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Drought Advisory: Watering Home Gardens and Landscape Plants - Washington State University

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Drought Advisory: Watering Home Gardens and Landscape Plants - Washington State University

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Drought Advisory: Watering Home Gardens and Landscape Plants - Washington State University

  1. 1. EB1090 WATERING HOME GARDENS AND LANDSCAPE PLANTSWatering home landscape and garden plants Soil-Water-Air Relationshipsproperly is one of the most misunderstood prob-lems facing the average homeowner. Most Establishing the correct water-air relationships inhomeowners are aware of the droughty condi- the soil is essential for the best growth of all planttions in eastern Washington, but much of western types. Oxygen in the soil is necessary for plants toWashington also can be extremely dry during the grow. Watering too often or too much is likely tosummer months. In most areas of the state, there exclude the necessary oxygen from the soil poreis not enough rainfall to support plant growth spaces. Without enough oxygen, plant rootsduring the period when water is critically suffocate and die, preventing water uptake. Plantneeded. If landscape plants are water stressed parts aboveground exhibit symptoms of thisduring the summer, they may experience severe stress; wilting, yellowing, and drying foliage, leafproblems during the rest of the year, such as drop and twig dieback may all occur. Constantincreased insect and disease susceptibility and overwatering kills most plants.decreased winter hardiness. Too little water, on the other hand, does not allow theWater Loss From the Soil roots to replace water lost by the plant through transpiration. The roots may dry up and die, and theThere are several ways in which water is lost top growth begins to show abnormal symptoms. Infrom the soil. Rain, melted snow, or water applied both cases, either too much or too little water, theby the homeowner may percolate down through plant suffers from lack of moisture in its tissues.the soil beyond the root zone. This water isuseless to growing plants. Heavy clay soils are much more likely to be overwatered than light soils. Conversely, lightWater also may evaporate from the soil surface, sandy soils are droughty and tend not to be wa-leaving it dry. Water from lower layers in the soil tered enough. Although light soils allow deeperis drawn to the surface by capillary action and and quicker water penetration, they dry out morealso evaporates. This continual evaporation may rapidly because they hold less water. Heavy soils,deplete water from quite deep in the soil. on the other hand, are slower to allow penetration but also dry out much more slowly.Transpiration is the process by which a plant loseswater through its leaves. This is a necessary A good rule-of-thumb to follow in wateringprocess for plant growth. A large tree may lose plants is to fill the entire root zone with water,hundreds of gallons of water a day in the summer. and then allow the soil to dry out partially beforeWater lost from the soil by evaporation and tran- the next irrigation. The amount of drying de-spiration must be replaced by precipitation or pends on the plant species and size. Large treessupplemental irrigation. and shrubs can be allowed to dry several inches down in the soil before rewatering. A small or newly established plant will need watering before very much soil drying takes place. COOPERATIVE EXTENSION
  2. 2. It is essential that gardeners become familiar with efficient pentration of water. Compacted soils inhow long it takes the root zones of the various which a vegetable or flower garden is to be plantedplants in their gardens to become completely should have organic matter incorporated into themoistened, and then, how deeply they can allow top 6-8 inches. This allows easier water penetrationthe soil to dry before the plants begin to show after the garden is established.stress and need rewatering. It is also necessary tounderstand that quick, light sprinkling will not do Wateringthe job of wetting the entire root zone. Vegetables, bedding plants, and perennials areWater Penetration usually small when planted and have compara- tively shallow roots. These plants may have to beSoil type or texture is a major determining factor watered more often to ensure a consistent waterof how much water a soil will hold, or how supply. Check the soil with a trowel or spade toquickly a soil can be irrigated. For example, one the depth of the expected root zone. Moisten theinch of water applied to a sandy soil will penetrate entire root zone before the plants show signs of12 inches. It will move anywhere from 6–10 inches wilting. If the plants are allowed to wilt a fewinto a good loam soil, and in a clay soil it will times, growth will be stunted and crop yieldspercolate down only 4–5 inches. reduced. Be careful not to overwater. Porous-wall hose and drip irrigation systems can provideTime Required adequate water more efficiently to the vegetable garden than sprinkler systems can.Sandy soils allow water to penetrate more quicklythan will heavy, dense soils. Wetting the entire root Plants in containers need special attention. Bothzone of plants growing in heavy soils takes much volume of soil and total water available for plantlonger than wetting