regardless of any particular explanation, the stress on trees in the coastal forests, as virtually everywhere in the world currently, the stressors are creating a selection event, and, in places where both Q. agrifolia and Q. parvula var. shreveii grow, it is shocking to see, over and over, the Q. parvula var. shreveii is thriving as the Q. agrifolia around them weaken and die. This observation is repeated over and over again in the coastal zones occupied by both species.

But please don't take this as some kind of blanket recommendation to plant Q. parvula var. shreveii, because this tree is unlikely to thrive outside of its native habitat, due to the unusualness of the microclimates that make up it's native habitat, characterized both by enhanced rainfall - sometimes nearly ten times the amount seen in the dry inland valleys - and the rapid soil drainage of its native range.

Interior Live Oak - Quercus wislizenii

Native throughout mountainous areas of California, with occasional forays onto the valley floors in areas of particularly good soil drainage.

Quercus wislizenii is arguably the most widely distributed of the California "Live" oaks. This is the Interior Live Oak, as opposed to the Coast Live Oak (Q. agrifolia). Q. agrifolia is typically found within 50 to 100 miles of the Pacific Ocean, in both mountainous (fast soil drainage) and valley (slower soil drainage with some clay content) areas. Within the Q. agrifolia zone, Q. wislizenii can be found only in the mountainous regions, and not in the valley conditions.

But within the mountainous coastal zones, Q. wislizenii exists within a truly and wonderfully vexing swarm of native Black Oak subgenus species: Q. agrifolia, Q. kelloggii, and Q. parvula var. shreveii. When venturing into these mountainous areas to observe California native Black Oaks, prepare to be bewildered by the almost infinite hybrid forms expressed, and learn to prize those trees that you can truly identify, being either true to species type, or readily confirmable hybrids.

Q. parvula var. shreveii drops out of the swarm quickly as one moves inland, only thriving in areas with powerful marine influence, along with elevated mountainous rainfall levels, typically 2 to 5 times greater than the inland valleys below. The next loss from the swarm, moving inland, is Q. agrifolia. By the time one travels to the foothills of the Sierras, only Q. wislizenii and Q. kelloggii remain, sometimes creating what is arguably the most famous of all California hybrid oaks, the Oracle Oak. In these far inland mountainous areas, identifying Q. wislizenii becomes much more straighforward than in the coastal mountains.

Q. wislizenii is a poor performer, and rarely survives, in locations with anything less than very fast soil drainage, and low clay content. If true hybrids between Q. wislizenii and Q. agrifolia can be found, and they are grown fairly commonly without the growing nursery having any idea (or tag) that the trees are hybrid, there will be a better chance of survival, but good soil drainage will remain a must. Q. wislizenii possesses a more upright growth habit than Q. agrifolia, and also tend to have much stronger and smoother branch attachments.

True to species Q. wislizenii is a tree best grown as part of restoration projects in proper habitat/geographical locations in northern California. The hybrid Q. wislizenii x kelloggii, aka Oracle Oak, is highly prized, but incredibly difficult to capture genetically, as acorns from hybrid trees, in a mixed wild or swarm setting, will typically revert to one parent or the other.

Arizona White Oak - Quercus arizonica

Quercus arizonica is a native tree of the Madro-Tertiary Geoflora, and is very closely related to California native Quercus engelmannii. Q. arizonica is a common, widely dispersed, and almost infinitely variable species within its range from west central Mexico north to the American Southwest.

Q. arizonica is one of two confusingly overlapping and intrograding White Oak subgenus trees in the American Southwest, and southward into Mexico. Quercus grisea is the other White Oak subgenus species which co-occurs with Q. arizonica in many locations. Many guidebooks and signs in the Southwest have given up telling the two apart, simply lumping them together in a sad commentary on our vast societal ignorance of all things horticultural.

But the real oak hunters in the Southwest know the difference, and have shared some advice about telling the two apart.Q. arizonica has foliage that is more generally oval rather than pointed, more generally green in color rather than gray, and is found growing in moister areas. Q. grisea foliage is generally more gray in color than green, and the leaves are more toward being lance-shaped, and even pointed at the tip. Fittingly, as gray is a color of drought adaptation, Q. grisea will be more likely to be found in drier upland locations, away from obvious sources of water.

When two or more related oak species occupy the same range, they often co-exist both as each individual species, and also as hybrids. A grouping such as this, with both true species and hybrids, is often referred to as a "swarm", and such swarms are quite typical for oaks. Because it turns out that oak swarms have profound evolutionary meaning.

Given that variability is the nature of climate even in the best of times, these species swarms have moved, over geological time, up and down the mountains, leaving behind slightly to highly hybridized offspring. This is how oaks actually work in the wild - good luck finding definitive species specimens - many oaks are far from the idealized vision of what a particular oak species "should" look like.

Oaks move through time and space as genetic swarms - infinitely variable slight variations on a given species theme. If one goes oak exploring thinking that identification will be straightforward and definitive, then one can look forward to having their expectations shattered by infinite shades of gray.

Nature creates diversity. Humans with our plant names attempts to capture and catalog that diversity. But nature just keeps producing variety and diversity, without regard to sameness and category. Nature loves controlled chaos - oaks are a fantastic example of this phenomenon.

Emory Oak - Quercus emoryi

Quercus emoryi is a very common low altitude oak species in the American Southwest into northern Mexico. No known specimens exist in California at this time. This is very much a monsoonal oak species, which drops it's acorns right into the heat of the Southwestern summer, in mid-July. Observation in Arizona landscapes suggests that Q. emoryi performs poorly outside its very specific climate and soil adaptations. Not recommended currently for any locations in California, but certainly worth limited experimentation to solidify these hypotheses.

Escarpment Oak - Quercus fusiformis

In Texas, all the trees in the Q. fusiformis/Q. virginiana lineage of closely related species are simply called "Live Oaks". Looking at the genetics of these trees with greater care, an interesting pattern emerges. In the wet eastern portion of Texas, with a climate similar to the rest of the southeastern US from Louisiana to Florida, the live oak is Q. virginiana, whose range continues right to the Atlantic Ocean and the Gulf of Mexico.

But as you head westward in Texas, the climate become much more arid and drought-prone, and the soil pH begins it's change from acidic to alkaline. The Texas Hill Country can be considered the buffer region between the wetter climate zones to the east, and the drier climate zones to the west. Moving west, another live oak species emerges, subtly different from Q. virginiana.

This is Q. fusiformis, and it's range continues into West Texas, and down into northern Mexico. But in the transition zone between Q. fusiformis and Q. virginiana, mixtures of the two forms are common. Q. virginiana live oak types tend to be in the wettest locations, while Q. fusiformis types will be found on the driest locations. In between, all manner of hybrids between these two can be found.

In Austin and other central Texas cities, it's quite hard to tell exactly which group any individual tree belongs to. Thus, practical Texans simply call them all live oaks, and they all seem to grow equally well.

In California, Q. virginiana types have been planted, in limited numbers, since the late 1800s. They have performed surprisingly well, and Q. virginiana is still commonly planted today in California cities.

But Q. virginiana has a very horizontal, spreading growth habit, with no discernible apical dominance, and might be described as having a mushroom growth habit. So while Q. virginiana will indeed grow under a wide variety of California urban conditions, it requires quite a lot of clearance pruning in the first decade or so of its urban California life.

Gray Oak - Quercus grisea

Extensive range from southern Colorado in the US south nearly to Mexico City.

Q. grisea is one of two confusingly overlapping and intrograding White Oak taxa on the Sky Islands of Arizona, and south into Mexico. Q. arizonica is the other White Oak, and many guidebooks and signs in the Sky Islands have given up telling the two apart, simply lumping them together into one big chaotic genetic mess, in a sad comment on our vast societal ignorance of all things horticultural.

