EDITOR'S ARMCHAIR: So Many Oaks, Such Little Gardens!
By Tony Gurnoe, for Let's Talk Plants October 2026
So Many Oaks, Such Little Gardens!
Oaks are not the most approachable organisms, whether speaking botanically or horticulturally. I personally spent the first several years of being a professional horticulturist doing what I could to avoid them, because they scared and annoyed me. Nobody around me could tell me exactly what kind of oak was in front of us. Their reputation among horticulturists was being extremely slow growing and unexciting plants. Plus, despite oaks supposedly being highly durable, I had killed the first few endangered Engelmann oaks I was in charge of caring for! Another thing that everybody seemed confident of was that oaks were among the most important plants on our planet in terms of their ecological interactions and the way that they act as a hub for enhancing broader biological diversity.
As fortune would have it, I necessarily got over all of those quibbles and hang ups and have since become one of our region’s most knowledgeable botanists regarding the Quercus genus, and probably Southern California’s most experienced horticulturist and arborist when it comes to working with oaks. In some cases, such as with Quercus cedrosensis, I led the effort to cultivate that endangered species from the small and highly threatened U.S. population immediately north of the border with Mexico for the first time ever. On the other end of the scale:rarity dynamic I’ve also worked on oak inventories as large as 13,000+ individuals. Accidentally, over the last decade, I became one of our region’s go-to people for knowledge about oaks. My hope here is to distill that professional and academic understanding into useful and usable highlights for gardeners in San Diego.
The September meeting of the San Diego Horticultural Society included a last minute change in presenter which meant that I had to very quickly throw together a presentation on a subject that I knew well enough to improvise but that would hopefully be engaging for SDHS members. Oaks are like a language that I feel comfortably fluent in, but they also embody such an enormous subject that initially I was unsure how to circumscribe something useful without losing the interesting dynamics of the overall storyline about the talent these plants have for interaction and connectivity. As I started to think through what kind of information would be useful for gardeners I realized that in a presentation format it’s impossible for me to neglect the fun storytelling elements about such storied trees, so I focused on creating horticultural profiles for several of the oaks relevant to managed landscapes in San Diego.
Since I threw so much information at the audience that day, I’m taking the opportunity of this article to summarize some of the pertinent or interesting bits about the following species of Quercus, native to our region.
Quercus agrifolia
Coast live oaks (Q. agrifolia) are the critical staple tree of the western portion of California, without the slightest bit of exaggeration. A massive band from south of the border to beyond the bay area is, or at least was, dominated by these stately, spreading trees. These trees can be identified by their cup shaped leaves with prickly points at the end of each lateral vein. For a deeper dive, look for the tufts of fuzz in the axils of the veins on the underside of the leaves - this is one of the ways that you can differentiate Q. agrifolia from Q. wislizeni.
This is the species that most people think of when they think abiout oaks in California, and as a consequence of their ubiquity, they’re definitely the most planted oak in San Diego. Setting aside the major intrinsic benefits to growing coast live oaks, there are potentially negative things to consider. One might be size. For those with space, a large tree is of incomparable benefit. For those of us with small suburban lots, a coast live oak can be a challenging roommate. They insistently grow wide and low. Calscape says this species gets to be up to 35 feet wide but I have certainly been under the canopy of many 60 foot across coast live oaks. Calscape is a great resource, but consider it a database still in development.
The other major, and novel challenge to growing coast live oaks is in their susceptibility to the recently introduced gold spotted oak borer (GSOB) (https://cisr.ucr.edu/invasive-species/goldspotted-oak-borer). I am sad to say that we are seeing significant losses among San Diego’s most majestic coast live oaks to this small insect. Hopefully we get to the point of some kind of new homeostatic balance, but what started as an inland, wild tree problem has been spreading and horribly marring, if not outright killing, many of the coast live oaks throughout our cities. This pest problem, along with the size of these trees, have been among the motivations prompting gardeners to explore other oak choices.
