Trees--Also Rans and Bad Actors!
written for the Davis Enterprise, October 24, 2002
More Shade Trees....the Also-Rans and the Bad Actors
Our September 2002 column, Some of Our Favorite Trees, got great reader response, pro and con. Here's a list of some others we like--and some we don't. No tree is perfect and every situation is different. Just like choosing a puppy: there are no "bad" trees, but some are more suitable to your needs than others. If you live in a condo, don't buy a St. Bernard, and if you want shade with zero maintenance and zero litter...build an arbor!
The criteria used to select a shade tree include
- growth rate,
- deep roots,
- low maintenance,
- low litter,
- fall color or seasonal flowers,
- pest resistance,
- tolerance of lawn watering or drought,
- and of the high salt content and high pH of Davis water.
Additional factors may include low allergy potential, and whether the tree is native. You decide which of these criteria are most important, and which tree will be appropriate to your location. Availability may be an important factor, as some trees are very hard to obtain.
- Dawn redwood (Metasequoia glyptostroboides) is an example of a good tree with limited availability. This deciduous conifer has many of the attributes of the evergreen Coast redwood (Sequoia sempervirens). Bright, soft green spring growth, fast-growing and narrow, with nut-brown fall color before the needles drop. Very nice planted in groves and tolerant of lawn watering. Bald cypress (Taxodium distichum) is very similar, with a more weeping growth habit. I have seen nice specimens in Davis, but it is scarce in the trade.
Also-Rans....
Four popular trees have drawbacks that may limit their usefulness:
- Birch trees grow fast, with fluttery leaves and bright yellow fall color. European birch (Betula pendula) is often planted in groups: very pretty, but trees weakened by drought are prone to borers and get burnt leaves during hot spells. Love lawn watering but develop surface roots. Japanese birch and Betula jacquemontii also grow here, having more upright growth habits that resemble alders. Note: the Western Garden Book mentions some species as being "more borer resistant" than the European birch, but that borer is not the one we get here.
- Chitalpa, a hybrid between Desert willow (Chilopsis) and Catalpa, has become popular. Pretty pink blooms spring through fall attract hummingbirds, and the tree is drought tolerant. Some training is needed for good branch structure. The leaves get a blight in spring and mildew in summer (especially in lawns), which can make the trees unsightly.
- Mulberry (The male version of Morus alba is the Fruitless mulberry) deserves special mention. This is the poster child for how NOT to prune. Very fast-growing with the densest shade of any tree, it has aggressive roots that make it impossible to garden under, produces prodigious amounts of pollen, and needs lots of thinning (NOT heading). Incredibly tough and great for rural areas but not for the typical back yard.
- Silk tree (Albizia julibrissin) has soft green, ferny foliage, a unique umbrella-like canopy, pretty pink blooms for a long time, and attracts hummingbirds. Even the pods are kind of pretty. But it reseeds everywhere and the flower litter looks like pink mold. Very drought-tolerant.
Bad Actors....
Some trees are prone to life-threatening or annoying pests, or have other significant drawbacks. Some of these should not be planted or sold, while others may be suitable as long as you are aware of the problems.
- Alder: White alder (Alnus rhombifolia) is being killed by clearwing borers all over the Valley. Don't plant it. The Italian alder resists the borer, but is not very attractive.
- Ash trees (Fraxinus spp.) are prone to pests and have surface roots. Raywood ash has been attacked by the Ash/lilac borer (which also attacks olives and privets) in recent years. Modesto ash gets anthracnose blight, which causes leaves to drop all spring, and mistletoe.
- Elm trees were once America's stately shade trees until Dutch elm disease decimated several species. Elm leaf beetle is a messy pest on some species. Chinese elm has a very graceful semi-weeping habit and is not prone to beetles or disease, but it drops leaves all summer and fall.
- Honey locust (Gleditsia triacanthos inermis): The gall midge began attacking in the late 1970's, an insect that causes defoliation of most of the summer's growth, and mistletoe is common.
- Locust (Robinia x ambigua; Idaho and 'Purple Robe') have showy Wisteria-like flower clusters in the spring, are fast-growing, and tolerate drought. But the wood is brittle and branch angles are often narrow, so they break in high winds; the roots are aggressive and suckers are profuse.
- Maples (Acer species). Some desirable maples were mentioned last month, and the Japanese maple is very popular in spite of not liking hot sun, dry wind, heavy soil, or hard water. Silver maple (Acer saccharinum) breaks readily in the wind, has very aggressive roots, and is prone to aphids and chlorosis (yellow leaves due to hard water). Our native Box elder (Acer negundo) suckers profusely--fine for a thicket and wildlife cover in riparian areas--and reseeds prolifically. It is also prone to boxelder bugs, insects which populate by the thousands and crawl all over everything nearby.
- Pears (Pyrus species). Bradford pear and its cousins can be recommended, although with the caveat that they require careful training (and the flowers, though pretty, do smell like old socks). Aristocrat was highly recommended when first introduced--more open growth habit and better branch structure, nice fall color. But the fruit is pulpy and stains, and it proved to be prone to mistletoe and fireblight. The Evergreen pear (Pyrus kawakamii) is a graceful little tree, but is so prone to fireblight and leaf-spot fungus that wholesalers have stopped growing it.
- Poplars and Cottonwoods (Populus spp.) become liabilities and are rarely recommended, except in rural areas. Even there, keep them well away from septic tanks, leach lines, and irrigation systems. Very prone to borers and several diseases, with extremely aggressive roots. When you cut them down, a forest of suckers emerges from the widespread root system. Good luck!
- Tallow tree (Sapium sebiferum, Chinese tallow) is the common streetside tree in Village Homes. Advantages include fast growth, moderate height (good for solar homes), very nice fall color, and drought tolerance. The downside: prone to surface roots, and the litter of pollen and flowers from male trees and seeds from females is annoying over a long season. The roots are aggressive and sucker freely. Sapium is reseeding into riparian areas and is classified as an invasive tree in southern states. Don't plant them anywhere near drainage ponds, sloughs, or creeks.
- Willow trees (Salix spp.) are nostalgic favorites, especially the weeping willows. There is a beautiful specimen on the corner of Eighth and Oak Streets. Fast growth and graceful habit are offset by aggressive roots and brittle wood, plus susceptibility to borers. Rural areas only, and keep well away from sidewalks, water lines, septic and leach lines.
Trees that don't like our water include Beech (Fagus spp.), Dogwood (Cornus spp.), Sour gum (Nyssa sylvatica, also called Tupelo), and Sweet gum (Liquidambar). These trees all get very anemic and sickly looking due to the high salt content and pH of our groundwater in Davis and Woodland.
Space considerations limit these lists...for more information you can look at our table here, and I welcome your feedback at redbarn@davis.com. Let's keep the discussion going!