But the real oak hunters in the Southwest know the difference, and have shared some advice about telling the two apart. Q. grisea foliage is generally more gray in color than green, and the leaves are more toward being lance-shaped, and even pointed at the tip. Fittingly, as gray is a color of drought adaptation, Q. grisea will be more likely to be found in drier upland locations, away from obvious points of water collection.

Q. arizonica has foliage that is more generally oval rather than pointed, more generally green in color rather than gray, and is found growing in moister areas. Further discussion of Q. arizonica can be found in that species section of this monograph.

Is Quercus grisea, in its various forms, worth growing in California? Yes, limited experimentation has certainly been positive. The trees will grow. This is a qualified recommendation for Q. grisea. Qualified in that it's a tree unlikely to catch on in the California ornamental landscape industry. Q. grisea is a pleasant but rather generic evergreen oak, without any particular ornamental character to recommend it.

Quercus albocincta

iNaturalist

https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=275459

Northwestern to Central Mexico

This is an oak identified for potential use in coastal California simply due to its size and attractivenss, combined with the fact that it grows in association with other oaks which have proven successful in coastal California. In this case the associated oak species include Quercus arizonica, Quercus grisea, Quercus hypoleucoides, Quercus oblongifolia and Quercus rugosa. There are a number of oaks in a similar situation - they are present only in northwestern Mexico, without range extension into the United States, but with a large distribution within Mexico, and strong overlap with oak species whose range does extend into the United States. Highly recommended for trial throughout coastal California.

Brandegee Oak - Quercus brandegeei

Shields Oak Grove, UC Davis, Baja

This rare oak is closely related to the "Live Oaks" of the American South, Quercus virginiana in the wetter area, Quercus fusiformis in the drier areas. A pair of Q. brandegeei have performed well in Davis for nearly 60 years now, surviving the decades in which the Grove was "neglected" in fine style. The mature tree is incredibly pendulous, sweeping foliage to the ground. For this reason, Q. brandegeei has a very limited applicability in ornamental landscape locations in California. The pruning needed to grow these trees with urban pedestrian and vehicle clearances will be incessant and expensive, and the tree will be substantially disfigured. But these warnings only apply to highly used urban locations - on large sites with good soil drainage, Q. brandegeei should be given much wider testing in California, given the fact that current Baja climate may be where much of coastal California is heading. The excellent performance of Q. brandegeei thus far in Davis only reinforces that recommendation. Q. brandegeei should likely be used in carbon/habitat plantings in California, especially southern California.

Encino Aguacatillo - Quercus candicans (Now renamed Quercus calophylla)

San Francisco Botanical Garden

There are several trees marked (when last seen) Quercus candicans in San Francisco Botanical Garden, where it performs well in the moist and mild conditions there, which, sadly, are not a good mimic of growing conditions more generally throughout California. Judging from the native climates where Q. candicans comes from, Q. candicans is well adapted to California coastal mountains and ocean plains, but rather water needy for typical coastal cities. Quercus candicans is worthy of further trial in moist locations throughout coastal California, largely in the northern two thirds of the state. But caution is urged in such locations, as these trees could become a naturalization risk.

Chihuahuan Oak - Quercus chihuahuaensis

iNaturalist

https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=167643

Northwestern to Central Mexico

This is an oak identified for potential use in coastal California simply due to its size and attractivenss, combined with the fact that it grows in association with other oaks which have proven successful in coastal California. In this case the associated oak species include Quercus arizonica, Quercus grisea, Quercus hypoleucoides, Quercus oblongifolia and Quercus rugosa. There are a number of oaks in a similar situation - they are present only in northwestern Mexico, without range extension into the United States, but with a large distribution within Mexico, and strong overlap with oak species whose range does extend into the United States. Highly recommended for trial throughout coastal California.

Sartor's Oak - Quercus sartorii

Apple Park, Cupertino, Homestead Road

An oak from the mountains of eastern Mexico, closely related to Quercus canbyi. There are specimens growing at Peckerwood Gardens outside Houston. Offspring of these trees grow along Homestead Road at Apple Park, intermixed with Q. canbyi, Q. gravesii, and a few odd hybrid red oaks. So far, performance mimics the more widely planted Q. canbyi. Performance thus far in irrigated locations on quality soil is good. The trees have remained disease and pest free, despite very shady growing conditions. Quercus sartorii is unlikely to ever become an important tree in coastal California, though it is apparent quality specimens can be grown.

Quercus tuberculata

iNaturalist

https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=282411

Northwestern to Central Mexico

This is an oak identified for potential use in coastal California simply due to its size and attractivenss, combined with the fact that it grows in association with other oaks which have proven successful in coastal California. In this case the associated oak species include Quercus arizonica, Quercus grisea, Quercus hypoleucoides, Quercus oblongifolia and Quercus rugosa. There are a number of oaks in a similar situation - they are present only in northwestern Mexico, without range extension into the United States, but with a large distribution within Mexico, and strong overlap with oak species whose range does extend into the United States. Highly recommended for trial throughout coastal California.

Encino Sauce - Quercus viminea

iNaturalist

https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=167673

Northwestern to Central Mexico

This is an oak identified for potential use in coastal California simply due to its size and attractivenss, combined with the fact that it grows in association with other oaks which have proven successful in coastal California. In this case the associated oak species include Quercus arizonica, Quercus grisea, Quercus hypoleucoides, Quercus oblongifolia and Quercus rugosa. There are a number of oaks in a similar situation - they are present only in northwestern Mexico, without range extension into the United States, but with a large distribution within Mexico, and strong overlap with oak species whose range does extend into the United States. Highly recommended for trial throughout coastal California.

Boissier's Oak - Quercus boissieri (Quercus infectoria subsp. veneris)

SelecTree

http://selectree.calpoly.edu/tree-detail/1703

Shields Oak Grove, UC Davis

This highly adaptable Mediterranean oak is best represented in California by the set in the Shields Oak Grove, where they were able to thrive with no irrigation for decades. Although not a terribly striking tree, Q. infectoria subsp. veneris / Q. boisseri grows well, and makes quite a handsome young tree. The tree is deciduous to semi-evergreen., and of moderate size The trees in Shields Grove produce small to moderate numbers of acorns consistently, and the seedlings grown thus far have proven surprisingly true to parent. No powdery mildew has been observed. Branch attachments are strong, and the tree could be described as upright and spreading in growth habit. This oak is certainly recommended for further trial in California, especially as a street tree.

Mirbeck's Oak - Quercus canariensis

Shields Oak Grove, UC Davis

The Quercus canariensis growing in the Shields Grove has grown large, but declined severely when the nearby lawns were removed. Like most oaks, Q. canariensis appears to have a wide array of types, and the Shields tree doesn't appear to be typical. In fact, it is noted that Q. canariensis seed grown in cultivation is almost always hybridized with the vastly more common Quercus robur, and the Shields Grove tree appears to be of this type. Further exploration and experimentation with true species Q. canariensis is recommended, given its extensive distribution in climates very similar to those found in California.

Chestnut Leaf Oak - Quercus castaneifolia

Shields Oak Grove, UC Davis. Apple Park, Cupertino, Pruneridge Road and Tantau Road

Here is a prediction. By 2040 (if not sooner), the largest oak in California will no longer be a California native Quercus lobata. Instead, it will be one of the Quercus castaneifolia growing at the Shields Oak Grove in Davis. The tallest of these trees is well over 100 feet in height, in about 50 years. In 15 years of observation, the trees do not seem to be slowing their growth. Given that the largest Q. lobata in the state are quite old, and almost invariably in decline, it's virtually inevitable that the largest will be surpassed by the remarkably vigorous imports.

The Q. castaneifolia love the growing conditions in Davis, and, remarkably, showed no change in vigor and health status when the large lawns in the Shields Grove were removed and watering vastly decreased. The continued vigor shows clearly that the trees are tapped into underground moisture supplies, which tend to be abundant in the Davis area, and are mixed into coarse and deep alluvial soil characteristic of the Davis area.