Quercus wislizeni
It continues to surprise me how unfamiliar gardeners are with the interior live oak (Q. wislizeni). Just over the mountains from their coastal sibling, the larger iterations of these trees behave very similarly. Physical distinctions include leaves with entire margins rather than prickles, and smooth undersides rather than hairy. More to the point of horticultural usage, there’s an interesting feature available to us in Q. wislizeni through a natural variety. Q. agrifolia also has two natural varieties but they’re not substantially different in their application to the gardener. This, however, is the opposite. Q. wislizeni var. frutescens flips the size dynamic on its head. Frutescens essentially means “shrubby,” and that’s what this variety is. Instead of a large tree, this variety grows as a spreading dense shrub in the range of somewhere around 12 feet in height. Those of you with a big, dry, inland slope that you want to convert into erosion mitigating high value native habitat should really consider this one!
Quercus englemannii
Engelmann oaks (Q. engelmannii) are arguably not Californian oaks when it comes to their evolutionary history. Instead, this species is more closely related to oaks in southern Arizona and northern Mexico. Oaks which receive much of their moisture during summer monsoons. Very different from the long, dry summers of a Mediterranean climate. This isn’t just random naturalist trivia for you, this is the secret to successfully growing Engelmann oaks.
First let me hype them a little bit more. Blue leaves on gorgeous spreading trees. Almost the entire remaining natural population of this endangered species exists in San Diego, with small exceptions in Los Angeles. Other countries, including Australia, have found such favor in using Englemann oaks as street trees that there are beautiful allees of this rare species but it doesn’t naturally grow anywhere in the world outside of southern California.
Let me spare you the heartbreak of killing such a wonderful local native species by saying that if you want to grow Engelmann oaks you will need to water them. My mistake early on, and one that I see often with home gardeners, is treating Engelmann oaks like coast live oaks in the sense of intentionally avoiding providing them water during the summer. The “avoid summer water” trope with coast live oaks is more dogma than based on data anyway, but with Engelmann oaks it’s a recipe for disaster.
Related to that monsoonally influenced history, Engelmann oaks actually like a bit of summer water. Not a lot, but maybe a deep irrigation cycle each month that we don’t receive substantial rainfall. This was a working theory based on my experience cultivating that species, but it has been borne out in nature by some of our recent weird weather. When hurricane Hilary swept through our area, bringing dramatically unusual summer deluges, the Engelmann oaks couldn’t have been happier. When they have enough moisture available they’ll actually put on two full vegetative growth cycles each year. In drier conditions, or in unirrigated gardens, they tend to only produce one growth cycle any year. It’s an unscientific assertion yet, but I strongly suspect that the notion in the literature about this species only growing in rich, deep soil like in the Santa Ysabel valley is actually related to that soil providing a better moisture reservoir rather than anything innate to the geology.
In garden settings this expresses itself as Englemann oaks being more tolerant of classic mesic garden conditions. It’s not something I recommend, but I’ve seen Engelmann oaks thriving in the same planter beds as tropical palms and bamboo - a moisture regime that would likely kill most of the other oaks in this article.
The last thing I’ll mention for now about Engelmann oaks is that although they are large trees, comparable to coast live oaks in overall scale, they are part of a different section of the genus and so far seem resistant in some fashion to the GSOB. In San Diego these days that’s a major plus if you’re going to invest decades into growing an oak. I’m biased, but of all the trees that I work on, old-growth Engelmann oaks might be the most majestic.
Quercus tomentella
The island oak is unique among this list in a variety of ways but most notably in its origin on the Channel Islands. Humans and this species have a deeply intertwined history but despite ancient distribution by gardeners, these oaks have since gained a California Rare Plant status and remain remarkably uncommon in private cultivation. I was also born and raised looking out at the Pacific Ocean, and have had the privilege of visiting some of the Channel Islands. That shared experience makes the advocacy I do on behalf of these trees more effective and in recent years they’ve become adopted by several of our coastal cities.
Some of the features that I really like about this species for coastal gardens derives from their growth habit and what they can put up with. Don’t let the big, glossy, almost subtropical looking leaves deceive you. These oaks are intensely salt tolerant after millions of years of evolving in the salt spray of the Channel Islands. Perhaps that’s not a decisive factor in your garden but for those where it is, it can really limit planting options. I’ll spare you too much taxonomic jargon, but this species is in a different “section” of the oak genus and it appears that they’re not prone to becoming afflicted by the GSOB.
