Drought and Landscape Plants - Bartlett Research

1. Drought and Landscape Plants
Bruce R. Fraedrich, Ph. D., Plant Pathologist
Moisture deficiency is the most common stress Restricted growth and vitality are manifested by
encountered in the landscape. Usually, this is a stunted chlorotic leaves; premature defoliation,
temporary condition and has minimal impact on crown thinning and poor shoot growth. This
plants. Periodically, drought conditions may smaller photosynthetic area further impedes the
persist for several months to years and can plants ability to produce carbohydrates even
significantly affect plant health and survival. after the drought has ended. Landscape plants
Unfortunately extended droughts have become typically require several years to recover fully
the norm. In the past decade, significant rainfall from drought due to a lesser capacity for food
shortages have persisted for multi-year periods production. Growth reduction is often most
on at least two occasions in the eastern portion severe the year following drought. In
of the United States. combination with other stresses such as
transplanting, root damage from construction,
EFFECT FROM PROLONGED DROUGHT soil compaction, defoliation and old age, trees
ON PLANTS can decline and die
from drought injury.
Moisture stress adversely affects virtually every
physiological process in the plant. The first
response is closure of leaf stomata. This
reduces transpiration water loss and acts as a
defense mechanism. However, with the Trunk splitting is a
stomata closed, carbon dioxide is not absorbed possible tree reaction
and photosynthesis is curtailed. The reduction to drought stress
in photosynthesis limits growth and increases
susceptibility to insect and disease pests.
Water stress also inhibits the production Drought-stressed trees
(synthesis) and translocation of essential often attract insect
materials in the plant. In addition to less pests and disease
carbohydrate (food) production, proteins, organisms. Common
enzymes, growth regulators, mineral nutrients pests on drought
and other essential materials for life are not stressed plants include borers, bark beetles and
produced and/or translocated in the plant. canker and root disease fungi. These
These factors further contribute to reduced secondary invaders accelerate decline and may
growth and development. ultimately cause death of the plant.

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SYMPTOMS OF MOISTURE STRESS
Drought Severity - The length and severity of
Symptoms on broadleaf plants usually are first the drought are perhaps the most important
evident on older foliage. Typical symptoms factors influencing plant survival. Drought in
include wilting, chlorosis, browning of leaf early spring when water for growth is critical has
margins (scorch) and premature defoliation. the greatest impact on plant health and survival.
Eventually shoot growth and new foliage
become stunted and chlorotic. Plant Species - Some plants are inherently
more tolerant of drought stress. Drought
tolerance may be attributed to anatomical
structures such as an aggressive, deep root
system or thickened, waxy leaves. Drought
tolerance may also be attributed to physiological
responses within the plant.
Soil Conditions - Soil type, organic matter
content, fertility levels and other soil factors
affect drought tolerance. Plants growing in
sandy soils, which have low moisture holding
capacities, are most sensitive to drought. On
heavy clay soils or those that are compacted,
root growth is restricted, which can predispose
plants to drought damage. Loam soils with at
least 5% organic matter are conducive to root
development and water retention.
Nutrient deficiencies can intensify the effects of
drought stress. A deficiency of nitrogen or
Dieback symptoms in moisture stressed micronutrients can further impede photosynthate
Japanese maple production. Phosphorus deficiency can restrict
root growth. A deficiency of potassium can
interfere with normal functioning of the stomates
On conifers, wilting and premature browning and that affect internal water relations.
defoliation of older needles occurs. New growth
is stunted and needle tips become brown. Additional Stresses - Landscape plants are
subjected to a wide range of stresses that can
During prolonged droughts, twig and branch intensify the effects of drought. Root damage
dieback may occur. Plants stressed by root from construction, transplanting, soil compaction
loss may suddenly wilt and die. and pavement over the root system have
profound effects on plant survival. Landscape
FACTORS AFFECTING PLANT SURVIVAL plants must compete with turf or other ground
covers for water and nutrients, which also
The ability of landscape plants to survive intensifies the effects of drought. Other
drought depends on many factors including: stresses that weaken landscape plants include
Severity and timing of drought old age, defoliation from pests, bark wounds,
Species of plant reflective heat from pavement and buildings and
Soil conditions chemicals such as air pollutants, herbicides and
Additional stresses deicing salts.