Interestingly, and typically, European oak experts who have seen the trees claim they are not true Q. castaneifolia, but instead hybrids between Q. castaneifolia and Q. cerris. Since the seed for the Shields trees originally came from Kew Gardens in England, and, sure enough, there are numerous Q. cerris at Kew, close to the large Q. castaneifolia, which was grown from acorns originally collected in Iran in the 1830's. Properly marked, the huge trees in the Shields Grove are Quercus castaneifolia x cerris.

Regardless of exact parentage, the Davis trees are regular producers of medium to large acorns. Hundreds or thousands of seedlings from the Shields nominal Q. castaneifolia have been propagated by California nurseries, and the offspring are invariably hybridized. Some of these offspring are marcessant. Some of the offspring are very upright in growth habit, while others are quite pendulous. Virtually all offspring are resistant to powdery mildew. On good quality sites, the seedling are among the fastest growing oaks that exist, commonly growing 4 to 6 feet each year.

So, yes, like many oaks, you can grow superior specimens derived from these trees in California. The problem with the seedlings is the marked variation in traits and performance from individual to individual. The Q. castaneifolia planted at Apple Park appear to be carefully selected offspring of the Davis trees, but, despite clearly careful selection, there is still a wide variation from tree to tree. Some individuals display an enhanced susceptibility to Phythophthera, but this could also be a response to root defects in the form of girdled or circled roots started early in the life of the tree.

Chestnut Leaf Oak - Quercus castaneifolia

Shields Oak Grove, UC Davis. Apple Park, Cupertino, Pruneridge Road and Tantau Road

Here is a prediction. By 2040 (if not sooner), the largest oak in California will no longer be a California native Quercus lobata. Instead, it will be one of the Quercus castaneifolia growing at the Shields Oak Grove in Davis. The tallest of these trees is well over 100 feet in height, in about 50 years. In 15 years of observation, the trees do not seem to be slowing their growth. Given that the largest Q. lobata in the state are quite old, and almost invariably in decline, it's virtually inevitable that the largest will be surpassed by the remarkably vigorous imports.

The Q. castaneifolia love the growing conditions in Davis, and, remarkably, showed no change in vigor and health status when the large lawns in the Shields Grove were removed and watering vastly decreased. The continued vigor shows clearly that the trees are tapped into underground moisture supplies, which tend to be abundant in the Davis area, and are mixed into coarse and deep alluvial soil characteristic of the Davis area.

Interestingly, and typically, European oak experts who have seen the trees claim they are not true Q. castaneifolia, but instead hybrids between Q. castaneifolia and Q. cerris. Since the seed for the Shields trees originally came from Kew Gardens in England, and, sure enough, there are numerous Q. cerris at Kew, close to the large Q. castaneifolia, which was grown from acorns originally collected in Iran in the 1830's. Properly marked, the huge trees in the Shields Grove are Quercus castaneifolia x cerris.

Regardless of exact parentage, the Davis trees are regular producers of medium to large acorns. Hundreds or thousands of seedlings from the Shields nominal Q. castaneifolia have been propagated by California nurseries, and the offspring are invariably hybridized. Some of these offspring are marcessant. Some of the offspring are very upright in growth habit, while others are quite pendulous. Virtually all offspring are resistant to powdery mildew. On good quality sites, the seedling are among the fastest growing oaks that exist, commonly growing 4 to 6 feet each year.

So, yes, like many oaks, you can grow superior specimens derived from these trees in California. The problem with the seedlings is the marked variation in traits and performance from individual to individual. The Q. castaneifolia planted at Apple Park appear to be carefully selected offspring of the Davis trees, but, despite clearly careful selection, there is still a wide variation from tree to tree. Some individuals display an enhanced susceptibility to Phythophthera, but this could also be a response to root defects in the form of girdled or circled roots started early in the life of the tree.

Turkey Oak - Quercus cerris

Lake Merritt, Oakland. Shields Oak Grove, UC Davis Arboretum. Shields Library, UC Davis. Stanford University. Street Trees: Berkeley. Davis. Fallen Leaf Road, Arcadia. Apple Park Perimeter, Tantau Road, grafted trees.

Quercus cerris is a drought tolerant, near-Mediterranean oak which has historically been planted in more northerly European cities. It did not prove a good timber species for northern Europeans, so there it is limited to ornamental use. The oldest Q. cerris so far identified in California is a tree on the Stanford University Campus, near the former home site of Jane Lathrop and Leland Stanford. The tree likely was brought to Stanford in the early 1880's, as part of the extensive Arboretum planting of the time, of which this tree is one of the few survivors.

Q. cerris is widely introgressed with other oak types, leading to a wide variation in leaf shape amongst the specimens. Sooty mold has shown itself to be a consistent problem with Q. cerris in coastal California locations, and in locations with poor soil drainage.

There are a pair of Q. cerris in the Shields Oak Grove in Davis. They are both lovely specimens. In 15 years of observation, one of the two trees produces acorns in many years, while the other tree has never produced a visible acorn. Hundreds of seedling were grown around 2010 - unfortunately, every seedling grown proved steadfastly marcessant.

There are several marcessant Q. cerris planted as street trees in Davis, and interviews with residents who have the trees in front of their houses found people frustrated by the marcessance. Having a tree filled with dead leaves all winter, especially when they are south of the house and blocking the warming winter sun, can be a huge bummer. Marcessance is truly an unwelcome trait to most people. But not every Q. cerris is marcessant, and identifying and reproducing such trees is likely a valuable exercise.

Q. cerris is but one species out of a group of closely related trees which grow throughout southern Europe, North Africa, and Asia. Q. cerris comes from a somewhat wetter climate than most of California, and the true Mediterranean oak from this lineage, particularly Q. trojana, are more consistent performers in California, though they are smaller trees at maturity.

Q. cerris is not highly recommended for most locations in California, but is certainly worth further investigation, especially for the inland northern half of California.

Laucombe's Oak - Quercus x crenata

Quercus crenata is a rare oak closely related to Quercus suber. It can be found growing in southern Italy, and a few locations in France. Recent genetic evidence suggests this oak species is a remnant of the ancestral population from with Q. suber evolved. Strongly resembles Q. x hispanica, the natural and artificial hybrid of Q. suber and Q. cerris. Given the consistent success of Q. x hispanica in varied California locations, it can safely be assumed that Quercus crenata will perform well. Q. crenata is highly recommended for further trial in California.

Portugese Oak - Quercus faginea

There is one example of Q. faginea in the Shields Oak Grove at UC Davis. Seedlings of this tree have proven to be, as one often finds with oaks from arboreta setting, a morass of variable hybrids. While Q. faginea certainly survives in Davis, the tree itself isn't terribly compelling, but is probably worth further trial.

Eurastian Mesobalanus Oaks - Quercus frainetto / Quercus macranthera / Quercus pyrenaica / Quercus dalechampii

Visalia. Palo Alto. Apple Park, Cupertino

The 'Forest Green' cultivar of Quercus frainetto has seen limited planting around California since the late 1990's. The trees grow well, with rapid growth, attractive foliage, strong branch attachments, decent apical dominance, and low to nonexistent acorn production, a bonus for urban oaks.

Unfortunately, the 'Forest Green' cultivar is grafted by J. Frank Schmidt nursery, and they have historically used Q. robur and it's cultivars as rootstock. Sadly, Q. frainetto and Q. robur are in different oak subgenera, and delayed graft incompatibility is common.

In Visalia, Palo Alto, and Cupertino, beautiful 'Forest Green' trees have been seen to decline and die from delayed graft incompatibility, anywhere from years to decades after planting. This has been a disaster for the early adopters of this otherwise attractive new tree.