plants growing in lighter soils. use are limited. These plants have to be wateredSandy loams will accept from 1/2–3 inches of water more often than plants growing in the ground.per hour. A clay-loam may absorb only 1/10–3/5 Check for soil moisture by sticking your fingerinch of water in the same amount of time. A very dry into the potting soil. When it becomes dry belowclay-loam soil could take as long as 120 hours to the surface, begin watering. Do not depend oncompletely wet to a depth of 12 inches. A sandy the appearance of the surface of the potting soil.loam, however, might take as little as 4 hours. It may look dry, but feeling the soil can reveal that the mix below the surface is still moist. If youOrganic Matter allow the potting soil to dry out completely, you may need to soak the pot in water to rewet theSoils to which organic matter has been added will soil.behave differently. For example, clay soils withadded organic matter will accept water more Frequency and amount of water depend on thequickly. Organically amended sandy soils hold potting mix, location, size, and type of the pot, sizewater longer and, consequently, do not need to be and type of plants, amount of exposure to sun andirrigated as frequently. wind, and temperature. Plants in plastic pots or glazed ceramic containers will have to be wateredCompaction and Thatch less often than plants in clay pots or nonglazed, porous ceramic containers. A plant that uses a lotWater will not soak into compacted soils, or soils of water, such as fuchsia, or a potbound plant, mayoverlaid with a thatch accumulation, particularly if have to be watered daily or several times a day.water is applied too quickly. For compacted or But most container plants will need less frequentthatch-choked areas, the best treatment is to aerate watering. When watering, water the plant thor-the soil by removing plugs. Wetting agents can help oughly, allowing water to drain out of the bottomwater soak through dry organic layers, like thatch, of the pot. Be careful not to keep the roots con-so that it moves into the soil. Mulches placed over stantly wet. Root rot and other disease problemsthe root zone of trees and shrubs help restructure can result if the potting soil is waterlogged and airthe surface layer of compacted soils to allow more is excluded from the roots. 2
  3. 3. Trees, shrubs, and landscape plants should be nursery soil. Plants in such a situation may ulti-watered just inside and outside the dripline, or mately girdle themselves and die.outer edge of the plant. In foundation or borderplantings, it may be more convenient to water the Container soils, in particular, have a bad habit ofentire area. A hose, soaker hose, or various kinds drying out much faster than the surrounding orof sprinklers are commonly used. For deep-rooted backfill soils. Both media should be adequatelytrees, a root needle or fertilizer feeding needle moistened to prevent newly installed plants from(minus the fertilizer) may be used for deep water- being injured or dying of drought. But be carefuling. This is a tedious process, but it works. Pen- not to overwater.etration is important. Mulching newly established shrubs and treesFor recently planted trees and shrubs a dished- or helps prevent moisture loss. Moisture-demandingberm-enclosed area constructed around the base of plants, such as rhododendrons, azaleas, and ferns,a tree or shrub may be filled with water. This have to be irrigated more often during warm,allows for slow percolation into the root zone. sunny weather.However, on heavier soils during the rainy seasonor in the winter, these basin rims are best removed Many native woody plants should not receiveto avoid concentrating too much water. summer watering. Once they are established, they are drought tolerant in the summer, and some mayShrubs and trees near house foundations, under eaves, be damaged by moisture at this time. It is especiallyor in southern, southwestern, or western exposures important to keep water away from the crowns andhave to be watered more frequently. They may get larger roots of madronas and western dogwood inlittle water from precipitation, and reflected heat western Washington. They often succumb to rootfrom walls leads to increased water and heat stress. rot problems with summer watering. Avoid plant- ing moisture-demanding plants underneath them.Capillary action can cause dissolved salts to becarried from moist zones into the dry soil under Other drought tolerant shrubs and trees also doeaves. A salt concentration is then left behind as the not need to be watered. For lists of drought toler-water evaporates. Thorough leaching of such areas ant shrubs and trees, consult a good referencemay occasionally be necessary, particularly in the work. Many plants in the following genera havedrier regions of the state, to remove salt buildup. proven themselves drought resistant; Caragana, Ceanothus, Cotoneaster, Cytisus, Eleagnus,Mounds or berms in which landscape plants have Genista, Juniperus, Koelreuteria, Pinus, Quercus,been installed have much more soil surface ex- and Robinia. There are many more.posed to evaporation than the natural soil profile.Therefore, these areas will have to be checked and Lawns are best watered by overhead sprinklers.watered more frequently. The deeper the wetting, the deeper the roots will grow. Deep-rooted grass plants are much healthierRecently