Posted by Mary Small and Curtis Utley, Jefferson County Plant Diagnostic Clinic
Ah, sweet mid-summer! Long, warm evenings spent on the veranda listening to the sounds of leaves dropping. Huh?! leaf drop in late July/early August?
Yep, you read that right. We’ve been getting inquiries and emailed photos about, and observing this very phenomenon. Well, “phenomenon” probably isn’t quite the right term when you consider we see it every year, usually beginning mid-July. Several situations cause this by themselves or combine with others into a “perfect storm”. Tree leaf drop is probably the number one kasus we encounter in our clinic every summer. So why do trees drop leaves in the summer?
Trees often form more leaves in the spring than their systems can support later in the season. The hotter, dryer weather of summer signals plants to drop leaves as a defense mechanism against water loss. There may not be enough available or transpiration rates are so high that trees just can’t keep up. So they drop leaves to cut water losses and keep the rest of the system going. Drought stress causes leaf drop. Some of the tree canopy is shed to balance water intake and “outgo”, similar to the above situation. And, even though it looks like a lot, as much as 10% of leaves can drop without causing serious injury.
Newly transplanted trees (including those planted last year) frequently drop leaves to compensate for root loss and reduced water absorption.
Overwatering, poor soil drainage and too-deep planting results in root oxygen deficiency. Poorly functioning or dying roots can’t absorb needed water, so again the tree drops leaves to reduce transpirational losses. 
Insect pressure causes leaf drop. It’s not unusual for so-called “healthy-looking” trees to drop leaves all of a sudden. Lilac-ash borer and the banded ash clearwing are common culprits. These insects spend a good chunk of their lives tunneling in and through wood underneath the bark a couple of inches deep. In addition to physically weakening the tree, the activity can interfere with the flow of water, stressing the tree and inviting attack. Leaves are shed as (you guessed it) trees try to cut their transpirational losses. Then there’s hail and wind that physically knock leaves (and branches) out of trees. Sometimes the full extent of damage doesn’t show up until a bit after the storm, when damaged petioles and leaf blades succumb to dehydration following tears or dings.
Aspen trees begin their Marssonina-infected leaf drop in mid to late July. Infection occurred in spring, so there’s nothing to do until leaf raking season.
Trees lose leaves for many reasons in late summer but the question behind the question for most folks is, “is the tree okay”? Check the quality and quantity of new growth by looking at the annual growth increments, the general size of the leaves and remaining canopy density. If all of these things check out, then the tree is probably going to survive to see another spring…and summer.