Q.
I
notice that trees in my yard are already dropping leaves, and I see
the same thing along roadsides and in the parks. Is there some kind
of insect problem or blight, or is it just the dry weather?
A. What you are
seeing is an environmental problem rather than an insect or disease
issue. They are usually pretty species specific, and you would not
see similar damage on so many unrelated plants.
Spring started out fairly warm and dry, and then we started getting
a lot of rain, but it remained warm. Temperatures remained above
average through spring and beyond. The growing season seemed to
speed up, with plants blooming earlier than we remember from past
years. Some vegetables and annual flowering plants do very well in
this weather, as long as they have sufficient irrigation to make up
for the lack of rain. It is a different story for trees and shrubs.
This has been a rough growing season for many woody ornamentals. The
stress they are under shows up as yellowing and falling leaves, and
premature fall coloration. Much of the stress was caused by the wet
weather we had in May and June followed by hot, dry conditions since
then. The saturated soil early on may have damaged the root systems
of many plants by causing some root rot.
The rain also promoted a lot of quick, succulent growth. When those
plants were hit by the hot, dry weather, that succulent growth lost
moisture faster than more moderate growth would have. Their
compromised root systems were unable to take up sufficient moisture
to support that growth after the rains stopped coming so frequently.
If your established woody ornamentals are showing signs of stress
there are a few things you can do to help them. Provide an inch of
water a week if there is no rain. Soaker hoses and drip irrigation
are the most efficient ways to water. If that is not an option,
remove the nozzle from your hose and allow it to trickle slowly
around the base of the plant. Move the hose every hour so, until you
have watered the entire circumference of the plant. Start near the
trunk and work your way out to the drip line, if possible. Even if
you cannot water, trees and shrubs have set their buds for next
year’s growth by this point in the growing season. Although it is
not ideal for them to go into winter drought-stressed, they should
be fine next year.
 A two to three inch layer of mulch around the circumference of a
tree, out to the drip line if possible, helps conserve soil moisture
and moderate soil temperature. Replacing competitive lawn grasses
with mulch reserves water and nutrient resources for the tree. Mulch
also helps keep weeds in check that would otherwise compete with
your trees for water and nutrients. Just as important, a circle of
mulch around the base of trees and shrubs keeps lawn mowers and
string trimmers away from their trunks. Many woody plants
(especially young ones) are killed outright by lawn mowers and
string trimmers damaging the bark. The thin, light green layer of
tissue under the bark is called the cambium. This where water and
nutrients absorbed by the roots are transported up to the top of the
tree and nutrients produced by photosynthesis are transported down
to the root system. If this transport system is damaged around the
circumference of the trunk (girdling), the plant will die. Even less
severe damage from mowing equipment creates an entrance into the
plant for insect and disease problems.
Avoid mounding excessive mulch up around the trunk of trees and
shrubs. Doing so can cause the bark to rot, and it creates a perfect
environment for insect and animal damage to occur out of sight.
Mulch can be close to the trunk as long as it does not physically
touch it. Thick mulch also tends to become water repellent
(hydrophobic) once it dries out completely. When it does rain,
plants mulched in this manner may not benefit as much as plants
mulched more moderately.
Avoid pruning or fertilization now, since this will push new growth
that will not harden off before winter arrives. It is never wise to
fertilize drought-stressed plants – the concentration of salts in
most fertilizers can send stressed plants over the edge. It is
always best to take a soil test and fertilize according to the
resulting recommendations. Trees growing in a lawn situation receive
more than enough when you fertilize your lawn. They should not
require additional fertilization unless a soil test reveals a
deficiency. If you do not fertilize your lawn, or if the trees are
growing in a bed, they may benefit from a late fall application of
fertilizer. The roots of woody ornamentals put on a lot of growth
late in the season. The best time to fertilize is when they have
lost their leaves and gone dormant, but the soil is still warm
enough for root development. This is usually late fall, right around
Thanksgiving.
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