As the grafts on 'Forest Green' fail, shoots begin to emerge from the rootstock - a sign that failure is inevitable - and those shoots clearly show the differing genetics of the rootstock and mother tree. 'Forest Green' Quercus frainetto is not recommended due to these failures.

It is recommended that other forms of Quercus frainetto (along with the closely related Mesobalanus oak species Q. macranthera and Q. pyrenaica) should be identified and brought to California, to serve as genetically appropriate seed sources for future use of this species, mostly for the northern half of the state, and for locations farther to the north.

Holly Oak or Holm Oak - Quercus ilex

Common through much of California and the Mediterranean into southern Europe and the British Isles.

Among oaks from other parts of the world historically planted in California, Q. ilex is clearly the tree that has been used the most, and results have been less than ideal, for a variety of reasons. Some of the key problems include intense aphids and sooty mold in many locations, a destructive root system when used in proximity to infrastructure, unpleasantly heavy shade for urban locations, near-agricultural levels of acorn production, major stunting of planted trees due to girdling and circling roots in nursery production, and, finally, invasiveness.

Quercus ilex has been a sad tale for the California landscape industry. After decades of scattered use in California, Q. ilex became heavily planted in rapidly urbanizing post-war California from the 1950's to the 1980's. One prominent set of such urban Q. ilex grew in the California Avenue commercial area in Palo Alto.

These trees became the center of a civic firestorm in 2009, when the 50 or so Q, ilex, planted in the 1980's, were suddenly removed by the city of Palo Alto, with virtually no public process. This act was part of what eventually led to the retirement of both the head of public works and also the head arborist. City officials had relied on the agreement of a small number of business owners on the street who hated the trees for several of the reasons cited above.

But these were highly visible, and even beloved, business district trees, and the public outrage was immediate. A replacement tree plan was pulled together, and new trees installed within months. Most of these trees have performed well, and, of course, did not feature Q. ilex. The story of California Avenue, which can be found detailed in newspaper stories from the time, is truly a cautionary tale for those not yet convinced to leave Q. ilex out of California.

Almost certainly, the trait that served as the "last straw" for the California Q. ilex was acorn drop. A bit of study into the history of Q. ilex in the Mediterranean shows that it was a critical component in the dehasa production system, which specialized in the fattening of the famous Iberian swine. Quercus ilex was a primary provider of acorns on the dehasa, with particularly productive strains likely selected for 1,000 human generations, all choosing to propagate the Q. ilex which produced the most abundant and largest acorns.

Trojan Oak/Lebanon Oak - Quercus trojana/Quercus libani

In Eurasia, there are a series of oaks similar to Quercus cerris, mostly from more arid climates. These trees are mostly smaller than Q. cerris, with leaves also smaller and more generally pointed. This series includes Q. libani and Q. trojana, and used to include Q. macaedonica, which has now been subsumed into into the species Quercus trojana.

Several of these trees are represented in the Shields Oak Grove, but the nomenclature on these trees in the Shields Grove has changed over time. The two larger trees used to be called Q. macaedonica, now changed to Quercus trojana on signage in the Grove.

Also at the Grove there is a much smaller oak with similar leaves, located on the former lawn at the edge of the current grassland, which has been marked on Shields Grove maps as Quercus trojana. The smaller tree is quite distinct from the larger trees, though they now have the same name. The smaller tree actually fits descriptions of Q. trojana better, and it is a much more valuable tree.

The larger trees, formerly Q. macaedonica, are powerfully marcessant, holding all their dead leaves all winter since planting. In the Shields Grove, these trees are located adjacent to a pair of true Quercus cerris, one of which produces acorns with regularity. These two Q. cerris both share an upright to spreading growth habit, some yellow fall color, and the leaves drop cleanly after coloration. In other words, these two Q. cerris show no marcessance.

But in growing 400 of those Q. cerris acorns into seedlings, all were found to be marcessant for at least the first 5 years of their life. There are oaks, individuals spread amongst various species and subgenera, which are marcessant when young, and then transition to normal leaf drop with age. The best know of this type is Quercus coccinea, well known to hold dead leaves (be marcessant) for roughly the first decade of their lifespan, then change to a normal leaf dropping deciduous tree for the remainder of their lifespan.

This explains why so many of the older Q. coccinea seen in the landscape are cleanly deciduous trees, while the young Q. coccinea planted are invariably marcessant. This change in leafing behavior has compounded the epic confusion regarding the so-called Eastern Red Oaks in California.

Caucasus Oak - Quercus macranthera

Located almost entirely in the Mediterranean the oak subgenus Mesobalanus is a small group of closely related oaks that, so far, show good adaptation to the northern two thirds of California.

The best known of these is Quercus frainetto, a stately, upright late deciduous species native to the Balkans and the wetter parts of the Mediterranean itself. In more arid zones nearby, Q. macranthera can be found. In even more arid areas, and with a small distribution, can be found Quercus pyrenaica.

While we have many thousands of Q. frainetto in the California landscape, there are currently no known Quercus macranthera. Extrapolating from its native environments, Q. macranthera may prove even better adapted to California microclimates than Q. frainetto.

Quercus macranthera is highly recommended for further trial in California once confirmed propagules can be secured.

Eurastian Mesobalanus Oaks - Quercus frainetto / Quercus macranthera / Quercus pyrenaica / Quercus dalechampii

Visalia. Palo Alto. Apple Park, Cupertino

The 'Forest Green' cultivar of Quercus frainetto has seen limited planting around California since the late 1990's. The trees grow well, with rapid growth, attractive foliage, strong branch attachments, decent apical dominance, and low to nonexistent acorn production, a bonus for urban oaks.

Unfortunately, the 'Forest Green' cultivar is grafted by J. Frank Schmidt nursery, and they have historically used Q. robur and it's cultivars as rootstock. Sadly, Q. frainetto and Q. robur are in different oak subgenera, and delayed graft incompatibility is common.

In Visalia, Palo Alto, and Cupertino, beautiful 'Forest Green' trees have been seen to decline and die from delayed graft incompatibility, anywhere from years to decades after planting. This has been a disaster for the early adopters of this otherwise attractive new tree.

As the grafts on 'Forest Green' fail, shoots begin to emerge from the rootstock - a sign that failure is inevitable - and those shoots clearly show the differing genetics of the rootstock and mother tree. 'Forest Green' Quercus frainetto is not recommended due to these failures.

It is recommended that other forms of Quercus frainetto (along with the closely related Mesobalanus oak species Q. macranthera and Q. pyrenaica) should be identified and brought to California, to serve as genetically appropriate seed sources for future use of this species, mostly for the northern half of the state, and for locations farther to the north.

Spanish Sweet Oak - Quercus rotundifolia

With its widespread adaptation in the Mediterranean, Quercus rotundifolia is certainly a promising species for coastal California. Unfortunately, these trees are exceedingly rare. A dozen or so trees were grown in Central Valley nurseries for the Apple Park planting, and their performance in the ground in Cupertino has been superior.

Though Q. rotundifolia is a typically variable oak, superior small to medium size tree selections should be sought out for inclusion in future California plantings. Q. rotundifolia has real promise as a habitat enhancement and migration species, due to its consistently low-tannin acorn production. Q. rotundifolia has been a critical species in the Spanish de hasa system, which produces both fattened Iberian swine, and also cork from the rightly famous Quercus suber.

Quercus rotundifolia is highly recommended for further trial in California once confirmed propagules can be secured.

Trojan Oak/Lebanon Oak - Quercus trojana/Quercus libani

In Eurasia, there are a series of oaks similar to Quercus cerris, mostly from more arid climates. These trees are mostly smaller than Q. cerris, with leaves also smaller and more generally pointed. This series includes Q. libani and Q. trojana, and used to include Q. macaedonica, which has now been subsumed into into the species Quercus trojana.