Tomentella refers to the rich, velvety fuzz on the underside of the leaves of this species. As mentioned, the topside of their large (for Californian oaks, at least) leaves are a rich, glossy green. The overall aesthetic of these trees comes across as a bit more refined and perhaps less rangy in a respect when compared to the coast or interior live oaks. In this case I have saved perhaps the best characteristics, in gardening terms, for last. In dramatic contrast to the species mentioned so far, island oaks tend to remain much, much narrower relative to their height. This, combined with their higher tolerance for irrigation compared to the coast or interior live oaks makes for a beautiful and usable garden oak for coastal gardens of our region.
Coastal is something that I mention a lot here, because while island oaks make incredible planting options for gardeners along the coast, I have unfortunately witnessed many of them simply burn up when gardeners try to introduce them too far inland.
Quercus chrysolepis
For those with hot and cold, inland gardens, canyon live oaks (Q. chrysolepis) are worth considering… If you have the space. Canyon live oaks might be the best gigantic upright oak to put into a Californian landscape today considering the lack of pest pressure evident in the Protobalanus section of the oak genus. I’m trying to keep taxonomy out of this particular article, so suffice it to say that there are different sections within the oak genus (ie: white vs. red/black) and both Q. tomentella and Q. chrysolepis are in a smaller section which seems to be less susceptible to our current array of pests than other sections of the oak genus.
Let me reiterate about their size, though. The largest live oak known in the world is a canyon live oak. They’re beautiful trees, with different colors to the underside of their leaves and massive acorns with large wooly caps. They also tolerate extremes of not only heat but colder temperatures than other large evergreen oaks. Maybe not something you’ll see planted throughout the suburbs and cities of San Diego, but incredible trees nonetheless, and a species to keep a keen eye out for as you hike through areas like Volcan Mountain.
Quercus lobata
Other resources are likely to have better information for those gardeners growing valley oaks (Q. lobata) in suitable locations, but I don’t consider San Diego to be such a region. Discussions over plant nativity are complicated and vary quite a bit in their scale. In legislative settings I have found people to be very squeamish about circumscribing a narrower notion of “what is native” to a given location based on ecological data and instead we end up with broad and relatively meaningless anthropocentric lists. What am I going on about? Despite cities like Encinitas legally defining valley oak as “native” here, there are zero natural individuals of this species in the entirety of San Diego County. Valley oak does not naturally grow anywhere in San Diego, but I get why people want it to.
Valley oak is a deciduous, white oak with an offering of fall color in a nicely structured, upright tree. They are admittedly gorgeous and these are horticultural features not provided by any other oak species in our region. Areas where our native black oaks (Quercus kelloggii) have been decimated by the GSOB lost the splendor of their fall foliage color so understandably people have been trying valley oaks, with white oaks not appearing susceptible to GSOB, but it just doesn’t work in terms of horticultural practices relative to climate. We are too hot and dry for them for the most part. In more moderate coastal areas they suffer from the same limitations as Ginkgo trees - they don’t receive a strong enough cue that winter is coming and leaves just become ugly before falling haphazardly rather than putting on the show they do in areas with colder winters.
The two points I have to offer regarding valley oaks in San Diego are that they are not native to San Diego and more importantly in this conversation, they just don’t tend to like it here.
Scrub Oaks:
Somebody asked me after the presentation about what makes something a scrub oak. Scrub oak is not a specific term but more of a statement about growth habit. Some species of oaks tend to stay smaller overall and if they grow from a burl as a multi-stemmed thicket people tend to refer to them as “scrub oaks.” Scrub is not quite a pejorative term, though. Just don’t dare call these beautiful organisms brush or fuel!
The degree of “scrubbiness” is also not a fixed state. Eventually some of these species become small trees and a deft gardener can encourage that habit through strategic pruning. If I had to attribute something of a “scale of scrub” to the following species, it would be: Q. dumosa - Q. berberidifolia - Q. acutidens in order of decreasing “scrubbiness.” Perhaps a better way to label that scale would be “which species serves best as a small tree?” With Q. acutidens as the best option.
