Most plants can withstand short periods of drought without significant damage. The same is not true for their ability to withstand excessive water for extended periods of time. Standing water can cause drowning due to oxygen depletion and poor drainage can cause root decay and root rot diseases.

Watering is not as simple as waiting until a plant wilts or using some predetermined schedule. Watering is influenced by a number of factors. Soils differ in how quickly they dries out after a rain or watering. A clay soil will not need watering as frequently as a sandy soil since clay soils drains slowly and sandy soils drain quickly. The addition of organic matter will increase drainage in clay soil and moisture retention in sandy soil. Subsoil also affects water drainage. Mulches help keep the soil cool and reduce water loss through evaporation thus extending the time between waterings.

Different plants have different water needs --- some are quite drought tolerant. Plants with large leaves (hydrangea, rhododendron) or with shallow root systems (azalea, dogwood) are usually the first to suffer during drought periods. Shrubs under large trees are especially susceptible because of the large volumes of water taken up by tree roots. Plants are more prone to suffer from drought during their first two growing seasons after transplanting. Late spring or summer transplants are the most susceptible, because their roots have a shorter time to become established prior to summer stress. When there is an extended period without rain during the summer, new plants should be watered at least once a week.

Fertilization also affects the tolerance of plants to dry weather. Recently or heavily fertilized plants can easily suffer root damage if water becomes depleted. Severe pruning results in vigorous, tender growth that is more dependant on uniform moisture. Weed-free areas will have more water available since weeds use water that might have been available to landscape plants. Low humidity, wind, and bright, sunny days will increase the need to water.

It is important to water thoroughly and to allow the soil to dry between waterings. Frequent, light watering wastes water, does little to satisfy the water requirements of most plants, and leads to development of a shallow root system thus increasing susceptibility to drought. By allowing the soil surface to dry out somewhat between waterings, major root development will be at greater depths where soil moisture is higher. For well-established woody plants, watering should be done every 10 days during prolonged dry spells. During cool seasons, less watering is necessary because evaporation from the leaves and soil is slower.

The best way to determine if adequate water has been applied is to dig in the soil and check how far the water has moved. An alternative is to use a tensiometer. This water-measuring probe should be pushed into the soil to a dept of at least 12 inches. If the meter reads 50 to 70 centibars the soil is moderately dry and should be irrigated. Check the soil again 12 to 24 hours after watering. If the reading is 5 to 15 centibars the soil has been adequately watered.

Wetting the soil to a 6 inches deep requires 1 to 2 inches of surface water (will vary with soil type, compaction, and slope). The best time to water is between 9 pm and 9 am. During this time there is generally less wind and lower temperatures therefore less water is lost through evaporation. Irrigating during the evening after dew develops and before it dries in the morning does not increase disease development. Watering in late afternoon, however, lengthens the time the foliage is wet and can lead to disease development.

Watering methods
Hand watering - is often an ineffective method of watering plants because water is applied at a rate that exceeds the ability of the soil to absorb the water or not enough water is applied. To reduce runoff use a water breaker nozzle that divides the spray into rain size droplets (Figure 15-22). Do not use a trigger-type nozzle. Apply a minimum of 5 gallons of water per 10 square feet which is approximately the amount delivered by a 5/8 inch hose operating for 1 minute at medium pressure (water pressure from a municipality can range from 30 to over 100 psi). Water small shrubs (under 4 feet) for at least 1 minute. Larger shrubs will need more water; add 15 seconds for each foot of height exceeding 4 feet. For large trees, apply 6 to 7 gallons for each 10 square feet of canopy area. For best results check the output of your faucet by determining the time required to fill a 1 gallon container and then estimate the output per minute. If runoff occurs before you have applied the correct amount, move to another spot and come back after the water has soaked in.

Sprinklers - There are many types of sprinklers on the market. Many gardeners use an inexpensive hose-end sprinkler that can be moved form place to place. Hose-end sprinklers come in many sizes and shapes. Because of their limited range, they are not convenient for watering large areas.

A variety of sprinkler heads can be used. Rotary sprinklers generally operate at 30 pounds of pressure per square inch (psi) and cover a 30 to 50 foot radius delivering 0.2 to 0.5 inches of water per hour. Spray heads are designed to operate at low pressure (15 to 35 psi) and cover a smaller area (10 to 20 foot radius) than rotary sprinklers. They apply from 1 to 2 inches of water per hour.

An underground permanent sprinkler system is more expensive to install but is normally easier to operate. Pop-up rotary sprinkler heads are installed flush with the ground and pop up above the ground when water pressure is applied. Frequently gardeners with an underground sprinkler system will use a timer to water plants on a set schedule that does not take into account rainfall or other weather conditions. It will also be harder to water plants that might have different needs since the water is apply to a large area or the entire yard.

Since sprinklers apply water into the air a considerable amount of water can be lost to evaporation. This is especially true on windy days. Sprinklers do not apply water in a uniform pattern; most apply more water in the center of the pattern. When you use several sprinklers there should be overlap in their distribution area. The sprinkler should be turned off if runoff occurs. Check the dept of water penetration and turn the sprinkler on again in half an hour if adequate water has not been applied.

Other methods - Soaker and ooze hoses are ideal for watering beds of plants. They emit water slowly and economically. They do not wet the foliage or disturb the soil structure since water does not hit the ground with any force. Many gardeners leave the hoses in one location for an entire season or longer. Several drip or trickle irrigation systems are available for home gardeners. They emit small amounts of water at very precise location. One potential problem is they can become clogged easily and may require a filtration unit. Rabbits sometimes chew on the tubing.

Water Stress Most plants wilt when they experience inadequate moisture. Leaves on river birch and poplar trees turn yellow and drop. Sycamore leaves turn brown and fall, while other trees develop premature fall color or shed small branches. A condition known as leaf scorch can develop on plants, such as dogwood, maple, beech, ash, elm, and rhododendron, when their root systems can not replenish moisture as fast as it is lost from their foliage. This disorder can be caused by a limited or damaged root system, drought, and hot, windy conditions. The leaf tissue on the edges and between the main veins dies and turns brown. Eventually the entire leaf except for a narrow band along the veins die. Plants can be affected uniformly or only on one side. Plants growing near reflective surfaces such as roads and patios, as well as, recent transplants are more likely to be affected. A similar condition can develop during the winter from desiccation. It is particularly important to provide relatively high soil moisture for evergreen plants prior to a severe freeze.

Prepared by: Erv Evans, Consumer Horticulturist, NC State University

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