Fifteen years ago, I asked myself a couple of simple questions. What are the best trees to plant in the cities of the San Francisco Bay Area? And what is the best way to grow those trees? My motivation was—and still is—a response to the many stunted, deformed, and ill-adapted trees littering the roadsides of the Bay Area like bad biological graffiti or publicly funded vandalism.
I spent the previous decade learning to identify trees, both common and rare, throughout the Bay Area. At the time, I didn’t own a car and travelled almost exclusively by bicycle. This form of transport was uniquely useful for learning trees as it allowed me to move more quickly than on foot and with a better view than traveling by car, yet slow enough to make such observation safe.
In 2000, I was hired to rewrite the street tree list for the city of Santa Cruz. To do this I got a map, hopped on my bike, and rode every street within the city limits, and well beyond, in order to identify every visible tree and evaluate the performance of each tree type in as many micro-environments as possible. I believed the ideal way to determine what was going to succeed in the future was to collect the data from 150 years of past tree planting experiments. From that exercise, two trees stood out as stellar performers in many, if not all, locations around the Santa Cruz area.
For years I’d observed hideously disfigured examples of ‘Bloodgood’ London plane trees (Plantanus ×hispanica, syn. P. ×acerifolia ‘Bloodgood’) stunted by powdery mildew, a yearly occurrence throughout the Bay Area caused by enhanced humidity from proximity to large bodies of water. Plantings established in the 1980s along El Camino Real in Palo Alto have performed miserably, and you can see the same syndrome at work along Market Street in San Francisco.
By the early 1990s, as problems with ‘Bloodgood’ became evident, ‘Yarwood’ London plane (Plantanus ×hispanica ‘Yarwood’) was introduced and touted for its immunity to powdery mildew. For the next decade, ‘Yarwood’ was the most commonly planted tree in the urban Bay Area. But by the early 2000s, astute observers began to notice that ‘Yarwood’ was highly susceptible to anthracnose, a fungal counterpart to powdery mildew that occurred in the spring and resulted in the tree dropping its leaves by July. But what finally killed ‘Yarwood’ in the industry was its awful growth habit. The trees commonly suffer major branch failure and look unattractive because of the absence of a central leader.
My survey revealed ‘Columbia’ London plane (Plantanus ×hispanica ‘Columbia’) to be a workable compromise in the Bay Area. ‘Columbia’ shows good resistance—not immunity—to both anthracnose and powdery mildew. However, the infections do not substantially affect the growth habit or performance of the tree. The trees have a uniform, attractive, and strong shape. They even have decent fall color, which is rare for London plane trees. ‘Columbia’ has now been planted extensively in the Bay Area, and the trees are, in general, performing very well.
The Achilles heel of London plane is that to perform well and fill in with a dense, shady canopy, the trees require either permanent supplemental irrigation or ready access to groundwater. For assuredly arid sites, a tree with better drought tolerance is a must. This is where the second tree shines.
Although cork oak (Quercus suber) isn’t a common tree in Santa Cruz, the ones that do exist are quite old, look excellent, and grow in a variety of locations. These observations led me to recommend cork oak for planting on the streets of Santa Cruz. This advice turned out to be less than successful for reasons that have become painfully clear in recent years.
What I didn’t realize at the time is that cork oak, like many drought-adapted trees, cannot tolerate circling or girdling roots created when trees are left too long in their growing containers. Container-grown tree stock has been the rule since California’s post-World War II housing boom, which in turn has lead to an increase in this poor nursery growing practice. Some trees—typically those native to (and only really appropriate in) moist environments—are often able to outgrow these damaging conditions by pushing lateral roots beyond the circled or girdled ones. Most drought-adapted trees cannot do this and cork oak is perhaps the most affected tree in the nursery industry. This situation has resulted in decades of terrible performance of cork oaks in California. When every old cork oak you see looks great, and every young cork oak you see looks terrible, that’s a clue that something has gone decidedly awry in our cultural approach to this particular species.
Today, air-pruning containers are being adopted by the California tree nursery industry and defective root systems may become a thing of the past. We are again entering an era where cork oak can be used to its full potential in the California landscapes. This is good news.