It happens that the following species are all part of the “Quercus” section of the oak genus, aka the white oaks, but that’s not true of all scrub oaks. There are scrubby oaks from the section containing island oaks, and as mentioned, there’s even a variety of the interior live oak with a scrubby growth habit. Part of why it matters that these are all in the Quercus section, aside from the GSOB consideration, is that oaks within a given section will hybridize readily and these scrub oaks are as promiscuous as anything! As I mentioned during the presentation, scrub oaks can even provide a reasonable argument that biological species are not a real thing.
Quercus berberidifolia
Sometimes referred to as the California scrub oak, or just scrub oak, Q. berberidifolia remains a pretty poorly understood character in San Diego’s Flora. Presumed to be the most common species carved out of the old catch all version of Q. dumosa, it seems that many of the plants attributed as Q. berberidifolia are partly or entirely something else. As you move north along the coastal portion of California, this will be the most abundant scrub oak by far but in San Diego there’s quite a bit of mystery remaining behind such an assertion.
In gardening terms this is a habitat providing powerhouse that becomes nearly invulnerable to drought, fire, boring insects, or any other environmental stresses once established. The aesthetics here can be challenging to some, because it takes them a long time and a lot of encouragement to resemble what most people think of as a “tree.” Only the most die hard native plant lovers are going to consider this species for a spot in a small courtyard outside of their front door, but anybody with an open spot less front and center that they wish to gift to nature would do well to look at this species, and that is true almost anywhere in San Diego west of the mountains.
Quercus dumosa
Nutall’s scrub oak (Q. dumosa) holds a dear spot in my heart because they come from the same place that I grew up and because they were the oak that was originally so confusing for me and others that I wanted nothing to do with oaks. That was actually true. I wasn’t simply confused by them, the taxonomic history of this species is legitimately confusing and has been the subject of debate among botanists for well over a century. In 2012 a new edition of the Jepson Manual, California’s floristic bible essentially, was released along with a new treatment for Q. dumosa which dramatically narrowed what it means to be a Nutall’s scrub oak. Unfortunately it is extremely difficult to filter out outdated floristic data which means that people continue to be confused by old, inaccurate information about this species to this day. In an effort to resolve this, I designed a range-wide population genetics research project that is currently underway but as is always the case in botanical taxonomic research, we need more money!
The simplest way that I can characterize this species is by pointing you to small oaks with tiny leaves that grow in a less “whippy” fashion than Q. berberidifolia, often with red leaf petioles, and with leaves that have trichomes, the minute hairs on the leaves, which are visible to the naked eye. Those leaves also tend to be somewhat tormented in their overall shape, rather than being perceivable as flat blades. I also look for narrow, pointed, “torpedo shaped” acorns but I get a bit of pushback on that particular physiological feature from other botanists. My research experience in Baja, though, where other white scrub oaks don’t interfere as much, makes me confident in narrow acorns as a defining trait of this species.
So what? They’re all oaks. They’re all awesome. The main reason we bother to attribute a name to a plant is so that we might better understand what to expect from it in a garden. True-to-type in my fastidious fashion (there are A LOT of hybrid oaks with Q. dumosa genes out there - way more than there are plants that meet the description of the species itself!) Q. dumosa is the smallest oak that gardeners grow in San Diego, and among the rarest. In some cases, plants that are hundreds of years old still remain beneath head height. The tallest plants that meet a strict species definition tend to be in the 10 feet range, as dense twiggy shrubs.
No other oak from our region provides as much habitat bang for the amount of space as Q. dumosa can, and they’ve proven surprisingly resilient in cultivation to a range of soil and climatic conditions.
