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Drought and the Landscape Garden

Reviewed 1996 by J. R. Nuss, Professor of Ornamental Horticulture

Drought Tolerant Annuals

It will be difficult to find an annual flower that is able to survive in totally dry soil or one having no available moisture. However, through proper soil modification and management, chances of survival increase. The careful use of any available water to assist establishment and the application of a mulch material will also increase the chances of success.

An additional point also involves the selection of a plant that is known to be drought tolerant. The quality of the transplant or seedling at the time it is set out will also determine its survival. When purchasing annual plants choose those that have been hardened off by the grower. These are plants that have been given less and less water prior to sale. If you grow your own plants, gradually cut back on the water before setting them into the garden. Buy only dark green and sturdy looking plants. Generally, tall spindly plants will not tolerate dry conditions all that well once they are set out, even if there is adequate organic matter and water in the soil.

The sponge-like nature of organic matter will improve the water holding capacity of both sandy and heavy clay soils. It is one of the more valuable components in a soil. The incorporation of 4 to 5 bushels of organic matter into the top 8 to 10 inches of soil per each 100 square feet of area will be helpful. Compost, peat, rotted sawdust or similar aged organic matter will help to create a suitable root environment. The absorbing capacity of the organic matter retains water while the improved pore space supplies air essential for root growth.

If soil moisture is low at planting time and traditional supplies of water are not available, it will be necessary to use alternate sources of water. Collected rain water, household grey water, or rinse water are possible sources of water to establish seedlings and transplants as long as some natural water is available. Water containing high concentrations of soap or detergent may injure young plants. If possible, attempt to plant annuals during periods of higher soil moisture after a rain or shower.

You may be able to increase the moisture supply of the soil prior to planting with an early season mulch material. Several inches of compost, peat, or organic matter on the soil surface will prevent excessive water run-off from rains while slowing surface evaporation of water. Plastic sheets on the soil surface can also be used to channel rain water and prevent evaporation. Plastic has to be positioned in such a way that water is not diverted to the sides and away from the root zone.

As soon as temperatures are suitable for planting, you can consider using any of the drought tolerant annuals in the following list. For best results, plant directly through the mulch layer, making certain that the root system is set into the moist soil below the mulch. If there is any clean water available, use it to settle the soil around the root system. Where early plastic mulch has been used it may be desirable to spread a thin layer of organic material over the plastic after planting. This will improve appearances and reduce the chances of high root temperatures that may injure the seedlings.

A 2 to 3 inch layer of mulch is essential for success if you plant directly into bare soil and water is not available early in the growing season. Once the soil has been mulched, it will be possible to apply collected water to the mulch layer between the rows of plants. The water will move downward into the root zone.

You may wish to consider growing some of the following annual flowers if your conditions are dry and supplemental water is not available in the coming months.

Amaranthus species-Amaranth Mirabillis jalapaQFour oUclock
Anagallis linifolia-Pimpernal Nicotiana alataQNicotina
Aster amellus-Italian Aster Pelargonium hortorum-Geranium
Calendula officinalis-Pot Marigold Petunia hybridsQPetunia
Clome spinosa-Spider flower Phlox drummondiQAnnual phlox
Cosmos bipinnatus-Cosmos Portulaca grandifloraQRose moss
Dianthus chinensis-Annual pink Rudbeckia speciesQGloriosa daisy
Euphorbia marginata-Snow-on-the-mtn. Senecio cinerariaQDusty miller
Gypsophila elegans-Annual baby breathTagetes hybrida-Verbena
Lantana camara-Lantana Zinnia hybrids-Zinnia

Keep in mind that the above plants do not require dry conditions for survival, but they will do better than most if moisture is not available all the time. To promote good branching, pinch out the terminal buds or shoots on most of the young plants in order to develop bushier specimens, reduce ultimate height, and cause more abundant flowering. Make certain that you remove all the faded flowers which will prevent seed development. Once seeds start to form, most annual flowers will stop blooming. Gardening during dry periods will be a challenge but is possible with adequate planning and understanding of the problems.

