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Monday - June 15, 2009

From: Leander, TX
Region: Southwest
Topic: Diseases and Disorders, Trees
Title: Problems with Mountain Laurel in Leander TX
Answered by: Barbara Medford

QUESTION:

My 13 yr old Mountain Laurel tree is sick. It has lost most of its leaves after blooming this spring. The twigs seem to still be pliable so I assume alive. I saw no moth worms or bags this year, just the loss of leaves. Shall I just have patience and it will return to its glorious state?

ANSWER:

Sophora secundiflora (Texas mountain-laurel) is truly one of the treasure native plants of Texas. It is puzzling that it would have done so well up until it bloomed this Spring, and then began to decline. We did some research trying to find a clue to what has happened and didn't come up with the perfect answer. All we can do is give you some questions to ask yourself about something that has or has not happened recently that might have caused this. First of all, a Texas mountain laurel will not tolerate compacted soil. Over time, has the soil around this tree's roots been walked on a lot, maybe some vehicle is being parked nearby and perhaps over the roots? If so, stop whatever has been causing the compacting, and spread a good quality shredded hardwood mulch over the root area. This will shade the roots, protect the ground surface and, as it decomposes, add to the drainage capability of the soil. We are sure you already know that this plant cannot tolerate wet feet, and needs a well-draining alkaline soil. We found no pests nor diseases of "any major concern" of this plant. The Genista caterpillar can chew the leaves, but said you found no indication of insect damage. You should also know you should not ever fertilize this plant. Most native plants are so well-adapted to their area the don't need fertilizer, and this is certainly one of those plants.

Finally, and we sure hope this is not what happened, the Texas mountain laurel is very susceptible to phenoxy herbicides. These are herbicides used for killing broad-leaf plants, weeds, and usually applied by spraying. The Sophora secundiflora is, of course, a broad-leaf plant. Is there any possibility that a herbicide was applied in the area by you or a neighbor? Perhaps a landscaper putting down "weed and feed" fertilizer? That fertilizer feeds the grass, which is a monocot, and kills the broad leaf plants, which are dicots. Application on a windy day (of which we have plenty in Central Texas) could cause the dissemination of the herbicide beyond the target area. If that happened, we're not at all sure the tree can be saved.

Obviously, for whatever reason, the plant is stressed. Although it has been in the ground for 13 years, about all we know to suggest is that you treat it as though for transplant shock. Trim off the upper 1/4 to 1/3 of the plant. Make sure the drainage of the soil around it is still good, then water it deeply but infrequently. This is a tough desert plant, well-adapted to our conditions in Central Texas, and we can hope it will pull through.