Sunday, April 22, 2007

Watering Plants

Watering home landscape and garden plants properly is one of the most misunderstood problems facing the average gardener. If landscape plants are water stressed during the summer, they may experience severe problems during the rest of the year, such as increased insect and disease susceptibility and decreased winter hardiness.

Water loss from the soil

There are several ways in which water is lost from the soil. Rain, melted snow, or water applied by the gardener may percolate through the soil beyond the root zone. This water is useless to growing plants.

Transpiration is the process by which a plant loses water through its leaves. This is a necessary process for plant growth. A large tree may lose hundreds of gallons of water a day in the summer. Water lost from the soil by evaporation and transpiration must be replaced by precipitation or irrigation.

Soil-Water-Air relationships

Establishing the correct water-air relationships in the soil is essential for the best growth of all plant types. Oxygen in the soil is necessary for plants to grow. Watering too often or too much is likely to exclude the necessary oxygen from the soil pore spaces. Without enough oxygen, plant roots suffocate and die. Plant parts above ground exhibit symptoms of this stress: wilting, yellowing, and drying foliage, leaf drop and twig dieback may all occur. Constant overwatering kills most plants.

Too little water does not allow the roots to replace water lost by the plant through transpiration. The roots may dry up and die, and the top growth begins to show abnormal symptoms.

In both cases, either too much or too little water, the plant suffers from lack of moisture in its tissues.

Heavy clay soils are much more likely to be overwatered than light soils. Conversely, light sandy soils are drought susceptible and tend not to be watered enough. Although light soil allow deeper and quicker water penetration, they dry out more rapidly because they hold less water. Heavy soils, on the other hand, are slower to allow penetration but also dry out much more slowly.

It is essential that gardeners become familiar with how long it takes the root zones of the various plants in their gardens to become completely moistened, and then, how deeply they can allow the soil to dry before the plants begin to show stress and need rewatering. It is also necessary to understand that quick, light sprinkling will not do the job of wetting the entire root zone.

Organic matter Soils to which organic matter has been added will behave differently. For example, clay soils with added organic matter will accept water more quickly. Organically amended sandy soils hold water longer, and consequently do not need to be irrigated as frequently.

Compaction and thatch Water cannot soak into compacted soils, or soils overlaid with a thatch accumulation, particularly if water is applied too quickly. For compacted or thatch-choked areas, or possibly under the canopy of trees and shrubs, the best treatment is to aerate the soil by removing plugs. Mulches around trees and shrubs help restructure the surface layer of compacted soils to allow more efficient penetration of water.

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Plants in containers need special attention. Both volume of soil and total water available for plant use are limited. These plants have to be watered more often than plants growing in the ground. Watering should begin when the soil surface feels dry to the touch, but not before. Frequency and amount of water depend on media, location, amount of sun, temperature, type of plant, etc. Containers which have been allowed to completely dry out may need to be soaked in water to rewet the soil.

A plant which uses a lot of water, such as Fuschias, or one that is potbound, may have to be watered daily or several times a day. But for most container grown plants, a thorough watering once or twice a week will be sufficient. Plants in plastic or solid containers will have to be watered less often than plants in porous containers or clay pots. Be very careful not to keep the root system constantly soaking wet. Pathological (disease) problems will occur if air is excluded from the soil.

Trees, shrubs, and landscape plants should be watered just inside and outside the dripline, or outer edge of the plant. In foundation or border plantings, it may be more convenient to water the entire area. A hose, soaker hose, or various kinds of sprinklers are commonly used.

A dished- or berm-enclosed area constructed around the base of a tree or shrub may be filled with water. This allows for slow percolation into the root zone. However, on heavier soils during the rainy season or in the winter, these basin rims are best removed to avoid concentrating too much water.

Shrubs and trees near house foundations, under eaves, have to be watered more frequently. They may get little water from precipitation, and reflected heat from walls leads to increased water and heat stress.

Capillary action can cause dissolved salts to be carried from moist zones into the dry soil under eaves. A salt concentration is then left behind as the water evaporates. Thorough leaching of such areas may occasionally be necessary, particularly in the drier regions of the state, to remove salt buildup.

Recently transplanted woody plants need special attention. The soils in which balled and burlapped and containerized plants have grown are often radically different than the soils into which they are planted in the home landscape. When this occurs, interfaces are created between the original nursery soil and the soil at the new site. Because of these interfaces, water does not move readily between the different media. Therefore, it is most important that water be applied to both the nursery soil and the surrounding soil during the critical establishment period. Roots grow only where there is moisture, and unless both media are moist the roots may never grow out of the original nursery soil. Plants in such a situation may ultimately girdle themselves and die.

Many native woody plants should not receive summer watering. Once they are established, they are drought tolerant in the summer, and some may be damaged by moisture at this time. It is especially important to keep water away from the crowns and larger roots of madronas and western dogwood in western Washington. They often succumb to root rot problems with summer watering. Avoid planting moisture demanding plants underneath them.

Other drought tolerant shrubs and trees also do not need to be watered. For lists of drought tolerant shrubs and trees, a good reference work should be consulted. Many plants in the following genera have proven themselves drought resistant: Caragana, Ceanothus, Cotoneaster, Cytisus, Eleagnus, Genista, Juniperus, Koelreuteria, Pinus, Quercus, and Robinia. There are also many more.

Lawns are best watered by overhead sprinklers. The deeper the wetting, the deeper the roots will grow. Deep-rooted grass plants are much healthier and better able to withstand drought stress.

Grass should be watered when the soil begins to dry out, but before the plants actually begin to wilt, and certainly before they begin to desiccate. Grass should be irrigated when it begins to be less resilient and springy and does not bounce back up after being walked on. The amount of water to wet the root zone is determined by soil type, amount of thatch accumulation, and several other variables. To determine when a sprinkler has put out an inch of water, or any specific quantity, simply use several coffee cans or jars spaced at intervals from the sprinkler itself to the edge of the watering pattern.

Important factors to remember
  • Frequent, shallow waterings lead to shallow roots. Shallow roots lead to more rapid stress under drought or hot conditions.
  • Too much water is as bad as, or worse than, too little. Rate of water application should be no more rapid than the rate at which the soil can absorb it.
  • Fertilizer spread around plants (including lawns) does absolutely no good at all unless it is dissolved in water. Therefore, fertilizers have to be watered in, and soils have to be moist to get the full effect of the fertilizer application.

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