For The Answer
By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist
Week of November 14, 2005
“Why They are Called Trash Trees”
This is a great time to plant a shade tree. My favorite trees for long life and value are live oak, Texas red oak, Mexican white oak, Montezuma cypress, bur oak, cedar elm, chinkapin oak, and Chinese pistache. There are a number of tree species that are not usually recommended. Each has some value for wildlife food, fall color, or fast growth, but they each also have some negative characteristics that usually outweigh the good features. Hackberry, cottonwood, Arizona ash, mulberry, chinaberry, and Chinese tallow are in that category.
Hackberries lead the list of “trash trees.” Hackberries are undesirable because they reseed prolifically, do not usually develop an attractive crown, and can be short-lived. The species produce huge quantities of seed. It is a favorite food of the birds. The birds consume the berries and then excrete the seed along fence lines, in your rose bushes, and every place else that they roost. The germination rate is very high.
On our typically poor soils the hackberries do not reach a large size and often have a raggedy appearance. They also seem to be susceptible to diebacks and are short-lived on many sites. This is not always the case, however, on deeper soils hackberries reach over 50 feet tall and if they have room to develop and receive full sun, they can form a handsome tree.
Cottonwoods make a pleasant sound when the leaves blow in the wind. Unfortunately the trees become very large, very quickly and they do not live a long time because they are susceptible to borers and other stresses. Instead of leaves blowing in the wind you are more likely to hear chain saws and your money blowing way from your pocketbook. A large cottonwood requires the attention of a professional when it dies and the removal is very expensive. I do not find the cotton produced to disperse the seed as a problem, but others do.
One tree that many landscapers designate as a “trash tree,” but is for sale in most nurseries is the Arizona ash. The tree grows very fast and unless you prune it on a regular basis it can become a tangled mess with frequent branch dieback. Arizona ash is attacked every spring by anthracnose (a foliage disease) and is a favorite food source for insects. The seeds are formed in large numbers in the spring (a similar, but less common ash has seeds in the fall). The seeds are eaten by birds with the result that Arizona ash spreads around the neighborhood. A well cared for Arizona ash often only lives for 25 – 30 years before anthracnose and other stresses end its life. At that time it can be quite large and expensive to cut down.
Mulberries are also usually described as a “trash tree.” They grow very fast and reseed nearly as aggressively as hackberries. Mulberries are one of those tree species that will drop their leaves early during droughty summers. With conscientious pruning they can be shaped to form an open crowned shade tree, but they grow so fast that most people do not prune them adequately and they become quite tangled. Mulberries are almost as good as hackberries as a wildlife food source. In my neighborhood the edible berries are all stripped from the trees by the birds before they even ripen in the spring.
Chinaberry has a purple bloom every spring and is a bountiful berry producer in late summer. The berries are as large as grapes and produced in clusters. Birds eat a few and spread enough that the seedlings are a nuisance, but it is not a favorite bird tree. The berries are messy. Pool owners find Chinaberry as especially pesky. In its favor the crown has a pleasing shape and is sometimes called umbrella tree. Chinaberry is sensitive to cold and relatively sensitive to drought so trees in the San Antonio area are usually short-lived.
Chinese tallow is short-lived and spreads far and wide like the other “trash trees.” It has an autumn berry that is white and decorative. The birds eat the berries, but not with the same relish as mulberries or hackberries. Foliage color is the tallow’s claim to fame. It is an attractive light green in the spring and can have red purple foliage in the autumn. The trees are sensitive to cold and brittle in the wind.