I have seen a
number of small trees and shrubs severely declining, and even dying
this year. Many are newly planted, but some were well established.
Despite our plentiful – sometimes too plentiful – rain this summer,
woody plants carry the insults of previous growing seasons with them
for life, particularly the dry years. The damage caused by drought
often does not show up until the following growing season. Some of
the declining and dead plants were well established and had lived
through more than one drought.
To their owners, they seemed to die overnight, but in reality they
had been slowly declining over time. Such symptoms can be subtle and
may not be noticeable unless you know what you are looking for. Some
died from insect or disease problems that they were susceptible to
because of drought stress; others because of difficult site
conditions made worse by drought.
And although we
did have more than enough rain in July, much of it came as driving
rain that does not soak into the ground as well as more gentle,
prolonged rains do. And the dense foliage of mature trees often does
not let the rain reach the ground. Amazingly, I am seeing
Early fall color is a trouble sign
If you can
remember back to last year’s weather, it got very dry in mid-May and
stayed that way until July. We had above average rain in July, only
to go back into hot, dry weather from August into September. Fall
was pretty average until we got to November – last November was the
third driest since the weather service began keeping records. By
November, hoses and irrigation systems are drained for winter, and
no one is thinking about watering. Except for those thirsty trees
and shrubs, that is.
Drought is most likely to have severe impacts on
trees and shrubs that are:
planted, especially container-grown plants. Those in containers
are often grown in an artificial soil mix that dries out much
faster than the surrounding soil.
- planted on
stressful sites – those with compacted soil, heavy clay,
restricted root zones, or reflected heat from buildings and/or
and/or mulched too deeply. The root flare of trees should be
visible at ground level, and crowns of shrubs should not be
buried by mulch.
from mechanical damage. Wounds to the trunk from lawn mowers,
weed whackers, deer, rodent activity – anything that damages the
bark can impact a tree’s vascular system.
species such as flowering dogwood, redbud or crabapple. They
have smaller root systems than large shade trees, and therefore
less drought tolerance.
- those with
typically shallow root systems, such as conifers, maples and
honeylocusts. Their shallow root systems are less
drought-tolerant than species with deeper root systems.
- those with
Girdling roots grow across the side of a tree
trunk causing constriction
All of a
plant’s physiological functions depend on water – food production
and storage, manufacture of defensive compounds, and growth and
reproduction. Drought itself can severely damage plants, but it also
makes them more susceptible to insect and disease problems.
Drought on Trees
several impacts on plants. One is that their roots develop a waxy
layer to help prevent moisture loss. But when the rain returns, that
works against plants since the waxy layer limits their ability to
take up water. Furthermore, the fine root hairs that are most
responsible for absorbing water and nutrients are the most
susceptible to damage and death from drought because they tend to
grow in the upper layers of soil. This double whammy to the root
system, especially for plants growing on difficult sites, can be all
it takes to push them into decline. When such a drought is followed
by a period of wet weather, plants with compromised root systems may
decline steeply because the saturated soil does not allow enough
oxygen to reach the roots.
limits photosynthesis, the process plants use to manufacture
carbohydrates from carbon dioxide and water in the presence of
sunlight thanks to the chlorophyll molecule. When plants suffer from
drought stress, the pores in their leaves (stomata) stay closed to
conserve moisture, which reduces the amount of carbon dioxide they
can absorb. When both water and carbon dioxide are limited, so is
photosynthesis. Woody plants that cannot build up life-sustaining
carbohydrate reserves are in trouble. That is why trees and shrubs
that are defoliated by insects for several years in a row often die
– they use up their carbohydrate reserves when they have to send out
new leaves to replace the damaged ones. Drought often causes trees
and shrubs to drop their leaves prematurely, which also limits
We also had
several frosts after plants started leafing out. If the tender new
leaves were killed, affected deciduous trees put out a second flush
of growth. But that second flush of growth forced them to deplete
their carbohydrate reserves more than would have been necessary
otherwise. That is enough to send already-stressed plants over the
Frost on a Laceleaf Japanese Maple is
If you have
trees and shrubs that appear stressed, a little TLC can help. If
they have more than two or three inches of mulch over their root
systems, pull it back. Deep mulch becomes water-repellant once it
dries out thoroughly and can prevent rain from reaching the plant’s
roots. This also allows very saturated soil to air out more
efficiently, in situations where drainage is not good. If the soil
is dry, remove the nozzle from your hose and allow water to trickle
slowly around the base of the plant, moving it every twenty to
thirty minutes, until the plant is watered deeply around the entire
circumference of its root system. Make sure trees and shrubs do not
go into winter under drought stress.
temptation to fertilize stressed plants until they recover a bit,
especially if insect or disease problems are involved. Fertilization
can make some pest problems worse. Also, if their roots systems have
been compromised, they might not be able to keep up with the new
growth forced by fertilization.
and stems should be pruned out over the dormant season when you are
less likely to spread disease-causing organisms. As added insurance,
disinfest pruning tools between cuts by dipping them into 70 percent
alcohol and allowing them to air dry.
If you have
very large trees that need attention, it is wise to hire a certified
arborist for help diagnosing any problems and developing a
management plan to get them healthy and keep them that way.