Several of these trees are represented in the Shields Oak Grove, but the nomenclature on these trees in the Shields Grove has changed over time. The two larger trees used to be called Q. macaedonica, now changed to Quercus trojana on signage in the Grove.

Also at the Grove there is a much smaller oak with similar leaves, located on the former lawn at the edge of the current grassland, which has been marked on Shields Grove maps as Quercus trojana. The smaller tree is quite distinct from the larger trees, though they now have the same name. The smaller tree actually fits descriptions of Q. trojana better, and it is a much more valuable tree.

The larger trees, formerly Q. macaedonica, are powerfully marcessant, holding all their dead leaves all winter since planting. In the Shields Grove, these trees are located adjacent to a pair of true Quercus cerris, one of which produces acorns with regularity. These two Q. cerris both share an upright to spreading growth habit, some yellow fall color, and the leaves drop cleanly after coloration. In other words, these two Q. cerris show no marcessance.

But in growing 400 of those Q. cerris acorns into seedlings, all were found to be marcessant for at least the first 5 years of their life. There are oaks, individuals spread amongst various species and subgenera, which are marcessant when young, and then transition to normal leaf drop with age. The best know of this type is Quercus coccinea, well known to hold dead leaves (be marcessant) for roughly the first decade of their lifespan, then change to a normal leaf dropping deciduous tree for the remainder of their lifespan.

This explains why so many of the older Q. coccinea seen in the landscape are cleanly deciduous trees, while the young Q. coccinea planted are invariably marcessant. This change in leafing behavior has compounded the epic confusion regarding the so-called Eastern Red Oaks in California.

White Oak - Quercus alba

Shields Oak Grove, UC Davis

The single Q. alba in the Shields Oak Grove is notorious for chlorosis. Which makes perfect sense, since the tree grows all over the wet summer eastern half of the US, where soils are generally acidic. The native range of Q. alba stops abruptly where alkaline soils and intermittent to non-existent summer rains become the norm. Q. alba is a tree from cold winter, wet summer climates which makes little sense for a climate changed California. If you are looking for the future adapted climate for Q. alba, perhaps plant them in eastern Canada, north of their current range. Q. alba is sometimes marcescent, adding another reason to limit use. Overall, Q. alba is a poor choice for widespread use in California. But, as with oaks of many kinds, superior specimens can be grown in California locations with deep alluvial soils and favorable groundwater access. Not recommended., including any oak hybrids containing Q. alba as a parent.

Swamp White Oak - Quercus bicolor

Stanford University test plantings across Galvez Road from the football stadium.

The common name for this tree is Swamp White Oak. Swamps are, obviously, wet places, and they have acidic soils. Q. bicolor is also from quite cold winter climates. None of these features suggest widespread success in California. The other "swamp" oak from the eastern United States that has been utilized, extensively but unsuccessfully, in California historically is Quercus palustris. As is typical for all types of oaks, you can occasionally find good specimens of Q. palustris on favorable sites, largely in the central third of California. Unfortunately, we can't see all the Q. palustris which have died or been removed over the years, which includes virtually every Q. palustris ever planted here. Chlorosis is common with Q. palustris in heavier soils, most trees are marcessant, and life span on all but the best sites is reduced from centuries to decades. One can expect the same from Q. bicolor give its similar adaptation. Not recommended. Also, beware any oak hybrids containing Q. bicolor as a parent.

Texas Red Oak - Quercus buckleyi

Apple Park, Cupertino. UC Davis Campus

In California, there is undeniable need for moderately sized trees with good drought adaptation and superior fall color. Headed southward (and a bit east) from California, Quercus buckleyi becomes an obvious candidate for experimentation. Limited use of Q. buckleyi in the cities of the American Southwest, like University Avenue in Tucson, has encouraged this experimentation. Unfortunately, after a decade of observing Q. buckleyi in Tucson, we have learned that the tree has a very limited drought adaptation, because the trees on University Avenue look tattered and terrible, even after receiving good establishment care in their early years.

Quercus buckleyi is closely related to Q. rubra and Q. shumardii, and many other of the so-called Eastern Red Oaks. Quercus buckleyi has begun testing in California, but problems are becoming rapidly evident. At Apple Park, poor soil drainage has led to the stunting of dozens of the test trees, which show their unhappiness with yellow, chlorotic foliage during the growing season.

An additional demerit to Q. buckleyi in California can be found in its rather remarkable ability to reproduce in California. Dozens of large Q. buckleyi were grown in the Farmington BrightView nursery for several years, fully accessible to the tree buying public. During the several year grow period, Q. buckleyi seedlings from the heavy acorn producing trees began to sprout in the coarse nursery mulch, often far from the mother trees. These are very forbidding conditions for most oak acorns to germinate, yet hundreds, if not thousands of seedlings were readily visible.

During this grow period at BrightView Farmington, many other types of oaks not typically grown in California were evident in the nursery. Many of these unusual tree types produced acorns, often in quantity. But only Q. buckleyi seedlings were seen to be sprouting vigorouly in the nursery rows. And the only other oak to approach the reproductive fecundity of Q. buckleyi? That would be the California native Quercus agrifolia, a tree that some people jokingly refer to as an invasive native plant.

After initial enthusiasm, Quercus buckleyi has proven a poor choice for widespread use in California. But, as with oaks of many kinds, superior specimens can be grown in California locations with deep alluvial soils and favorable groundwater access. Not recommended.. If Q. buckleyi is used despite these warning, please keep in mind the tree is likely to spread, especially into ecologically fragile riparian areas. Not recommended.

Overcup Oak - Quercus lyrata

Menlo Park, California

The only currently-known true species Quercus lyrata in California stands in a front yard in Menlo Park, in the very deep alluvial fan adjacent to San Francisquito Creek, a large creek draining the redwood-festooned Santa Cruz Mountains.

These fans, which extend into Palo Alto and Atherton, support some of the largest trees of their types in the entire Bay Area. These deep gravelly soils, with the enhanced water table that inevitably occurs adjacent to major drainage corridors, provide growing conditions that allows virtually any non-tropical tree species to thrive. Oaks in particular thrive in these soils, with their profound deep rooting potential.

Keeping this underground context in mind, we can understand why the Quercus lyrata in Menlo Park is a very healthy, symmetrical tree that is likely nearing a century in age. Q. lyrata is known to grow best in deep, well drained soils with access to soil moisture. The Menlo Park tree produces acorns regularly, displaying the overcup habit that gives the tree its common name.

Fittingly for its location here, native Q. lyrata grows along creeks and rivers in the southeastern United States. The "overcup" acorn evolved to fall from the tree into flowing water, then float to it new potential home, somewhere downstream on the banks of the watercourse. Quercus lyrata is known for its tolerance of flooding in its native range. Functional flood tolerance in California is unknown.

Another key to the success of the Menlo Park Q. lyrata is the surrounding well-watered landscape, including an abundant green lawn, all of which appear to have been present with throughout the trees lifespan. Without the lawn, in addition to the deep alluvial soils, it is doubtful that this Q. lyrata would be even a fraction of the specimen it is.

Bur Oak - Quercus macrocarpa

Fremont. Davis. San Jose. Sacramento. Stanford. Likely many others

Quercus macrocarpa is likely the most widespread oak in the United States. Whether Q. macrocarpa appears as a species, as a hybrid, or as a key genetic component in other native oak species, it is likely that some substantial Q. macrocarpa genetics can be found in every state, perhaps with the exception of Hawaii, though this author suspects that a search of the Hawaiian Islands would yield - something. Probably twisted and riddled with mildew, but trying to grow nonetheless. Quercus macrocarpa is a tenacious oak without a doubt.

In the Bay Area, and other California locales subject to powdery mildew, virtually any true species Q. macrocarpa will be affected, and probably substantially, and yearly. Fast growing trees will be the most susceptible.