DROUGHT AFFECT ON NEW PLANTINGS

The drought tolerance of a given plant is its ability to survive under periods of moisture stress. The conditions needed to establish a plant will be considerably different than for survival. The speed with which a plant establishes at a site will in turn affect its ultimate survival.

The establishment of a plant will be dependent upon its ability to obtain enough water from the soil to sustain minimum moisture levels in the root system and plant. In addition, water will be needed for the development of the root system and its expansion into the surrounding soil. As available moisture is used to increase plant volume, additional replacement water will be needed from the soil.

Survival of any plant begins with the quality and water holding capacity of the soil. Neither light sandy or heavy clay soils hold sufficient available moisture for optimum establishment and growth. In such soils, organic matter can be a valuable component. The sponge-like nature of organic matter absorbs and holds moisture while creating pore space for the movement of both air and water through the soil.

Prior to planting anything into a particularly dry soil, it will be helpful to incorporate 4 to 5 bushels of organic matter into the top 8 to 10 inches of soil per each 100 square feet of bed area. Compost, peat, rotted sawdust, or aged tree bark are all suitable materials for soil improvement. Make certain the materials are worked into the soil evenly: layers of organic matter will inhibit internal water movement to the plant roots.

Soil fertility is also important prior to planting as long as the physical quality of the soil is satisfactory. Under drought conditions the plant should not be stimulated into vigorous growth from high fertility. It is possible that later in the season water will not be readily available to support the extra growth. A balance among the soil nutrients should be achieved for best results. A soil test is the only certain way to determine soil fertility. Your County Agricultural Extension office can supply information on this test for garden soil.

Once the quality and fertility of the soil has been adjusted the plant will be better able to utilize any available moisture: either added or from natural sources. After planting, a layer of mulch over the soil will retain soil moisture and reduce run-off from rainfall. A mulch will generally maintain a more uniform level of moisture in the soil by preventing rapid evaporation from the soil surface. Mulches also help to control weed growth; another source of water loss from the soil.

If you combine optimum cultural conditions with plants known to be drought tolerant, your chances of success will improve. However, even drought tolerant plants have to become established at the site. The following listing of drought tolerant plants is not complete, but does provide a variety of sizes and types of trees and shrubs for less than optimum sites.

Trees:Acer negundo Q Boxelder 60UAilanthus altissima Q Tree-of-heaven 60U
Albizia julibrissin Q Silktree 35U
Betula populifolia Q Gray birch 30U
Celtis occidentalis Q Hackberry 60U
Koelreuteria paniculata Q Golden-raintree 30U
Maclura pomifera Q Osage-orange 50U
Pinus thunbergi Q Japanese black pine 35U
Quercus macrocarpa Q Bur oak 40U
Robinia pseudoacacia Q Black Locust 60U
Sassafras albidum Q Sassafras 40U
Ulmus pumila Q Siberian elm 75U

Shrubs:
Acer ginnala Q Amur maple 20U
Berberis mentorensis Q Mentor barberry 7U
Berberis thinbergbii Q Japanese berberry 7U
Caragana species Q Pea tree 15U
Chaenomeles species Q Flowering quince 3-6U
Comptonia peregrina Q Sweet fern 4U
Cornus racemosa Q Gray dogwood 12U
Hamamelis virginiana Q Witch hazel 15U
Juniperus chinensis var. Q Juniper 10-15U
Ligustrum species Q Privet 15-20U
Myrica pensylvanica Q Bayberry 10-12U
Physocarpos opulifolius Q Ninebark 10U
Potentilla fruticosa Q Bush cinquefoil 4U
Rhus typhina Q Staghorn sumac 12U
Viburnum lentago Q Nannyberry 25U
Vitex agnus-castus Q Chaste-tree 8U

Very few plants require dry soil conditions for survival. Optimum soil and moisture conditions as well as mulches will enhance any plantUs quality and growth. The above plants are a few which can withstand drier soil conditions than most.

DROUGHT AND ESTABLISHED PLANTINGS

If you have an established garden or landscape containing plants that are not necessarily drought tolerant you may want to consider one or more of the following practices. These suggestions may help to reduce moisture stress in the plants and allow them to take advantage of any available moisture.

