WOODY PLANTS UNDER STRESS
Deborah B. Hill and William M. Fountain
When people see their woody plants (shrubs
and trees) decline, they often want a "magical" treatment to make them
lush, green and healthy again. But no magic exists. This publication should
help you understand some common causes of woody plant stress, how stress
leads to decline and what measures, if any, can make the tree healthy again.
Remember, however, that all trees and shrubs like every living organism,
will eventually die.
What Causes Woody Plant Stress?
All living organisms exist in a
world of stresses. Here we are emphasizing unusual stresses that negatively
affect normal health and growth of woody plants in a home landscape. Because
plant problems arise from physical, chemical, biological and climatic factors,
this publication defines stress as visible effects on your woody plants
of one or more of these factors. You may see abnormal color or color changes,
unusually small leaves, fewer leaves than normal, or a variety of other
symptoms. (See also Cooperative Extension Publication, ID-52,
Environmental Stresses on Woody Ornamentals).
Plant Ecology and Physiology
First, let's review a woody plant's
structure by looking at a tree. Each section of the tree performs unique
functions but all are necessary for it to grow in a healthy manner.
Photosynthesis or food-production takes
place in the crown. Foliage utilizes water and nutrients taken by
roots and combines them with carbon dioxide from the air and energy from
the sun to produce carbohydrates that fuel life processes of a tree. When
something interferes with normal photosynthesis, the tree produces less
food for ongoing life and less reserves for the future. The time of year
when photosynthesis is disturbed determines whether this year's or next
year's growth is affected. Early season problems tend to affect the current
year's growth, while late season problems tend to affect the next year's
The trunk (or bole) physically
supports the crown, conducts nutrients and water up from the roots to the
crown and conducts food down from the crown to the roots. Protected by
bark, it serves as the tree's vital circulatory system. Inner bark is where
new woody material is formed each year and also is where the transport
system is located.
The roots support the plant,
absorb water and nutrients from the soil for photosynthesis and "breathe."
Their location is important. Because 90% of a tree's small feeder roots
are in the top 6 inches of soil, the root system often extends outward
from the trunk much farther than the branches. Many people mistakenly think
that roots end at the dripline (the farthest edge of the branches), but
they usually extend much farther. Therefore, fertilizing and other cultural
practices should start within the dripline and extend outward at least
half again as far as the distance from trunk to dripline.
Note: Since roots do not observe
property lines, you may need to water and fertilize your neighbor's property
to help one of your stressed plants.
The Wrong Place
Consider a plant's native environment.
Species of plants have adapted to specific conditions over millions of
years. Take the dogwood: in its native habitat it grows under forest trees
or near the forest edge, always in partial or full shade. Leaves and stems
die when the tree grows in full sun, because of heat and the wind's drying
effects. Such constant stress may also make the tree more susceptible to
insect and disease attack.
Any time you place a plant into an
environment where it is not native, you are removing it from conditions
where it has thrived and are placing it in a stressful situation. In a
new location it may be attacked by new insects or diseases or it may affect
or be affected by other plants. Imported plants put in areas with soil
types and/or climatic conditions similar to their native environment will
probably have a better chance of surviving than ones not so well located.
Follow these guidelines as you plant:
•Don't put plants into locations
that they will outgrow. Be sure to ask how big your tree or shrub will
be when mature before you buy it from a nursery. How far will its crown
spread? How tall will it be? How big around will the trunk be? Because
tree roots extend far beyond the spread of the crown, you should expect
the root system to expand twice as far as the expected mature crown spread.
•Avoid physical barriers such
as building foundations and paved roadways. They inhibit the root system's
expansion and therefore shorten a tree's life span.
•Avoid planting large woody plants
in shallow soils with solid bedrock underneath. Such soils will force
the root system to stay abnormally close to the soil surface where it may
be more prone to drought and may require more regular watering during dry
periods. This situation also makes trees more prone to windthrow.
Good soil for plant growth must
be loose enough to provide spaces for air and water. Compaction is compression
of soil so that these spaces are reduced in size and number. Factors affecting
soil looseness include:
•Foot traffic from people and/or animals
(either livestock or pets);
•Paved driveways, sidewalks and roads,
that limit the effective functioning of tree roots beneath them;
•Construction, that often compacts
soil with heavy equipment;
Further construction problems include:
• Removing soil from the root zone,
which causes exposure and root mortality;
• Adding soil to the root zone which
causes overfilling and reduces the oxygen essential for root respiration
(breathing). If fill is unavoidable, use tile drainage with a tree well.
Consult a certified arborist or landscape contractor before beginning construction.
