Updated: Oct 23, 2021
The coast live oak is among other tree species noted for their size, often achieving heritage status within communities... witness trees to California's history, even before the land became a state. One such tree has been recorded by the Urban Forest Ecosystems Institute for its canopy reaching sixty feet in the air with a spread of one hundred and ten feet. That's a lot of shade! The trunk is also significant at three hundred and forty-eight inches in circumference (110" diameter, or about 9' diameter), and it is this measurement that I would like to discuss in a little more detail for landscape architects and arborists.
Part of my current work is reviewing development proposals for a major California city. My focus is primarily on what goes on in new sidewalks, which of course almost always includes street trees. The landscape architects do their creative best to integrate street trees while adhering to the city's standards and specifications. At this time, that standard for where to plant trees is a 4.5' x 5' tree well (other cities call it a tree pit, which I find quite depressing). This little cut out in the concrete, we can only hope, will be adequate for a street tree to grow to maturity. So, I am always surprised when I find Quercus agrifolia specified in these little spaces (remember how large the trunk can get when they're happy). For a coast live oak to grow to maturity, the tree well should be significantly larger, better if the impervious concrete can be avoided all together. Other trees can tolerate small tree wells, but this is not one of them.
When coast live oaks are provided room to grow, the reward is exponential, but there is another hurdle to cross: instant gratification. Clients and designers alike suffer from a desire to see a new landscape installation appear as if it has been there all along, lacking patients for trees to grow. Oaks, particularly our native species, are not known for their rapid growth...at least that is their reputation, but I will share a few not-so secret tips to designers and clients alike.
Young oaks grow fast. When given the right conditions, they can outpace other trees. Once they reach a good size, even for us instant gratification needy, they will slow down and focus on becoming more dense and shapely, things we do not notice so immediately.
The larger container at planting, the slower the growth over time. I have seen this with oaks, redwoods, and pretty much most trees. To solve the instant gratification twitch, designers will specify trees in larger boxes, for example 60" boxes (that's the square size of the wood box for the roots). To get to this size, nurseries "upsize" smaller trees over time, starting as early as a germinated acorn to larger plants brought in from elsewhere. If they are not paying attention, any damaged, rootbound, or circling roots of their youth will transfer the problem to the larger sizes. The consequence over time is that the tree is compromised underground, slowing its performance. Here's the kicker: planting a smaller tree, say for example a 15-gallon container, may not look like much when it is first planted (making the instant gratifiers ungratified); that little tree will likely surpass the larger specimen over a short amount of time. Remember the first rule above?
The instant gratification twitch will often disappear once the landscaping is complete...at least for the client. Designers want something they can photograph for their portfolio, and waiting for 15 years of growth is not in the program. For clients, however, they have just experienced a long process that may have started with building a home, office, or campus. The landscape is often last, and by that time they are relieved of the completion and excited to see it grow.
There are other tricks of the trade, but these top three highlight the need for strategy when planting trees like Q. agrifolia that need space and time.
Other challenges should be acknowledged when considering this tree, such as diseases, both native and introduced, and acorn drop that can be a liability if someone steps on them. The sharp leaves can be slippery if allowed to pile up. Despite these factors, the benefits outweigh the negatives, in my humble opinion. Q. agrifolia is iconic in the California landscape, provide huge benefits to wildlife while cooling the ground underneath them, and are a critically significant to West Coast First People. Or as the Los Angeles Magazine directs, Let's Take a Damn Moment to Appreciate the Coast Live Oak Tree.
West Valley College Campus Location: Quercus agrifolia
Child Development Center (parking lot CH2)
Long: 122° 0'27.79"W
Botanical Name: Quercus agrifolia
Quercus: Greek, kakhrus for acorn
Agrifolia: Having rough-textured leaves (probably the spiny margin)
Common Name: Coast live oak
Family Name: Fagaceae
Origin: Native; coastal, North Bay Area, central to Southern California
Positioning: Oak woodland, chaparral/scrubland, background, hillsides
Garden Themes: Native/Mediterranean/Dry, woodland, bird/butterfly, rain
Uses: Specimen, shade, utility (acorns, wood), street tree (with ample space), parks, habitat, hedge, bank stabilization, green stormwater infrastructure (requires ample rooting space)
Type: Evergreen tree
Form: Round, umbrella
Size: 70' tall and wide
Outstanding Feature(s): Size, density, acorns
Bark: Light gray and smooth when young, dark gray and furrowed with age
Margin: Spiny-toothed (similar to traditional English holly but more subtle)
Color: Dark blue-green to dark green
Surface: Smooth, glossy above
Flower: Spring. Insignificant (female); catkins (male)
Fruit: Autumn. Acorns are slender, green turning brown when ripe. With proper preparation, edible. Do not eat raw!
cultural requirements, tolerances & problems
Sunset Zones: 7-9, 14-24
USDA Zones: 8-10
Light: Full sun to partial shade
WUCOLS SF Bay Area Hydro Zone: Very low
Texture: Clay, loam, sand
Moisture Retention: Allow to completely dry for extended periods
pH: Highly acidic to slightly alkaline
Tolerances: Drought, verticillium wilt, deer
Problems: Acorns may be a slipping hazard on concrete. Sensitive roots.
Branch Strength: Strong
Insects: Carpenter worm, invasive shot hole borer, gold spotted oak borer, aphids
Disease: Oak root fungus, sudden oak death, crown rot, mistletoe, armillaria
citations & attributions
Bayton, R. (2019). The Gardener's Botanical: An Encyclopedia of Latin Plant Names. The Royal Horticulture Society. London: Quatro Publishing, plc.
Calscape. "Coast Live Oak." California Native Plant Society, Sacramento. Accessed on July 26, 2021, from https://calscape.org/Quercus-agrifolia-(Coast-Live-Oak).
Norris Brenzel, K. (Ed.). (2012). The New Sunset Western Garden Book. New York: Time Home Entertainment, Inc.
Urban Forest Ecosystems Institute SelecTree. "Quercus agrifolia Tree Record." 1995-2021. Cal Poly State University, San Luis Obispo. Accessed on Jul 26, 2021. https://selectree.calpoly.edu/tree-detail/1227.
Water Use Classification of Landscape Species. "WUCOLS IV Plant List." University of California, Division of Agriculture and Natural Resources, Davis. Accessed on June 28, 2021.