Chinese Pistache drops leave early
I bought a home with a mature Chinese Pistache tree (not quite as tall as our 2 story house) and each of the last 2 years it has dropped ALL of it's leaves in mid-August. It goes from beautiful, dense green foliage to completely bare within a week. The first year we assumed it was the heat and that it needed more water but it did it again this year and I can't imagine underwatering was an issue because I kicked up my sprinklers in June and forgot to turn them back down as the weather cooled. We were overwatering the whole yard, but not so much that anything else reacted badly. Anyone have any thoughts on what could be causing this or how I can fix it?
You are right to look first to water management, as I have seen the same premature leaf drop issue many times with drought stressed Chinese pistache. I have helped my customers pistache trees regain health with a watering program and fertilization.
I'm not sure what overwatering means to you, how frequently you irrigate, for how long and how you assess the depth of penetration. If its a superficial watering schedule for a few minutes, then there may not be enough for sustained soil moisture and the tree could be drought stressed in spite of frequent surface watering. On the other hand, if you are truly overwatering, then that could be a stress factor and could even cause root rot.
Ideal watering for a Chinese pistache would be infrequent, no more than once a week and allowed to dry in between waterings. And it should be deep, preferably 6 inches minimum to ideally 10 inches or more water infiltration into the soil profile. You can check this by digging a couple of holes the day after you water and look at how deep the wet layer is. Or use a soil sampling tube or soil moisture meter.
A common mistake is to water only near the tree trunk and not the entire soil surface under the foliar canopy and beyond, where the absorbing roots are. Mulching the soil surface helps to retain soil moisture. Drip systems are often inadequate for optimum tree health.
After ruling out water deficit/drought stress, the next thing to look at is the possibility of plant nutrient deficiency. Do you have good organic matter content in the soil (compost), is there mulch on the soil surface, and do you fertilize with a complete, mineralized, organic fertilizer.
Consider a fertility product like one of these. Agricolas is my favorite, but DrEarth or DownTo Earth all purpose fertilizer products are the next best and may be available at your local garden center.