Issue 9, June 26, 2017

Hot Weather and Foliage

During hot weather, even though it appears there may be moisture in the ground, many trees and shrubs will shed entire leaves to reduce the moisture stress and transpiration loss on the plant. Some may think autumn has come early.

You can view it the leaf drop as a "sacrifice some to protect the whole."  Older leaves, more toward the interior of the tree instead of the tip of the branches, are shed in order to protect the newer leaves.  In most cases, the entire leaf, including the veins, turn yellow before they drop.

Birch (Betula spp.), tuliptree (Liriodendron sp.), silver maple (Acer saccharinum) and cottonwoods and other poplars (Populus spp.) are common leaf-dropping trees.  It should be no surprise these trees are also lowland trees, commonly found more in flood plains than the upland oaks and hard maples.  Lowland trees tend to develop more surface roots.

Even if there seems to be sufficient moisture in the ground, the tree could be losing water faster through the leaves than it can replenish from the roots.  This usually occurs when the temperatures exceed 90 degrees F.  Part of the problem is the shallow nature of these water-loving trees; the upper soil levels dry out faster than those below.

Any large summer-flowering shrub is also prone to leaf drop.  Flowers double the stress on the plant, and seed production isn't far behind.  Sacrificing some flowers and/or flower buds puts less stress on the root system, though it is difficult to remove potential flowers, especially if that's the reason for the plant.

Scorch is another symptom of heat and water stress. 

Scorch shows up as drying of leaf tissue, usually starting along the margins and tips of the leaf and progressing toward the veins, which typically remain green.  The tissue starts out as a lighter green, progressing to yellow and then finally brown.  Leaves may remain on the tree, but seldom recover even with moisture.  If the scorch is severe, leaves will drop.  Scorch is common on dogwood (Cornus), maples (Acer), oaks (Quercus) and lindens (Tilia). 

A side effect with leaf drop scorch is an increase if leaf spot diseases, though these are more aesthetically unpleasing than harmful to the plant. 

Mulching definitely helps conserve soil moisture and keeps the root system cooler so it can extract more water for the plant.  Four to six inches of composted wood chips should be sufficient, ideally spread to the plant's dripline.

Deep watering is also recommended, encouraging roots to seek out the lower levels of moisture and cooler soil temperatures.  (David Robson)

David Robson

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