Mulches. Over the years various mulch materials have proven very effective in controlling moisture loss from the soil. They are widely used by both commercial and home gardeners to increase plant productivity and quality. Additional benefits from mulches include weed control (weeds may use more water than your garden plants), soil temperature modification, erosion control, and in some cases improved appearance of the mulched areas.

Mulch materials can be grouped into two major categories: organic and inorganic. A wide variety of organic materials are available and range from shredded newspaper to processed bark chips. Actually, any material that satisfies the requirements of a mulch, i.e. moisture and weed control, will be suitable. The selection of a specific material will be governed to a large extent on where it will be used, appearance, and possibly cost. Most organic materials have to be applied in a layer 3 to 4 inches deep to be completely effective.

Inorganic materials should also function in the same way as organic products in their control of moisture loss and weed growth. Unfortunately, some of the inorganic materials like black plastic are not too attractive alone on the soil surface and are generally covered over with a textured material. If appearance is important, topdress the plastic with gravel, bark chips, or shredded bark. As a mulch, gravel alone is not all that satisfactory since a layer 6 to 8 inches deep is often needed to slow moisture loss and weed control.

Additional information on mulching and home grounds weed control is available in Penn State Circular 563, RMethods of Weed Control for the Home Owner,S at your local Extension Office.

Anti-desiccants. Moisture loss from expanded leaves can be reduced through applications of an anti-desiccant. A majority of these materials form a water tight film over the leaf surface. They are most effective on evergreens and mature foliage. Expanding new leaves will need additional treatment as the leaf becomes larger: the film will not stretch with the leaf.

The anti-desiccants are applied to the plants as a spray any time the air temperature is above freezing. Total leaf coverage is essential for optimum results. Several applications will also be needed as the plant grows and weather wears the film off the leaf. For specific application schedules, consult the package directions for each material. Several advantages of such materials include their ease of application on smaller plants, their invisibility on the plant, and the fact that they do not alter the landscape appearance.

Wind Screens. A considerable amount of water can be lost from plant tissue by wind action. There is a constant layer of water vapor near the leaf surface caused by natural plant transpiration. As long as the layer persists near the leaf, it tends to slow the movement of more water from the leaf. However, as the vapor layer is removed by wind action it is replaced with more water vapor from within the leaf.

Any steps that can be taken to slow or reduce the amount of air movement over the plant and leaf surface will retard moisture loss. Installation of wind screens or barriers near a shrub bed can be effective. Snow fence alone or covered with burlap or plastic sheets can be used to block the air. Such barriers should be located close enough to the shrubs to be effective but not so close as to trap heat against the plants.

Most windbreaks or screens will modify air movement for a distance of about two times their height. A six foot screen will reduce the air flow for about 12 to 15 feet when placed in front of plants. Make certain, however, that such barriers do not channel the air through adjacent plants, which could increase their drought injury.

For long term wind protection, if space permits, it may be well to consider a shrub border of drought tolerant plants. Such a border will add to the appearance of the property and be functional at the same time. Several possible plants might be: Mentor barberry (7U), Pea tree (15U), Flowering quince (6U), Gray dogwood (12U), Privet (15-20U), Bayberry (10-12U), or Chaste-tree (8U).

Wetting Agents. The movements of rain and irrigation water into and through the soil can be effected by the surface tension of the water. The natural surface tension of water tends to slow its movement in soil. If movement is slowed, there may be instances where extra water may run off the surface rather than penetrating into the root zone. Water that does penetrate may not go deep enough to be of much help.

Wetting agents reduce the surface tension of water and allow it to spread further over a given surface. It is surface tension that causes water to bead on a waxed surface. Water containing a wetting agent should not bead on wax but will form a thin, uniform water film on the surface. If your soil tends to be hard to wet uniformly because of high clay or peat content, the use of a wetting agent may help you take better advantage of natural or irrigation water applied to the soil.

