Express-News
Saturday, September 18, 2004
By Calvin Finch, Conservation Director, SAWS, and Horticulturist

LEAVES, LEAVES EVERYWHERE

            Are your trees dropping their leaves? This is the year for it for several reasons. Because of the wet cool spring we had many trees put on full crowns of lush leaves. In July, August, and September there was some rain but there were also hot dry spells. Many trees, especially those in shallow or poor soils, were not able to pump enough moisture or nutrients to the crowns to support the heavy leaf load. The result is that the excess leaves are dropped to protect the tree from being over extended.

            Related to the leaf drop is a mechanism by some trees to drop their leaves early for the season. Cedar elms, sycamores, and mulberries are known for the strategy but bur oaks, red oaks, and other trees will also do it. It is as if the trees determined how difficult it is to support all the leaves on the crown, calculated how close it is to autumn, and made the decision to declare it is better to go dormant early, rather than try to fight for another month or two of leaf cover.

            Another thing that is causing major leaf drop is fungal leaf spot. Oaks have oak leaf spot, sycamores have anthracnose, and other trees are also affected. The wet and humid spells we have had throughout the year have been perfect for incubating fungus diseases. The weather has also been responsible for root damage. Soggy cool soil this spring was especially tough on drought-tolerant plants like desert willow but any plants could be affected. Fruit trees and other drainage-sensitive trees defoliated or even died because root rots developed. The rots kill a large portion of the feeder roots so that when it got dry and hot, the whole tree died. Crepe myrtles and oaks are not overly sensitive to soggy soil but even they have sustained root damage in some soils. The reduced root capability means that the plant cannot support all the leaves, causing leaf drop.

            Acid-loving plants are also very sensitive to root damage of the sort caused by soggy soil. Soggy cool soil kills a portion of the feeder roots again and the ill-adapted plants have an especially difficult time bringing up iron. The result is that severe chlorosis occurs. The symptom may just appear as light green foliage but in more severe cases the leaves turn yellow or even white. The next step is burnt-looking leaf edges, defoliation and even death.

            It is real tricky to nurse root-damaged plants back to health. The treatment is definitely not fertilization or heavy watering. Fertilizers are salts and salts are difficult to uptake by plants with damaged roots. In addition to the nutrients being wasted, they can damage the roots further.

            The injured plant is very susceptible to drying out, but excess water will also cause more of the same problems that killed the roots originally. Even in the best of times we kill more plants by over watering in South Texas than we do by under watering.  With a root-damaged plant it is especially important to monitor the soil to put the right amount of water on the plant. Use your garden trowel to penetrate 3 or 4 inches into the soil under the crown of a plant that is showing wilting or leaf burning symptoms. If the soil is dry at 3 to 4 inches, drought is threatening the plant and you should irrigate (water). If the soil is wet, however, the plant has root damage due to soggy soil and you must resist the urge to drown the root system further. Such a situation requires conscientious monitoring because the weakened plant is susceptible both to excessive watering and dry soil.