Poor pruning practices, especially
topping of trees, harm trees and shrubs. Aside from being unsightly, topping
removes all normal leaf-bearing woody material and causes heartrot. Trees
are weakened because they must take extra energy from their reserves to
produce new foliage-bearing branches. Topping trees to avoid power lines
simply aggravates the problem. Trees will form many fast-growing water
sprouts that grow vertically and soon interfere again with the lines. Careful
directional pruning, done by a trained arborist, encourages branches to
grow primarily away from lines and can permanently solve the problem.
Not only is the method of pruning important
but the time of year can also be critical to the plant's continued good
health. Know what methods are appropriate for the woody plants you have.
(For more detailed information see Cooperative Extension publications HO-59,
Pruning Landscape Shrubs, and ID-55, Warning:
Topping is Hazardous to Your Tree's Health.)
•Some deciduous trees are best pruned
during the dormant season (winter, early spring).
•Certain flowering shrubs and trees
should be pruned only after flowering.
•Pines, spruces, maples, elms, beech,
birches and yellowwood may "bleed" excessively if pruned in late winter
•Many shrubs and trees benefit from
shearing or trimming during the growing season, but usually not in early
spring when buds first break.
Bark and Wood Damage
Bark and wood on tree trunks are often
inadvertently damaged by bicycles chained to trees, automobile collisions
and carelessly operated lawn mowers or weed trimmers. Every nick on the
bark provides a potential entry site for insects and disease organisms
that are constantly present in the environment but normally excluded by
the physical barrier of intact bark. Further, removal of bark not only
interrupts circulatory activity at that point, but inhibits the tree's
ability to make new woody material for that year. If the wound is not fatal
(e.g. complete girdling of the tree) energy will go into its healing. Interruption
of the two-way transport of carbohydrates, nutrients and water disrupts
the ability of either crown or roots to perform effectively.
Plants can also be seriously damaged
by toxic materials in the atmosphere or soil. Air pollutants, such as sulfur
and nitrous oxides from fossil fuel combustion (e.g. vehicles, power plants,
industries) can cause localized problems such as "burn" on roadside plantings
of trees or shrubs constantly exposed to vehicular exhaust. Ozone, another
atmospheric factor that occurs naturally at low levels, damages leaf tissue
at high concentrations. Studies indicate that acid deposition, whether
as solid particles, fog, dew or other precipitation, does affect acidity
of soil, but it is still unclear whether such acidification is enough to
be harmful to roots or tree growth.
Salt applied to roads for snow or ice
control can infiltrate soil around woody plant roots. Salt can change the
soil pH and damage roots, in turn affecting leaves and branches dependent
on those roots for water and nutrients. Spray that cars whip up off salted
streets can float like fog for up to 100 yards, killing or damaging stems
and buds on nearby dormant plants. Evergreens suffer even more severe damage
from salt than broadleaved trees because they carry live foliage during
winter months. Likewise, excessive application of fertilizer or injection
of fertilizer into soil in localized spots can also kill roots they contact.
Some obvious problems for woody
plants are insects and diseases. Such stresses often create holes in leaf
tissue, obvious discoloration, or distortion of leaf shape in one form
or another. Defoliating insects, such as gypsy moth, are capable of completely
removing live foliage in spring and early summer. Often trees and shrubs
can produce a second set of leaves after such defoliation, but repeated
defoliations weaken plants considerably. Once weakened by such activity,
trees and shrubs often become more susceptible to secondary invaders such
as boring insects and canker fungi. (Several Cooperative Extension publications
are available on insects and diseases of shrubs and trees. Contact your
county Extension agents for details.)
Weather patterns are never stable.
Extremes in heat and cold or sudden temperature changes have occurred when,
normally, a slow warming or cooling trend might have been expected. Also,
long periods when the soil has been excessively dry have been followed
immediately by periods when the soil has been saturated. These stresses
have killed many established plants and badly damaged others.
The interaction of wide swings in weather
causes the greatest plant damage. For example, in 1983 Kentucky experienced
the second worst drought of the century and many trees and shrubs were
forced into dormancy during the growing season. Fall brought rains and
unseasonably warm temperatures. Instead of beginning the normal process
of hardening off for winter, many plants began putting on growth. When
all-time record lows were experienced in December and January these plants
were killed or severely damaged. For example, southern magnolia (at the
northernmost edge of its natural range in Kentucky) was severely damaged,
in most cases losing its evergreen foliage and dying. Many trees and shrubs
continued to show symptoms of decline in years following the 1983 drought.
