For The Answer
Weekly Express-News Article
Saturday, October 29, 2005
By Calvin R. Finch, PhD, SAWS Conservation Director, and Horticulturist
“Shade Trees for San Antonio”
Some of our soils are difficult to dig so select a tree from the nursery that is relatively small so you can manage digging the hole without the need for dynamite or heavy equipment. Dig the hole only as deep as the root ball and two or three times as wide. Never plant the tree too deep. It is best to have it stick out of the ground rather than have it in a sunken area. Do not waste your time or money adding additives (compost, planting fertilizer, or micronutrients) to the hole. As far as I know there is no research that verifies that the expensive additives contribute to survival or growth rate. What does help is to mulch over the root system three – four inches deep to keep the soil cool, reduce evaporation, and keep weeds and grass from competing with the tree. Shredded brush, leaves, or partially finished compost make good mulch. Do not put the mulch against the trunk.
There are many choices of shade trees that do the job in San Antonio. Some are most suitable for specific situations and others are good choices for many uses.
Live oaks are generally rated as the best shade trees for our area. They live a long time and prosper in our climate extremes and poor soils. Live oaks have a horizontal growth pattern that makes very attractive. The acorns are good wildlife food. Live oaks get very large in good soils (easily 65 by 65 feet), but may only reach 25 – 30 feet in poor soils. The only reasons to hesitate to plant a live oak would be lack of space and interest in diversifying a shade tree population already dominated by live oaks. Live oaks are not overly susceptible to oak wilt, but once it invades a neighborhood the disease travels 100 feet/year through the interconnected live oak roots. Live oaks grow at a moderate rate, one – three feet per year.
Texas red oaks are deciduous oaks that rate almost as high as live oaks in my book as a shade tree. They are nearly as attractive, sometimes have good fall color and grow faster than a live oak. Like live oaks they can become infected by oak wilt. Since the roots are unlikely to root graft as live oaks do; only individual trees become infected.
In the past, there have been problems from red oaks that originated from a seed source in East Texas or some other place with acid soils. When you are considering a red oak make sure the seed source is grown in an alkaline soil. The nursery should be able to guarantee the origin of the tree. If they cannot, buy from someone else. A red oak tree derived from a parent grown in acid soil will be chlorotic within two or three years and never grows well.
Mexican white oak is also called Monterrey oak. It is harder to find in area nurseries, but has many of the good traits of live and red oaks without being susceptible to oak wilt. Mexican white oak is as evergreen as live oak and grows nearly as fast as red oak. At first it grows more upright than live oak, but as it matures Mexican white oak spreads out. Like live oak and Texas red oak, Mexican white oak is a good xeriscape plant.
For an unusual shade tree consider a Montezuma cypress. They are related to our native bald cypress, but grow faster and have more drought tolerance. A Montezuma cypress will grow up to six feet per year on good soils, but will even grow fast in droughty situations. It is a conifer that has a Christmas tree shape for a few years, but eventually forms a shade tree like crown. The needles drop occasionally during drought periods, but the trees quickly recover.
Other shade trees to consider are cedar elm, Chinese
pistache, bur oak, chinkapin oak, and anaqua.