Calvin Finch: How to select and best grow shade trees in your San Antonio garden

Now is a good time to plant shade trees in San Antonio.

When you live in a climate like we do, shade trees are an important part of the landscape. Temperatures of 100-plus degrees are difficult to tolerate in the shade and are even more unpleasant if there is no shade.

Among the important issues to consider when selecting a shade tree species are its ultimate size, growth rate, appearance, drought tolerance, soil preference and susceptibility to pests and diseases. Quite often area gardeners remind me that we describe live oaks as “evergreen,” but they do, in fact, lose their leaves for a short time each March.

Live oaks are relatively slow-growing shade trees when compared to other choices, but they are held in high regard for their appearance, drought tolerance and longevity. If your landscape includes a 50-foot live oak, it may be 100 years old and is probably adding $30,000 to the value of the property.

A lot of attention is given to the live oaks susceptibility to the disease “oak wilt,” but it is recognized that the disease is relatively easy to detect and prevent if a homeowner does a limited amount of research and is alert to the situation with the trees in the surrounding neighborhood. One of the most effective ways to protect the value of live oaks and other shade trees is to establish a relationship with an arborist.

This week in the garden

 It is prime time to plant your winter vegetable garden. Prepare the soil by incorporating 2 inches of compost into the planting area. Also enrich the soil with 10 cups of slow-release lawn fertilizer per 100 square feet of bed. Plant broccoli, cauliflower, cabbage, chard, kale and Brussels sprouts with transplants. Use seeds for carrots, beets, radishes, turnips and lettuce.

 The fall tomatoes should be setting fruit. Support their production with side dressing with a winterizer fertilizer.

 Zinnias and other summer annuals will continue to bloom but the winter annuals such as snapdragons, dianthus, stocks, calendula and petunias can also be planted.

 Fertilize the lawn to prepare it to tolerate winter cold and to prepare the grass for a green-up next spring.

Texas red oak is like a live oak in its size, attractive shape and drought tolerance. It is different in its faster growth rate and that it is a deciduous tree, meaning it loses its leaves every winter.

Depending on the soil, it is not unusual for a Texas red oak to add 6 feet of growth each year for several years after it is planted. Texas red oaks also are susceptible to oak wilt as individual trees through wounds, but they’re not susceptible through the roots like live oaks.

Deer are common in many San Antonio neighborhoods and are a factor in successfully growing a shade tree. Bucks in their rutting season rub their antlers on the smooth trunk of young shade trees, often girdling the tree and greatly reducing its growth rate. The girdling involves damaging the tree’s vascular system.

In addition to deer, similar damage can occur when the young smooth bark of a shade tree such as Texas red oak is damaged by a string mower or other piece of equipment. Protect your newly planted tree from the deer with fencing and from string mowers with mulch.

Another species to consider for a shade tree in your San Antonio landscape is the Mexican white oak. It is also called a Monterrey oak. In terms of growth rate, ultimate size, drought tolerance and pest resistance, it is in the same league as the Texas red oak. Mexican white oak is generally evergreen but will sometimes drop its leaves in response to a drought period.

A deciduous tree with showy leaves and bark is the Mexican sycamore. The Mexican sycamore has the same large angular shaped leaves and patterned bark as the American sycamore, but it has much more disease and pest resistance. Sycamores grow even faster than Texas red oaks. Recognize the Mexican sycamore for its fuzzy coating on the underside of the leaf. The American sycamore has a smooth surface.

Cedar elm is a deciduous shade tree that grows more upright and even taller than the other recommended shade trees. In my experience they seem to grow faster than live oaks but not as fast as Texas red oaks or Mexican sycamore. Cedar elm is a good shade tree choice if your neighborhood is dominated by oaks and you want to diversify but not to the degree of planting a Mexican sycamore.

Calvin Finch is a retired Texas A&M horticulturist. [email protected]

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