Resistant hybrid forms of Q. macrocarpa can be found on Gerona Road on the Stanford Campus, and also adjacent to Galvez Road paralleling El Camino Real across from the Stanford football stadium. (Some old northerly-genetics Q. macrocarpa can be found nearby at the entrance to Frost Amphitheater.)

But even if cleanly foliated examples are found, there is no guarantee that they can be reproduced. Oaks are known for both their tendency to hybridize, making acorn reproduction dicey at best, assuming the desired tree ever produces acorns at all. Many oaks, especially far from their native homes, never produce a single acorn.

And, of course, oaks are notoriously difficult to either graft or grow from cuttings. Difficult, but not impossible. Successful grafts can be made on many types using a hot callus grafting apparatus, but such set ups are extraordinarily rare, and are far from guaranteed from giving successful results.

Burgambel Oak - Quercus macrocarpa x gambelii

Shields Oak Grove, UC Davis

Oak explorers traveling deep into the Gila Wilderness in New Mexico discovered large single trunk white oaks, nominally Quercus gambelii, but highly distinct from species Q. gambelii in being singled trunked, much larger in size, and with leaves highly distinct from the species, but in fact resembling Quercus macrocarpa. Some of the trees located are 100 feet tall, or more. These are trees utterly unlike the shrubby Q. gambelii which characterize the vast bulk of the species throughout the entire Rocky Mountain area.

These trees are fascinating relict hybrids of Quercus macrocarpa and Quercus gambelii. A small number of wild offspring of these trees have been grown in California, and have performed surprisingly well, showing much greater resistance to powdery mildew and other fungal foliar diseases.

These native hybrids are smaller in stature and acorn size compared to Quercus macrocarpa, making this a natural hybrid worth of further investigation for central to northern California, and other coastal states northward from there.

In the Shields Oak Grove can be found artificial hybrids of Q. macrocarpa and Q. gambelii. These appear quite distinct from the natural hybrid forms, and are derived from shrub form Q. gambelii growing adjacent to Salt Lake City, where researchers at the University of Utah did the hybridizing.

This oak hybridization, along with many others, were carried out by Dr. Walter Cottham, his grad students, and associates at Universities throughout the western United States. Shields Oak Grove has many Cottham hybrids, though few if any any of the myriad forms have proven useful in California.

Macdaniel Oak - Quercus macrocarpa x robur (Quercus x macdanielli)

Shields Oak Grove, UC Davis. Stanford University

Once again, we have hybrids from the work of Dr. Walter Cottham growing in the Shields Oak Grove. There are nearly a dozen Q. x macdanielli in the Shields Grove, and offspring of these trees can be found growing across Galvez Street on the Stanford Campus across from the football stadium.

Since both parent trees have a strong history of leaf diseases in California (powdery mildew and others on Q. macrocarpa, with aphids and sooty mold being the norm on Q. robur in most of coastal California), the offspring are likely to have similar issues. The trees at Stanford have grown, but have little to recommend them in most of California. They may be useful in more northerly climates. Not recommended for coastal California.

Chinkapin Oak - Quercus muehlenbergii

Shields Oak Grove, UC Davis

Q. muehlenbergii is one of the most widespread oaks in the United States. Q. muehlenbergii can be found, almost invariably at low densities, in most of the states in the eastern half of the US. Hybrids are rather uncommon, and the range of Q. muehlenbergii abruptly ends moving westward across the US, as drought and alkaline soils are ill-suited to this species. Experience with Q. muehlenbergii in coastal California confirms the tree is highly susceptible to a variety of foliar diseases, and the leaves, along with the overall growth of the trees, is invariably stunted, twisted and misshapen. Planting of Q. muehlenbergii within California zones of marine influence is strongly discouraged.

Southwestern Chinkapin Oak - Quercus muehlenbergii var. brayi

Isolated pockets of the American Southwest, in Texas and New Mexico

Far cut off from the main distribution of Q. muehlenbergii can be found some very distinctive trees, with leaves smaller, bluer and thicker than is typical for the species. These trees grow in an arid, monsoonal climate, and appears to have endowed this tree with a thick, waxy cuticle on the leaves, great for protecting the leaves from fungal infection in the California coastal zone. The trees that have proven successful at Apple Park were from the Capitan Mountains of New Mexico, and are extremely rare. These rare trees have excellent fall color, and a wonderful upright growth habit. In the past, they have been sold by Trees That Please from New Mexico.

Water Oak - Quercus nigra

Frost Amphitheater, Stanford University

Near the main entrance to Frost Amphitheater at Stanford, an unusual variety of oaks can be found, a few recent, but mostly they appear to have been planted in the early years of the University. This test grouping has proved quite valuable to the handful of horticulturists aware of its existence. The old Quercus nigra which grows immediately adjacent to the main entrance has proven remarkably robust.

The tree is late deciduous to semi-evergreen, typically keeping a thin later of green leaves through the winter, but dropping the bulk of its leaves by New Years. The trunk and bark resemble the local Quercus agrifolia, showing their shared genetic heritage.

The native range of Quercus nigra ends abruptly in Oklahoma and Texas, as drought becomes more frequent, and soils become alkaline. The area where the Quercus nigra grows at Stanford has long been known to have an elevated water table, and USGS soil mapping shows deep alluvial soils. These are conditions where many trees can thrive, so the good performance of Q. nigra here needs to be taken with a grain of salt, due to the uncommonly good soil/water relationships in this location. Factors which could hinder performance of Q. nigra include lack of access to groundwater, heavier/clay soils, and greater heat typical of more inland locations.

An interesting test of Q. nigra occurred during Hurricane Katrina in New Orleans. Observers noted that the two most common trees at the time of the hurricane were Quercus virginiana and Quercus nigra. Q. virginiana is noted for both branch and wood strength, and, not surprisingly, came through the hurricane with minimal damage. Quercus nigra fared much less well during the hurricane, with many trees having their canopies literally blown off their trunks. This vulnerability can be clearly seen in the Stanford specimen, where the two main stems of the structure clearly show poor attachments, along with a codominant arrangement, with both traits predisposing their possessor to catastrophic failure.

Quercus nigra is a poor choice for widespread use in California, but superior specimens can be grown on deep alluvial soils, especially with supplemental summer irrigation. Not recommended, though there is no denying the charm of this oak, which the French call the duck-footed oak, for reasons obvious to anyone who has seen the tree.

Pin Oak - Quercus palustris

Many locations throughout central to northern California. If any large number were planted in southern California, they haven't been found, and likely died shortly after planting.

The real question is, why did someone think it was ever a good idea to plant Q. palustris in California? They are trees with a very limited range, and that range happens to be swampy ground in a climate with, at best a 4 month growing season (as opposed to 10 or more in coastal California!). Oh, and minimum temperatures down to -30 degrees Fahrenheit.

Oh, and there's the fact that almost all Q. palustris are marcessant, steadfastly holding their dead paper bag colored leaves throughout the winter. Growing up in the midwest of the US, the marcessant Q. palustris stood out in the landscape, which was otherwise utterly barren except for a handful of conifers. 'Twas a bleak landscape, teaching the sensitive to appreciate color and pattern. In such a bleak landscape, even dead leaves provide a measure of releaf from the visual monotony.

But here in coastal California, in our radically different climate, evergreen trees, be they conifer or broadleaf, are by far the norm. Sure, we have deciduous broadleaf tree species, but they are, more or less, Ice Age relicts. Deciduousness evolved because broadleaf trees have leaves containing substantial amounts of water. Water expands when temperatures fall below freezing, popping leaf cells and killing leaves.

But where temperatures rarely drop below freezing, such an adaptation has no value, and is, frankly, detrimental. Why detrimental? If you've experience a bright sunny day in mid winter in coastal California, which is quite common, you know that a tree with green leaves could be fixing away a nice amount of energy to fuel their future growth. This extra energy is the key reason that evergreen trees of all kinds dominate in coastal California.