Quercus acutidens
Torrey’s oak, Q. acutidens, is the species among these that comes with a bit of a sales pitch from me. For my entire career (and much longer prior to it) Torrey’s oak has been overtly neglected by both the scientific and horticultural communities because of a remark made by one oak expert suggesting it to be a hybrid. I doubt that his intention was to cast the entire population of oaks into the shadows with that assertion, but that is functionally what happened. Botanists eschew hybrids in many cases, and botanical garden botanists are perhaps the worst in this respect. I can state that because I am talking about my former self. In fact, at the highest level official meetings on the subject of international oak conservation, we myopic botanists collectively listed “hybridization” as an evident material threat to our native oaks. Not hybridization with exotic, introduced species, but two native oaks hybridizing, naturally, as they have done for millions and millions of years. It was an incredible oak geneticist, Victoria Sork, out of UCLA, who attended one of those meetings and said “Hey, wait… this is what oaks do!” Thankfully she broke me out of that spell of tunnel vision.
The fact is that hybridization is a mechanism which has contributed to the rich diversity of oaks native to our region, not a bottleneck detracting from it. When you really get into considering and understanding this stuff, to suggest that natural hybridization is a threat is to speak in opposition to the theory of evolution. More genes mean more for evolution to “select” for fitness from. This is the foundation of genetic resilience for any form of life, not something to be paranoid about. Sorry to personify a theory but I hope that makes sense to at least some of you. Phylogenetic research shows that a lot of the things that we call “species” in oaks today originated through hybridization.
That’s why I feel like the treatment of Torrey’s oak is a bit tragic. Somebody slapped an X in the name, indicating big bad hybrid, and it essentially disappeared right in front of us. Originally all of the scrub oaks in San Diego were called Q. dumosa. After the 2012 revision that I mentioned, what Q. dumosa is became clearer but then everything else got almost automatically moved into Q. berberidifolia. To this day, even some of the practicing professional botanists look at white scrub oaks in San Diego as “if it’s not Q. dumosa then it’s Q. berberidifolia.”
Hopefully you’ll feel a little bit of vindication in your confusion over scrub oaks, as I do, by realizing that the way botanists treated them historically made little sense and put us in this position of barely knowing our own plants. Trust me for now, because this is supposed to be about gardening, but Torrey’s oak is its own thing. I don’t care what you call it but it is its own thing, with significant implications for gardeners.
The throughline of this discussion is that we want oaks but increasingly they are hard to garden with on shrinking suburban lots. Q. dumosa is great for what it is, but what if we want a tree within our lifetimes, rather than a shrub? Q. berberidifolia is an incredible organism, but a wild one. They’re unruly and often prove so difficult to tame that people just give in to the scrub oak doing whatever it does. These reasons are why I am so insistent about Torrey’s oak.
Q. acutidens is kind of the ideal small tree oak taxon (whatever you want to call it) for San Diego County. They grow as multi-trunked small trees with admirable structure up to somewhere in the 16-20 feet range but often smaller. Bountiful in terms of ecosystem services and habitat offerings. Beautiful with their bicolor leaves which grow much larger than Q. dumosa and less prickly than these other scrub oaks. The funny irony of this is that as I really dug into the oaks of our region it became clear to me first that Q. dumosa in its “pure” form is particularly rare and endangered. Next was that Q. berberidifolia is really not anywhere near as common in San Diego as people proclaim. So what are all of these oaks out there throughout San Diego? A remarkable number of the things that have been confusing everybody since the 1800s succinctly and singularly match a species concept of Q. acutidens. All in, the truth appears to be that Torrey’s oak is probably the most common scrub oak in our county and that almost all of what seemed to be intractable confusion in botanical dealings with white scrub oaks just disappears when we give these oaks their due credit of being their own thing.
That may all seem immaterial to you and your garden, but this species (or whatever) is the number one type of oak that people in the cities of San Diego County should be planting! Yet we can’t even agree on the name, so of course nurseries remain wary to grow a crop of “whatever” oak. This is where you as the gardening consumer have a power that I don’t. All of the native plant nursery owners have already heard this tirade from me, so they are primed in a sense. But I am an admittedly terrible nursery customer because I like to grow my own plants. Please, do your garden, your surrounding environment, and all of us a favor - go to your local native plant nursery and demand that they sell Torrey’s oaks. Barring success in that, given all of this absurd confusion, the best bet for those of you wishing to establish your own amazing Torrey’s oak might be to just grab some acorns from a nearby tree and stick them in the ground yourself. Future generations will thank you!
Photos are mine unless otherwise noted. Thank you for reading and please keep the feedback coming!
