Drought and Landscape Plants Bruce R. Fraedrich, Ph. D., Plant PathologistMoisture deficiency is the most common stress Restricted growth and vitality are manifested byencountered 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. Thisplants. Periodically, drought conditions may smaller photosynthetic area further impedes thepersist for several months to years and can plants ability to produce carbohydrates evensignificantly affect plant health and survival. after the drought has ended. Landscape plantsUnfortunately extended droughts have become typically require several years to recover fullythe norm. In the past decade, significant rainfall from drought due to a lesser capacity for foodshortages have persisted for multi-year periods production. Growth reduction is often moston at least two occasions in the eastern portion severe the year following drought. Inof the United States. combination with other stresses such as transplanting, root damage from construction,EFFECT FROM PROLONGED DROUGHT soil compaction, defoliation and old age, treesON PLANTS can decline and die from drought injury.Moisture stress adversely affects virtually everyphysiological process in the plant. The firstresponse is closure of leaf stomata. Thisreduces transpiration water loss and acts as adefense mechanism. However, with the Trunk splitting is astomata closed, carbon dioxide is not absorbed possible tree reactionand photosynthesis is curtailed. The reduction to drought stressin photosynthesis limits growth and increasessusceptibility to insect and disease pests.Water stress also inhibits the production Drought-stressed trees(synthesis) and translocation of essential often attract insectmaterials in the plant. In addition to less pests and diseasecarbohydrate (food) production, proteins, organisms. Commonenzymes, growth regulators, mineral nutrients pests on droughtand other essential materials for life are not stressed plants include borers, bark beetles andproduced and/or translocated in the plant. canker and root disease fungi. TheseThese factors further contribute to reduced secondary invaders accelerate decline and maygrowth and development. ultimately cause death of the plant.
2SYMPTOMS OF MOISTURE STRESS Drought Severity - The length and severity ofSymptoms on broadleaf plants usually are first the drought are perhaps the most importantevident on older foliage. Typical symptoms factors influencing plant survival. Drought ininclude wilting, chlorosis, browning of leaf early spring when water for growth is critical hasmargins (scorch) and premature defoliation. the greatest impact on plant health and survival.Eventually shoot growth and new foliagebecome 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 orDieback symptoms in moisture stressed micronutrients can further impede photosynthateJapanese maple production. Phosphorus deficiency can restrict root growth. A deficiency of potassium can interfere with normal functioning of the stomatesOn conifers, wilting and premature browning and that affect internal water relations.defoliation of older needles occurs. New growthis stunted and needle tips become brown. Additional Stresses - Landscape plants are subjected to a wide range of stresses that canDuring prolonged droughts, twig and branch intensify the effects of drought. Root damagedieback may occur. Plants stressed by root from construction, transplanting, soil compactionloss may suddenly wilt and die. and pavement over the root system have profound effects on plant survival. LandscapeFACTORS AFFECTING PLANT SURVIVAL plants must compete with turf or other ground covers for water and nutrients, which alsoThe ability of landscape plants to survive intensifies the effects of drought. Otherdrought 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 BTRL TR-31
3Secondary Invaders - "Secondary Invaders" Canker diseases are common on droughtrefer to insect pests and disease organisms that stressed plants. Cankers are caused primarilyare capable of invading plants after they are by fungi, which invade through wounds in thestressed. Certain borers are very prevalent in bark. Most of these fungi are weak pathogensdrought-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 moistureleaders of weakened oak and beech. This stress, these fungi invade living tissue, causinginsect 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 inBorer larvae removed from under bark low maintenance areas, is an effective method of minimizing the effects of drought. Consult theBark beetle is a common pest of pines and other Bartlett Tree Research Laboratories orconifers. They feed in the cambium area of the Cooperative Extension Service for a listing oftrunk by constructing galleries. In addition to drought resistant plant for a given geographicimpeding water and nutrient transport, bark area.beetles may introduce a fungus that grows in thevascular system, which further blocks water Irrigation - Most large landscape plants requiretransport and accelerates decline. Certain one inch of water per week during the growingspider mites and aphids usually are more active season. This is equivalent to approximatelyon plants during droughts. Heavy rains tend to 750 gallons of water per 1000 square feetwash mites and aphids from leaves. Drought beneath the crown. For new transplants, rootalso 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 ofOld plants subjected to drought stress are apt to new plantings. On established plantings in claybe invaded by root disease fungi. Armillary is a or loam soils, the recommended quantity ofcommon parasite that affects a wide range of water should be supplied at least once eachtrees and shrubs that have been stressed by week. Drip irrigation systems or soaker hosesdrought and age. Phytophthora root rot also is usually are most efficient since they irrigate onlymore prevalent on plants when heavy rains the root zone and minimize runoff.follow severe droughts. Tensiometers are very effective tools for rapidlyArmillaria 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 BTRL TR-31
4organic mulch including wood chips, shredded reduce demands for water and nutrients. Avoidbark, bark nuggets, pine straw, leaves, etc. are significant pruning of live branches because thisconducive for mulching. Wood chips from tree will add additional stress from defoliation andpruning operations are particularly effective and wounding.inexpensive mulch. Anti-Transpirants - Anti-transpirants areMulches should be applied to a depth of 2-4 materials applied as a spray to the foliage andinches around landscape plants. Do not which provide a barrier to water loss. Theseexceed this depth around trees, as this could be materials can produce a short-term benefit bydetrimental. Preferably, mulches should be reducing transpirational water loss. Anti-applied to the "dripline" of the plant whenever transpirants may be useful on recent transplantspossible. However, a narrow mulch ring around or when trees cannot be routinely irrigated forplants 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 diseaseGuidelines” for more information. organisms weaken trees by defoliation or by causing stem and root damage that impedesFertilization - Maintaining adequate soil fertility absorption and translocation of water andwill help prevent nutrient stress and minimize nutrients. Drought stressed plants arethe effects of drought. Slow release fertilizers particularly prone to pest infestations. Pestsare generally optimum for growth of woody should be managed using integrated pestplants. Avoid agricultural grade fertilizers, which management (IPM) principles, a technique ofhave a high salt content that can intensify periodically inspecting plants for pests and otherdrought stress. For best results, base fertilizer plant health problems. When detected, peststreatments on soil analysis results. are maintained below levels that impact plant health through cultural, biological and/orFertilizer should be applied after drought has chemical treatments.ended and soils are recharged by rainfall.Applying nutrients during a drought will have SUMMARYlittle impact on plant growth because water isthe limiting factor. High salt fertilizers can Moisture stress from drought periodically affectsseverely injure plants if applied to dry soils. landscape plants. The impact of drought on plant growth varies with the severity andThe addition of commercially available duration of the drought as well as other factorsmycorrhizae products, that contain live spores of including plant species, soil conditions and otherthe fungus, may be beneficial especially on stresses and pests that also may affect thetrees affected by root problems. Research has plant. A comprehensive plant health careshown that these products stimulate root growth program featuring irrigation, mulching,especially when applied with fertilizer. For more fertilization, proper pruning and integrated pestinformation regarding mycorrhizae, consult the management is recommended to helpTechnical Report Mycorrhizal Inoculation of landscape plants withstand the effects ofResidential Trees published by the Bartlett Tree drought.Research Laboratories.Pruning - Dead and dying limbs on landscapeplants should be removed. These limbs mayharbor insect borers or canker disease fungi thatcan contribute to further dieback and decline. Ifcrowns are very dense, light thinning will help BTRL TR-31