Against the backdrop of evergreen trees, marcessant trees stand out. Unfortunately, the message they announce in coastal California says to people "I'm dead! Cut me down!" The message of death is unwelcome and troubling given the horticultural and ecological context.

Willow Oak - Quercus phellos

Willow oak grows in area with abundant summer rainfall, and generally acidic soils. Most Willow oaks grown in California have shown stunting and chlorosis. Some good specimens have been grown on sites with excellent soil quality, and good irrigation practices. Willow oak is not recommended for California.

Sandpaper Oak - Quercus pungens

Shields Oak Grove, UC Davis

A small Sandpaper oak grows in the Shields Oak Grove at the University of California, Davis. The tree grows well on the deep soils of Davis. Could be grown on sites with excellent soil drainage.

Red Oak - Quercus rubra

Commonly planted throughout the urban Bay Area. Rare in southern California.

Quercus rubra is one of the most commonly planted "foreign" oaks in California. This tree is the namesake of the Red Oak subgenus, and the central species among what is often known as the Eastern US Red oaks, which includes Q. coccinea, Q. palustris and a few others. Q. rubra has been the most successful of the group in appropriate climates of California. And the reason it is the most successful is completely predictable.

Why predictable? Quercus rubra has a native range which encompasses nearly half of the continental United States, with its range extending into the Midwest. This range shows a real tolerance of both soil and rainfall variations, tolerances which predispose Q. rubra to success when taken far from home. This type of broad-range tolerance is likely to be one of the key factors in choosing trees for climate change adaptation and migration.

As California's climate changes, and inevitably warms, most of the state will become unsuitable to Q. rubra, as southern California now is. But the overall lesson still applies - the near future of the biosphere will belong to generalist organisms as the climate continues unpredictably destabilizing.

Although Q. rubra has performed best amongst its close cousins, that is far from meaning it has been a perfect tree. Probably the biggest drawback to Q. rubra has been its large trunk and root flare. When planted as a street tree, particularly in sidewalk cutouts, Q. rubra has proven to be quite destructive. This phenomenon is readily visible along El Camino Real in Mountain View, where trees planted in the 1990's have grown large, and have raised, cracked and buckled large areas of sidewalk, creating a chaotically uneven walking surface.

Another structural aspect of Q. rubra again lessens the value of the tree in the urban context. Q. rubra is a very wide-spreading species, growing large horizontal limbs. For ease of maintenance, urban foresters strongly prefer upright growing trees, which naturally grow out of the way of traffic and sidewalks. But Q. rubra naturally grows as wide, or wider, than it is tall. This growth habit forces substantial early pruning for clearance and structure, tasks which the California landscape industry and practitioners rarely perform either well or consistently.

Post Oak - Quercus stellata

Shields Oak Grove, UC Davis

Quercus stellata is another oak whose native range ends abruptly moving westward across the United States, as the species does not tolerate the drier, alkaline soils of the American Southwest and California. While the Q. stellata growing in the Shields Oak Grove are performing adequately, it is widely known that soil conditions in Davis are among the best in California.

A deciduous species whose range extends up into the US midwest, Q. stellata is adapted to much colder climates than the vast majority of California, and comes from areas with typically abundant summer rainfall. Q. stellata is a tree with limited potential in California, a species unlikely to thrive in most areas of the state.

Shumard Oak - Quercus shumardii

Apple Park, Cupertino, Tantau Road. California Avenue, Palo Alto.>

As the so-called eastern red oaks make their evolutionary way south in the US, into areas with a longer growing season, another oak species jumps into the mix. This is Quercus shumardii, native in Texas, Oklahoma and areas nearby. Q. shumardii offers good fall color, and a better, more upright growth habit than Q. rubra. But unlike Q. rubra, Q. shumardii is prone to marcessance, but this trait varies literally from one tree to the next.>

Q. shumardii also shares the enlarged root flare/trunk base common to the other eastern red oaks, and a trait which make the tree very marginal for street tree use. Q. shumardii has also proven highly susceptible to stunting due to poor nursery root production techniques. >

After copious trialing, it's clear that Q. shumardii is less-than-ideal for widespread use in California. Yes, good trees can be grown, especially on sites with good soil drainage, lots of room for root expansion on the ground plane, and abundant establishment irrigation.

Wavyleaf Oak - Quercus x undulata

We include the Rocky Mountain hybrid that includes Q. gambelii genetics, along with Q. turbinella, Q. macrocarpa and potentially other oaks. This type is widespread throughout the Rockies in the US, and was the original inspiration for Dr. Walter Cottam at the University of Utah to begin his investigations into oak hybridization.

Dr. Cottam's experiments led to the creation of dozens of hybrid oaks, a surprising number of which ended up being planted at the Shields Oak Grove at the UC Davis Arboretum. Dr. Cottam was working closely with professors at UC Davis at the time of his hybridization experiments, and this was the time of the main plantings in the Shields Grove.

There are many different and odd hybrids in Shields which originate from Cottams work. I have grown seedling from many of these trees, and, unfortunately, every single one I've grown is highly susceptible to powdery mildew. Some of these Q. x undulata types are being grown by Heritage Seedlings in Oregon, and are beginning to be seen in the California nursery industry.

While these shrub/trees can grow in well-drained soil locations outside of current powdery mildew range in California, there is little need for large deciduous shrubs with high powdery mildew susceptibility in the California landscape. Q. x undulata is best used in cold northerly climates where more southerly oaks succumb to extreme cold.

Sawtooth Oak - Quercus acutissima

Shields Oak Grove, UC Davis

Native to wet summer Asian climates. Common in the eastern half of the United States. Native soils are on the acidic side, so chlorosis common in California, especially in heavier coastal soils, and with drought. A deciduous oak prone to marcescence. Q. accutissima is closely related to several Mediterranean-adapted oaks, especially Q. cerris, Q. castaneifolia, and Q. trojana. Any of these trees are proven to grow in climates much more similar to California than wet summer Asia. Overall, Q. acutissima, and the very closely related Q. variabilis, are both poor choices for widespread use in California. But, as with oaks of many kinds, superior specimens can be grown in California locations with deep alluvial soils and favorable groundwater access. Not recommended.

Daimyo Oak -Quercus Quercus dentata

Quarry Hill Botanical Garden, Glen Ellen

Like most true Asian oak species, Quercus dentata has a strong preference for acidic soils. Soils in California, and the American Southwest, are consistently alkaline due to lack of leaching from our minimal rainfall. Soil pH is very hard to change throughout the root zone of a mature tree. Trees native to acidic soil zones will often turn chlorotic in California's alkaline soils, particularly under droughty conditions, or if the tree is otherwise stressed. This reality, combined with the fact that acid loving plants are also nearly invariably moisture loving plants, suggests perhaps looking elsewhere on the planet for good candidate oaks for growing in California. Of course, even here in California, in locations where soils are deep, and oaks can root deeply down into a consistent water table, superior specimens of many of the acid-preferring oaks can be grown. Quercus dentata, along with many other acid-preferring oak species, are not suitable for widespread planting in California. Not recommended.

Mongolian Oak - Quercus mongolica

Quarry Hill Botanical Garden, Hybrid with Q. robur in Shields Oak Grove, UC DavisQuercus mongolica is a cold winter deciduous oak from areas of Asia with abundant summer rainfall. This is a tree for trial in far northern US States, Canada, and other cold winter climates with year round precipitation. Overall, Quercus mongolica is a poor choice for widespread use in California, but superior specimens can be grown on deep alluvial soils, typically in concert with summer irrigation. Not recommended.

