Ask Mr. Smarty Plants

While Pistacia mexicana (American pistachio) is native to Texas, this USDA Plant Profile shows it growing in only one county in the Big Bend area. That doesn't mean it won't grow in Austin, because obviously they do, at least for 13 years anyway. If they had not gone ahead and leafed out in the Spring, we would have blamed the very unusual cold temperatures we had this past winter. That could still be the problem.

What we have to do is try to discover what happened between last year and this year that could be causing this. For instance, has some kind of herbicide or "weed and feed" fertilizer been spread on your grass to get rid of broad-leafed weeds? Trees are also broad-leafed plants, and a too-vigorous application or (in the case of spraying) a brisk wind might have caused the leaf drop. Of course, if the same conditions don't apply to all the trees showing stress, that is probably not the only cause of the problem. Another clue we found said that the pistachio could flourish for years in the right location,and suddenly die of root rot from too much moisture in the soil. Have you put in a sprinkler system or started wateriing more often? We had several dry years and then a more normal rainfall. If the drainage in your soil is not good, the roots of the pistachio could be drowning. Desert trees do not flourish in lush conditions. 

Finally, take a good close look at the tree. While we found no documentation of disease or insect pests bothering this plant, it's always a possibility.  Beyond that, we are out of ideas. If you are watering more, we would suggest you direct it away from your trees. You might try treating it like transplant shock, and trimming away some of the upper portion of the trees. If the roots are struggling, this could help them get the needed moisture up to the leaves so the leaves can continue to manufacture food for the whole plant. Don't fertilize.The last thing stressed roots need is stimulation to put on new growth when they are just trying to stay alive. 

Fiinally, we suggest you contact the Texas AgriLIFE Extension Office for Travis County and see if they have encountered similar problems in the neighborhood.

From our Native Plant Image Gallery:

Pistacia mexicana

Pistacia mexicana

Pistacia mexicana

Pistacia mexicana