Wetting agents are usually applied to the soil surface as a water solution. Specific products may have crop or use limitations; however, these will be shown on the package label. In any case, a wetting agent may be an additional tool to help extend the benefit of available water.

Supplemental Water. Much of the waste water generated in the home can be used on plants if certain precautions are taken. Such waste water is called Rgrey waterS and comes from clothes washers and bathing. The use of toilet wastes for irrigation or fertilization is not permitted by law. Disposal of human wastes by any method other than discharge into an approved system is unlawful and very hazardous to public health.

There may be some local restrictions on the use of grey water in certain areas. If uncertain, check with your county health officials or local water district for their particular ruling. Where use is permitted, the following guidelines may be helpful.

The detergents and soaps in wash water are a major concern. A direct application in full strength detergent to a tree or shrub can be harmful. Any recycling system should use as diluted a solution as possible, at least mixing the wash and rinse waters together. It may be easier in some cases to discard the stronger wash water and use only the rinse water for irrigation.

A majority of our present soaps and detergents are biodegradable. The laundry additives, however, can cause problems. Products than contain chlorine bleach or boron can be harmful to plants and should be avoided in the wash water you want to use on your plants.

The sodium ions present in softened water can also cause plant problems and may damage soil structure. Extended applications of sodium to the soil causes the soil particles to break down, resulting in poor soil structure and possible root damage. Moderate amounts of sodium in wash water will be leached form the root zone by seasonal rainfall. If water containing sodium will be used for an extended period of time, you may wish to consider disconnecting the softener during periods of high use.

The liquid fabric softeners that are added to the laundry rinse cycle can be harmful to plants. Rinse water that contains these softeners should not be used on plants. Softeners that work in the dry cycle may be an answer to the problem. Most soaps and detergents are somewhat alkaline and their presence in rinse water may be detrimental to acid-loving plants such as azalea, rhododendron, and Japanese holly. However, one season of moderate use should not kill the plant, but may cause some chlorosis (yellowing of the foliage). An application of iron and/or ammonium sulfate or ferrous sulfate in the spring should help to restore foliage color. Survival is more important than color at this point.

Once grey water is available, some consideration must be given to its use. Health officials warn that the use of grey water on vegetables is not advisable for health reasons. This is particularly true for root crops and leafy vegetables eaten raw. Soil application may be practical between rows of fruiting vegetables such as tomatoes and beans, as long as there is no contact between plants and grey water. The filtering action of a mulch layer between the rows will also help.

There are few if any restrictions on using grey water for ornamentals. Do not apply the water directly to the foliage or base of the plant. Most of the water absorbing roots are in the soil under the ends of the branches; apply any available here for best results.

Storage of grey water also has its particular problems. County or local health codes may restrict storage, so check to be sure. Any suitably sized container will work: garbage cans, barrels, or metal drums.

If volume storage is possible, you may wish to consider a distribution system that will apply the water as uniformly and slowly as possible. A low volume discharge hose from the container(s) will trickle the water to the soil and result in deeper penetration with little or no run-off. By law, grey water must not run off your property and cannot be sprayed into the air. It will be necessary to filter the water through a piece of cloth or similar material to prevent clogging of a trickle system. Filtering can be accomplished as the water is placed into storage. A cover should also be placed on the storage container for safety reasons.

The following additional points may help you use grey water more effectively. When watering, apply enough water to wet the soil deeply. Frequent light watering may produce shallow roots that dry out easily. Do not apply grey water to the same spot all the time. Distribute it over a wide area to help dilute the affect of soap or detergent in the water and prevent a buildup in the soil. Do not use grey water on potted plants. Their restricted root systems make them more sensitive to damage.

The careful use and application of the above gardening techniques will help both you and your plants survive periods of moisture stress. Mulches are a proven garden aid in all instances. Use of anti-desiccants, wetting agents, and wind screens may help conserve what water you have for the plants.

Where trade names are used, no discrimination is intended and no endorsement by Cooperative Extension is implied.

J. R. Nuss, Prof. of Ornamental Hort
Department of Horticulture
Penn State College of Agricultural Sciences
103 Tyson Bldg., University Park, PA 16802
(814) 865-2571