Long term effects of severe climatic conditions are not unusual in woody
Periods of saturating rains in spring
when trees and shrubs are beginning their new shoot and leaf growth are
particularly damaging. Water excludes oxygen from soil and can result in
"suffocation" and death of the lower root system. This lower root system
is necessary for uptake of water and nutrients during dry periods when
feeder roots close to the soil surface may dry out and die. If drought
damages feeder roots after saturating rains have damaged the deep root
system, woody plants will suffer.
Too little water can be equally, if
not more, damaging. Although you cannot remove excess water from the soil,
you can water trees and shrubs during dry periods. One to 2 inches of water
must enter the soil each week during spring, summer and early fall. If
the 1-2 inches does not occur as natural precipitation, you must supplement
it. A little sprinkle each day (for example, a tenth of an inch or less)
will be more harmful than helpful and results in the formation of surface
How can you tell your plants are declining
from stress? Careful observation and common sense are the keys. Look for
dead limbs, leaves that are off-color (pale green or yellowish in summer),
leaves drying out or margins (edges) of leaves turning brown, premature
leaf drop, premature fall coloration in late summer, holes in leaves or
skeletonized leaves (no green part, just veins), very short areas of new
twig growth, and the presence of insects and/or diseases.
You can be organized about your observations.
•Watch newly established woody plants
carefully throughout the first two growing seasons.
•Take pictures of valuable trees and
shrubs annually to help you detect changes from year to year.
•Ask for help from your county extension
service. Submit samples (including branches, twigs and several leaves)
through your county extension office when the problem first occurs.
•If necessary, your county agent may
consult the Plant Disease Diagnostic Lab or one of the Extension specialists
in Horticulture or Forestry at the University of Kentucky. Careful and
frequent observation is important because once the problem is easily visible,
it has already become serious, if not irreversible.
What can you do once you know that
a plant is suffering from severe stress? The following list includes some
very practical steps. If you do not yet have serious problems, these suggestions
may be preventive medicine.
Avoid Drought Stress
If your plants have suffered from drought
stress, make sure they do not suffer that stress again. Landscape plants
should receive at least the equivalent of 1 ½ inch of rainfall per
week during the growing season. If there are dry spells, water with hose
or sprinkler until enough water has collected in a pan. If this level of
watering causes runoff, three shorter periods of 1/2 inch of water each
time during the day may be necessary. When watering, remember that tree
roots extend well beyond the dripline. Use a soaker hose or some other
slow-release method of watering, and move it around the tree area during
the soaking period.
Mulching 2 or 3 inches deep with some
organic material (compost, sawdust, woodchips, etc.) around newly planted
shrubs and trees will help retain moisture for roots and provide protection
to the trunk from lawn equipment.
Get the advice or services of a trained
certified arborist or qualified landscape contractor for corrective pruning
on your tree. You should remove dead limbs from the affected tree because
of potential liability from falling branches, and selectively remove other
limbs to improve the plant's form or health. Removing some live material
will reduce the demand on the root system and allow the whole tree to renew
itself. Never top the tree or remove more than a quarter of the live crown.
When deciduous plants go into dormancy
in the fall, apply fertilizer to improve their vigor the following growing
season. If you already fertilize your lawn around trees, additional fertilization
should not be necessary. Otherwise, you can apply (at normal lawn application
rates) ammonium nitrate or a complete fertilizer (e.g. 10-10-10) divided
into equal parts two or three times during fall or winter (November to
January). The ground should not be frozen and it should be free of snow
cover when applying fertilizer. Most woody plants will show better summer
and fall color, fuller growth and greater hardiness following this treatment.
Fertilization can be repeated annually or biennially for as long as rapid
growth is desired or the stress continues.
Some plants may be too old or severely
damaged to show positive results from the addition of fertilizer. Nitrogen
added to trees under severe stress may actually increase the rate of decline.
These severely stressed plants may put on more top growth than the root
system is able to support resulting in depletion of stored carbohydrates.
However, these severely stressed plants may recover on their own if watered
and protected from other injurious factors. Making a decision is a judgment
call in which you should consult a certified arborist or other green industry
professional who has had extensive experience in saving declining trees.
Good management of your valuable woody
plants may involve both "preventive" and "curative" medicine. Tips to remember
for maintaining healthy plants in your home or farm landscape include:
•Plant species that are adapted to
Kentucky's climate and plant them in situations that mimic their natural
•Monitor woody plants during the growing
season for signs of abnormal growth or general poor health, as well as
for signs of harmful insects or diseases.
•Be very careful with equipment such
as mowers and weed trimmers so that you do not damage tree bark.
•Keep trees and shrubs adequately watered
during dry periods.
•Fertilize lightly during late fall
or early winter to improve growth for the following year.
•If you are unsure how to prune trees
and shrubs correctly, have a trained arborist or landscape contractor do
corrective pruning of dead or dying limbs.