transplanted woody plants need special and better able to withstand drought stress. Waterattention. The soils in which balled and burlapped grass when the soil begins to dry out, but beforeand containerized plants have grown often are the plants actually begin to wilt, and certainlyradically different from the soils into which they before they begin to desiccate. Grass requiresare planted in the home landscape. When this irrigation when it begins to be less resilient andoccurs, interfaces develop between the original springy and does not bounce back up after beingnursery soil and the soil at the new site. Because of walked on. The amount of water to wet the rootthese interfaces, water does not move readily zone is determined by soil type, amount of thatchbetween the different media. Therefore, it is most accumulation, and several other variables.important that water be applied to both the nurs-ery soil and the surrounding soil during the To determine when a sprinkler has put out an inchcritical establishment period. Roots grow only of water, or any specific quantity, simply use severalwhere there is moisture, and unless both media are coffee cans or jars spaced at intervals from themoist the roots may never grow out of the original sprinkler itself to the edge of the watering pattern. 3
  4. 4. Conservation of Water Drip or trickle irrigation allows a steady supply of water to be delivered slowly to the soil around theWater is a scarce commodity and will continue to plant roots. Often a 60% or more savings in waterbecome more scarce in the United States. Washington usage may be realized using such a system. Whenstate is no exception. With a little care and prior using drip irrigation for watering trees and shrubs,planning, water can be conserved when used for consider the use of the newer mini-sprinklerhome plantings. Anything that can be done to reduce emitters that wet the soil in a larger area anddownward percolation, run-off, evaporation from provide more even watering for plants. Simplethe soil surface, or transpiration will conserve water. drip emitters restrict the soil wetting patterns and are primarily suitable for establishing young treesOrganic matter. Deep incorporation of some sort and shrubs, but do not meet the needs of plantof organic matter such as well-rotted compost or root systems as they grow larger.peat moss, will help reduce downward drainage(percolation) if done before planting. This may not Important Facts to Rememberbe feasible for shade and ornamental trees, but canbe done for vegetable gardens, flower beds, and • Most plants in most areas in Washington needfoundation plantings. Organic matter absorbs water in the summer.many times its own weight in water, which is then • Frequent, shallow waterings lead to shallowavailable for plant growth. roots. Shallow roots lead to more rapid stress under drought or hot conditions.Mulching materials placed over the soil reduce • Theoretically, outside watering can be accom-evaporation from the soil surface, may also reduce plished any time of day, but it is more efficientsome of the water run-off, allow better water to water at night or in the very early morning,penetration into the root mass, and limit weed when evaporation is low. During a drought,growth. Mulches may be organic (shredded leaves, watering may be restricted to specific times onbark, sawdust) or inorganic (gravel, etc.). scheduled days. • Too much water is as bad as, or worse than,Spraying. Little can be done to stop plants from too little. Rate of water application should betranspiring. However, newly planted plants no more rapid than the rate at which the soil(woody, bedding, vegetable) will benefit by occa- can absorb it.sional spraying of the foliage during the day, and • Fertilizer spread around plants (including lawns)by shading. does absolutely no good at all unless it is dis- solved in water. Therefore, fertilizers have toTrickle or drip irrigation systems allow slow be watered in, and soils have to be moist to getwater penetration into the root zone with mini- the full effect of the fertilizer application.mum surface wetting. Such installations may be • Conserve water where possible. It is a valuableworthwhile, particularly if large areas are to be resource that is becoming scarce.irrigated. A variety of kits and parts to make upsuch a system is readily available. Plastic tubing,emitters, filters, and pressure reducers are in- Drought advisories and other Washington Statecluded in these systems. They are easily attached University Cooperative Extension Bulletins areto an existing outdoor water supply. available online at http://pubs.wsu.eduBy George J. Pinyuh, former Washington State University Cooperative Extension Horticulture agent, and Ray R. Maleike, Ph.D., WSU CooperativeExtension horticulturist, emeritus, WSU Puyallup. Revised by Marianne Ophardt, M.S., WSU Cooperative Extension Area Agent, Benton County.Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regardingrace, sex, religion, age, color, creed, national or ethnic origin; physical, mental or sensory disability; marital status, sexual orientation, and status asa Vietnam-era or disabled veteran. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names havebeen used to simplify information; no endorsement is intended. Reprinted March 2002. Subject codes 255, 373. A EB1090

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