Shira-kashi - Quercus myrsinifolia

Shields Oak Grove, UC Davis

Two of these trees were originally planted in the Shields Grove in the 1960s. Several decades later, despite the excellent Davis soil, one tree was very close to dead, and the other was in very poor condition. There were several decades after the planting of the Shields Grove when maintenance, irrigation and pruning was minimal. This period served as a real test of the drought tolerance of these oaks, with the winners looking excellent despite the neglect. Q. myrsinifolia proved to be a poor performer under dry California conditions. Interestingly, very close to the Q. myrsinifolia in the Shields Grove grow numerous Q. phillyreoides, also from Japan, and they performed with surprising resilience during the dry years, and continue to grow well.

Ubamegashi Oak - Quercus phillyreoides

Shields Oak Grove, UC Davis. 101 Soundwall, East Palo Alto

Quercus phillyreoides is an unusual smaller evergreen oak from Asia. The smallish leaves and dense growth habit allows the tree to be used in highly formal topiary. They display some shade tolerance, and seem to have a higher level of drought adaptation than one might assume judging from rainfall in the trees native range.

Quercus phillyreoides is a poor choice for widespread use in California, but reasonable specimens can be grown on deep alluvial soils, especially with supplemental summer irrigation.

Monterey Oak - Quercus polymorpha

Stanford University, near Gerona Road and near Palo Road. BrightView Nursery, Fillmore

Quercus polymorpha is a tree in great demand in urban southern Texas, particularly in Austin and Houston. Q. polymorpha is widely distributed in eastern Mexico, the Atlantic Ocean side of Mexico, areas consistently wetter than western Mexico, where those in California have a much higher likelihood of discovering useful oak species.

Two small sets of Q. polymorpha have been growing at Stanford for more than a decade. One set is on a dry site near the Dish at Stanford. Although these trees have continued to grow, they rarely look good, and seem unhappy with such a difficult site. The other small set, near Palo Road, are in deeper soils on a lusher growing site, and have a small amount of supplemental irrigation. These consistently look better than the drier upland trees, but have proven less-than-inspiring in other ways.

We see a moderate level of powdery mildew on Q. polymorpha consistently. The growth habit is widespreading when the tree is young, forcing substantial pruning on young trees when clearance is an issue. Quercus polymorpha is a dubious choice for widespread use in California, but superior specimens can be grown on deep alluvial soils, especially with supplemental summer irrigation.

Pope's Oak - Quercus pontica

Portland and elsewhere in Oregon

Quercus pontica is a fascinating but rarely seen tree in the western United States. It is one of a group of two oak species, on opposite sides of the world. The sister species, Quercus sadleriana, grows in the mountains of northern California and Oregon. These are small to very small oaks with large leaves, and are quite unique amongst oaks. Limited numbers of Q. pontica have been grown in the Pacific Northwest, often as small, unique specimens in tubs or containers. They can likely be grown in California, but a tree so small has little public use. It can be considered a novelty ornamental species, a rarity among oaks.

English Oak - Quercus robur

Planted in small quantities throughout California. Examples can be found in: Golden Gate Park in San Francisco, Palo Alto, a dying old tree in the Stanford Arboretum, LotusLand and elsewhere in Santa Barbara, many other scattered locations.

Quercus robur is a classic generalist tree species with an enormous range in Europe and even into Eurasia. Q. robur grows in very northerly locations all the way far into the Mediterranean. Given this immense range of adaptation, it's no surprise that Q. robur can be found planted throughout coastal California.

In Palo Alto, a gentleman found Q. robur acorns in Golden Gate Park, and chose to grow a tree from those acorns for his own back yard. Interviewed 15 years later, this well-intentioned gentleman regretted his decision. You see his Q. robur grew over his back patio, and the tree dropped enormous amounts of aphid dew onto the pavement below. Q. robur is quite susceptible to aphids in most locations, and in most years, throughout coastal California.

This leads not only to consistently sticky sidewalks and patios, but it also leads to foliage covered in black sooty mold which has grown on the aphid honeydew. Hybrids containing Q. robur typically inherit the aphid susceptibility. Overall, Quercus robur has proven a poor choice for widespread use in California, but superior specimens can certainly be grown on deep alluvial soils, and typically survive even on poorer sites, though performance will be stunted, and pest and disease susceptibilities enhanced. Not recommended.

Fastigate English Oak - Quercus robur var. fastigata

Scattered in low densities throughout coastal California. Shields Oak Grove features numerous specimens. Also, large, old trees can be found on the campus of the College of Marin. Newer cultivars from Oregon nurseries are now being planted in urban plantings, such as locations at Stanford University.

Quercus robur var. fastigata has a different set of strengths and weakness than the parent species. With Q. robur var. fastigata, a key drawback is marcessance, discussed at length in other sections of this writing. The other critical drawback is the narrow, columnar growth habit. Despite urban foresters and some landscape architects claiming this columnar form is good for urban planting, their logic in practice shows critical flaws.

Essentially, every benefit a tree offers, from shade to carbon fixation to oxygen emitted all track canopy size. If you want a tree to produce copious shade, it needs to have a spreading canopy, not a narrow column. Columnar trees offer an unwanted exclamation point in the environment. And if it's winter, the exclamation point is the non-inspiring color of a paper grocery bag.

In California, you will often find people denigrating the form of Cupressus semperviren, the iconic Italian Cypress. People agree that C. sempervirens look best when planted as a group, but, when planted singly, look quite odd.

Yet some of the same people who denigrate the growth habit of C. sempervirens are recommending these columnar Q. robur var. fastigata. A key to the successful use of new oaks is to view as many of the new trees as possible in environments similar to proposal planting locations. After viewing a street planted with narrow, marcessant Q. robur var. fastigata, few people will want to repeat the challenging and frankly odd aesthetics.

Fastigate English Oak - Quercus robur var. fastigata

Scattered in low densities throughout coastal California. Shields Oak Grove features numerous specimens. Also, large, old trees can be found on the campus of the College of Marin. Newer cultivars from Oregon nurseries are now being planted in urban plantings, such as locations at Stanford University.

Quercus robur var. fastigata has a different set of strengths and weakness than the parent species. With Q. robur var. fastigata, a key drawback is marcessance, discussed at length in other sections of this writing. The other critical drawback is the narrow, columnar growth habit. Despite urban foresters and some landscape architects claiming this columnar form is good for urban planting, their logic in practice shows critical flaws.

Essentially, every benefit a tree offers, from shade to carbon fixation to oxygen emitted all track canopy size. If you want a tree to produce copious shade, it needs to have a spreading canopy, not a narrow column. Columnar trees offer an unwanted exclamation point in the environment. And if it's winter, the exclamation point is the non-inspiring color of a paper grocery bag.

In California, you will often find people denigrating the form of Cupressus semperviren, the iconic Italian Cypress. People agree that C. sempervirens look best when planted as a group, but, when planted singly, look quite odd.

Yet some of the same people who denigrate the growth habit of C. sempervirens are recommending these columnar Q. robur var. fastigata. A key to the successful use of new oaks is to view as many of the new trees as possible in environments similar to proposal planting locations. After viewing a street planted with narrow, marcessant Q. robur var. fastigata, few people will want to repeat the challenging and frankly odd aesthetics.

Quercus robur var. fastigata has proven a poor choice for widespread use in California due to poor ecological and aesthetic adaptation, but superior specimens can certainly be grown on deep alluvial soils. Not recommended.

Asian Cork Oak - Quercus variabilis

Shields Oak Grove, UC Davis. Native to wet summer Asian climates in Japan, Korea and China.

In addition to much higher than California rainfall, particularly in summer, Q. variabilis is native to acidic soils common to wet summer climates. Thus chlorosis on Q. variabilis is common in California, especially in heavier coastal soils. Additionally, this deciduous oak is highly prone to marcescence, which causes many laypeople who see the tree in winter to think the tree is dead. While marcescence is aesthetically acceptable in some areas, it is nearly universally hated in California. Overall, Q. variabilis is a poor choice for widespread use in California, but superior specimens can be grown in deep Central Valley soils, and on favorable soil/climate sites elsewhere. Not recommended.