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 problem
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.
What an excellent blog post and great lesson in Tree Physiology 101. Trees are astounding and they have so many ways they can adapt to our rapidly changing climate.
ReplyDeleteThere are many places I have visited for summer vacations and Colorado is among the best. I was mesmerized by the friendliness and the hospitable nature of the residents, the great climate and the great mountains and the springs. The residents of Colorado are the best part says Ron Virmani, they welcome all visitors. I started my hike at the Glenwood Springs. Glenwood boasts the largest hot springs pool globally. There is also the Yampah Hot Springs and the recently opened Mountain Hot Springs.
ReplyDeleteThis was delightfully informative. Thanks!
ReplyDeleteI have been noticing color change and leaf drop in hampton roads (South eastern Virginia) for the past week or two. We have been having temperatures in the 90's with heat indices in the 101-109 degree ranges for the past 2 weeks. This article helps make sense of the early leaf fall. Perhaps the trees are trying to prevent too much transpirational loss. Thank you for this information.
ReplyDeleteI have been noticing color change and leaf drop in hampton roads (South eastern Virginia) for the past week or two. We have been having temperatures in the 90's with heat indices in the 101-109 degree ranges for the past 2 weeks. This article helps make sense of the early leaf fall. Perhaps the trees are trying to prevent too much transpirational loss. Thank you for this information.
ReplyDeleteThis explanation dispelled worry regarding our beautiful Silver Maple trees!
ReplyDeleteThank you ever so much!
I planted a little grove of three. One of them has developed an odd discoloration of the leaves. At a distance it looks like a powdery substance on the leaves, but up close, the minor veins of the leaf is pronounced visually, it's lighter in color. Thoughts?
ReplyDelete