Some native trees may be waiting for their chance to reveal their resilience and beauty.
The park, garden, and street trees that make up the urban forest in California consist primarily of nonnative species. Though conditions in parks and gardens are more amenable to a wider variety of trees, urban trees have a hard life. Essentially, a street tree is a plant in a container. Its roots are laterally constricted, its branches are often pruned to create a lollipop structure, and it is usually cut off from connection with other plants in the mycorrhizal realm.
Like other trees, the roots of street trees spread outward farther than their leafy canopies. But though roots need air as well as water to thrive, the roots of street trees are often constricted by pavement, with only a meager circle too close to the trunk. And within that circle, the soil is often hard-packed and trampled, rather than being nourished with mulch and good soil.
Furthermore, pollution from passing vehicles slowly poisons the tree. Though a young tree needs branches low on the trunk to feed itself optimally, a street tree's limbs are hacked back so as not to impede pedestrians, structures, or trucks. Similarly, its upward progress is stunted if wires overhead mandate some pruning.
Of the 11 most widely cultivated urban trees in California, not one is a native, according to Matt Ritter, who teaches at Cal Poly in San Luis Obispo and wrote the engaging guidebook A Californian's Guide to the Trees Among Us.
Ritter traveled throughout the state, photographing urban trees. His book covers about 150 of the most widely cultivated trees. He purposely left out fruit trees and large shrubs.
According to Ritter, only about 20 species of native trees are commonly planted. The ten most commonly cultivated California native trees are California buckeye, white alder, incense cedar, Monterey cypress, Monterey pine, California sycamore, California fan palm, coast live oak, valley oak, and coast redwood. Only the last three appear on the list of approved street trees for Los Altos, and all three are listed as over 40 ft. high and requiring at least a 6 ft. wide planting area.
But at a talk last year, Ritter said that 58 more California native trees are undeservedly rare and could be suitable for urban spaces. He is writing a series of articles on undeservedly rare trees for the quarterly Pacific Horticulture magazine, although the series does not focus on natives.
Asked to select his top 5 underused natives, Ritter named island oak, Santa Cruz Island ironwood, Guadalupe palm, Catalina Island mountain mahogany, and desert willow. Some of these are not yet common in native nurseries.
Island oak is an adaptable evergreen, lusher than coast live oak and with a narrower form, that grows moderately fast to 40 ft. It tolerates a range soils and is drought-tolerant but accepts moderate water and does especially well with fog drip.
Santa Cruz Island Ironwood also has an upright profile, growing fast to about 20-50 ft. It does not do well in windy sites and needs good drainage. It offers abundant texture, with ferny leaves, large clusters of tiny flowers, and shredded bark.
Guadalupe palm is a well-behaved fan palm with self-pruning fronds, so it does not retain dead leaves as do more commonly grown palms. It grows slowly to 30 ft. and survives cold spells down to 20F, but needs protection from protracted freezing temperatures. It has done well in England. The fruits are edible, though reports on their taste vary.
Catalina Island mountain mahogany is more treelike and spreads wider than the more commonly found mountain mahogany. It's evergreen and grows 12-20 ft., sometimes with multiple trunks. The feather-tailed seeds glow when backlit by the sun.
Desert willow is a deciduous flowering tree, with large fragrant flowers in shades of pink to purple irresistible to hummingbirds all summer. It needs deep watering and the sunniest spot in the garden. Its open rounded form reaches 12-20 ft.
Each of these native trees could flourish in the right conditions, contribute to the diversity of the urban forest, and welcome wildlife.
Though conditions in parks and gardens are more amenable to a wider variety of trees, urban trees have a hard life. Essentially, a street tree is a plant in a container. Its roots are laterally constricted, its branches are often pruned to create a lollipop structure, and it is usually cut off from connection with other plants in the mycorrhizal realm.
Like other trees, the roots of street trees spread outward farther than their leafy canopies. But though roots need air as well as water to thrive, the roots of street trees are often constricted by pavement, with only a meager circle too close to the trunk. And within that circle, the soil is often hard-packed and trampled, rather than being nourished with mulch and good soil.
Furthermore, pollution from passing vehiclesslowly poisons the tree. Though a young tree needs branches low on the trunk to feed itself optimally, a street tree's limbs are hacked back so as not to impede pedestrians, structures, or trucks. Similarly, its upward progress is stunted if wires overhead mandate some pruning.
© 2012 Tanya Kucak