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Mr. Smarty Plants - Leaves dropping on native Texas Mountain Laurel in San Antonio

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Saturday - September 20, 2008

From: San Antonio, TX
Region: Southwest
Topic: Diseases and Disorders, Pruning, Watering, Trees
Title: Leaves dropping on native Texas Mountain Laurel in San Antonio
Answered by: Barbara Medford

QUESTION:

Please help. We have a beautiful TX Mountain Laurel in our front yard. This year the leaves are dropping like snow in the north. What do you think is wrong with our tree?

ANSWER:

Last year this time, when people were worried about their Sophora secundiflora (Texas mountain-laurel) trees, we were advising them that it was the result of too much rain, and the drainage in the soil the mountain laurel was planted in was probably poor. This year, we're thinking it may be because it has been so hot and dry all summer. Sometimes you can't win. However, we will try to find a possible cause for the leaves dropping, but we don't have all the facts we need. Has your tree been very recently transplanted? The Texas mountain laurel does not transplant well, it has a long taproot and, if that is damaged, the survival of the tree is adversely affected. Or, if you have been trying to overwater it to help the problem, and the soil is not draining well, the mountain laurel roots might be drowning, just as certainly as if it had rained too much. 

Other possibilities include over-fertilizing. The Sophora secundiflora is a legume, and has a mechanism by which it can fix nitrogen in the soil, for its own use and for other plants to borrow. This is a plant accustomed to growing in rocky, alkaline soil, and pretty well taking care of its own needs. For the time being, don't fertilize at all, no plant under stress should be fertilized. The mountain laurel, unfortunately, is not very resistant to disease, and one to which it might be prone is cotton root rot. This is a disease of warm earth and hot weather, and only occurs in the Southwestern United States. It is a soil-borne fungus, and will quickly take out any vulnerable plant growing in that soil. See this Texas A&M University website on Controlling cotton root rot on ornamental plants.That site will also give you instructions for trying to diagnose if that is the case. Should it prove to be cotton root rot, do not plant any other plants susceptible to the disease in the same soil.

If you don't feel that any of these possibilities is the cause, we suggest you treat the tree as though it is under stress, and treat like transplant shock. First, check the drainage of your soil. If you stick the hose down in the soil around the roots of your mountain laurel, let it drip slowly until the water appears on the surface, and then the water stays for a half hour or so, your drainage is poor. Without disturbing the roots any more than you have to, try to incorporate some organic material like compost or leaf mold into the soil to improve the drainage. Water deeply but infrequently. Trim off 1/4 to 1/3 of the upper structure of the tree, including the dying leaves. Leave as many green leaves as possible to improve the nutrition of the tree. 

And, then, it's just watch and wait. This is an incredibly tough tree, growing in the wild almost out of solid rock, and certainly without a sprinkler system. It may take it a while to recover, but hopefully by next Spring your beautiful tree will be coming back. 

 

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