Secondary insect and disease pests
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Secondary Invaders - "Secondary Invaders" Canker diseases are common on drought
refer to insect pests and disease organisms that stressed plants. Cankers are caused primarily
are capable of invading plants after they are by fungi, which invade through wounds in the
stressed. Certain borers are very prevalent in bark. Most of these fungi are weak pathogens
drought-stressed plants. The two-lined chestnut that live primarily on dead twigs and branches.
borer for example, invades the branches and When live plants are subjected to moisture
leaders of weakened oak and beech. This stress, these fungi invade living tissue, causing
insect feeds in the inner bark and sapwood by girdling and dieback of branches and stems.
constructing galleries Alleviating moisture stress usually prevents
(tunnels) that impede further ingress by the canker fungus.
transport of water and
nutrients. This results
in branch dieback, PREVENTIVE AND REMEDIAL TREATMENTS
decline and possibly
death of the plant. Drought Resistant Plants - Utilizing drought
resistant plants in the landscape, especially in
Borer larvae removed from under bark low maintenance areas, is an effective method
of minimizing the effects of drought. Consult the
Bark beetle is a common pest of pines and other Bartlett Tree Research Laboratories or
conifers. They feed in the cambium area of the Cooperative Extension Service for a listing of
trunk by constructing galleries. In addition to drought resistant plant for a given geographic
impeding water and nutrient transport, bark area.
beetles may introduce a fungus that grows in the
vascular system, which further blocks water Irrigation - Most large landscape plants require
transport and accelerates decline. Certain one inch of water per week during the growing
spider mites and aphids usually are more active season. This is equivalent to approximately
on plants during droughts. Heavy rains tend to 750 gallons of water per 1000 square feet
wash mites and aphids from leaves. Drought beneath the crown. For new transplants, root
also reduces beneficial insect populations that damaged trees or plants growing in sandy soil,
prey on mites and aphids. water should be provided at least twice a week.
Water should be concentrated on the root ball of
Old plants subjected to drought stress are apt to new plantings. On established plantings in clay
be invaded by root disease fungi. Armillary is a or loam soils, the recommended quantity of
common parasite that affects a wide range of water should be supplied at least once each
trees and shrubs that have been stressed by week. Drip irrigation systems or soaker hoses
drought and age. Phytophthora root rot also is usually are most efficient since they irrigate only
more prevalent on plants when heavy rains the root zone and minimize runoff.
follow severe droughts. Tensiometers are very effective tools for rapidly
Armillaria root rot fungus mushrooms assessing irrigation needs. Consult Technical
Report “Tensiomenters in Landscape Plantings
for information on tensiometer use..
Mulches - Mulches help conserve soil moisture
and reduces competition for water from weeds.
They add organic matter to the soil, which
promotes root development and improves the
soil s moisture-holding capacity. Mulched natural
areas eliminate competition for water and
nutrients from turf or other ground covers. Any
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organic mulch including wood chips, shredded reduce demands for water and nutrients. Avoid
bark, bark nuggets, pine straw, leaves, etc. are significant pruning of live branches because this
conducive for mulching. Wood chips from tree will add additional stress from defoliation and
pruning operations are particularly effective and wounding.
inexpensive mulch.
Anti-Transpirants - Anti-transpirants are
Mulches should be applied to a depth of 2-4 materials applied as a spray to the foliage and
inches around landscape plants. Do not which provide a barrier to water loss. These
exceed this depth around trees, as this could be materials can produce a short-term benefit by
detrimental. Preferably, mulches should be reducing transpirational water loss. Anti-
applied to the "dripline" of the plant whenever transpirants may be useful on recent transplants
possible. However, a narrow mulch ring around or when trees cannot be routinely irrigated for
plants is better than none. Do not apply mulches brief periods in summer.
against the stem and root collar of plantings.
Consult Technical Report “Mulch Application Pest Management - Insect pests and disease
Guidelines” for more information. organisms weaken trees by defoliation or by
causing stem and root damage that impedes
Fertilization - Maintaining adequate soil fertility absorption and translocation of water and
will help prevent nutrient stress and minimize nutrients. Drought stressed plants are
the effects of drought. Slow release fertilizers particularly prone to pest infestations. Pests
are generally optimum for growth of woody should be managed using integrated pest
plants. Avoid agricultural grade fertilizers, which management (IPM) principles, a technique of
have a high salt content that can intensify periodically inspecting plants for pests and other
drought stress. For best results, base fertilizer plant health problems. When detected, pests
treatments on soil analysis results. are maintained below levels that impact plant
health through cultural, biological and/or
Fertilizer should be applied after drought has chemical treatments.
ended and soils are recharged by rainfall.
Applying nutrients during a drought will have SUMMARY
little impact on plant growth because water is
the limiting factor. High salt fertilizers can Moisture stress from drought periodically affects
severely injure plants if applied to dry soils. landscape plants. The impact of drought on
plant growth varies with the severity and
The addition of commercially available duration of the drought as well as other factors
mycorrhizae products, that contain live spores of including plant species, soil conditions and other
the fungus, may be beneficial especially on stresses and pests that also may affect the
trees affected by root problems. Research has plant. A comprehensive plant health care
shown that these products stimulate root growth program featuring irrigation, mulching,
especially when applied with fertilizer. For more fertilization, proper pruning and integrated pest
information regarding mycorrhizae, consult the management is recommended to help
Technical Report Mycorrhizal Inoculation of landscape plants withstand the effects of
Residential Trees published by the Bartlett Tree drought.
Research Laboratories.
Pruning - Dead and dying limbs on landscape
plants should be removed. These limbs may
harbor insect borers or canker disease fungi that
can contribute to further dieback and decline. If
crowns are very dense, light thinning will help
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