News Release Distributed 07/12/13

By Allen Owings
LSU AgCenter horticulturist

HAMMOND, La. – As we reach the middle of summer in Louisiana, we need to keep in mind the importance of irrigating our landscape plants. When we irrigate, it is important to apply water needed and efficiently. In many cases, plants are commonly over-watered rather than under-watered.

Each year, the Irrigation Association names July “smart irrigation month.”

Of the tremendous amounts of water applied to lawns, landscape beds and trees, much is never absorbed and used by the plants. Some water is lost to runoff by being applied too rapidly, and some evaporates from exposed, unmulched soil. But the greatest waste of water is applying too much too often. By simply using effective and efficient watering methods, you can cut irrigation requirements by 10 to 30 percent and increase landscape beauty and quality dramatically while at the same time saving water.

Correct watering is vital for developing and maintaining a landscape planting. Lack of water can cause a plant to wilt, dry up and die. Excessive water can cause root rot; the plant wilts because it is oxygen-starved and, consequently, is unable to absorb moisture. As a rule, plants can withstand moderate drought more easily than too much moisture. For this reason, it’s important to water thoroughly yet allow the soil to become fairly dry between waterings.

Wilting is a condition brought about when plants’ roots are unable to supply sufficient moisture to the stems and leaves. Wilting for short periods will not harm plants, but over a prolonged period, it will cause permanent damage. Sometimes a plant will wilt on a hot day because moisture is evaporating from the leaves faster than the roots can supply it. If the soil has ample moisture, the plant will absorb water in the evening to firm up the stems and leaves. When the leaves remain wilted the next morning, however, it’s time to water.

Water established trees, shrubs and ground covers infrequently, yet thoroughly. In the absence of rain, most trees and shrubs benefit from a once-a-month thorough watering during the growing season. Remember, normal lawn watering is not necessarily a substitute for thorough tree and shrub watering.

The feeding root system of a tree or shrub is located within the top 12 to 18 inches of the soil and at the drip line of the plant. The drip line is the area directly below the outermost reaches of the branches. Apply water and fertilizer just inside and a little beyond the drip line, not at the trunk. An effective way to water trees and large shrubs is to simply place a slowly running hose on the ground at the drip line. Move the hose around the drip line as each area becomes saturated to a depth of 8 to 10 inches. For large trees, this watering technique may take several hours.

It’s difficult to make broad recommendations about when to irrigate because of the tremendous variations in climatic conditions. But during an extended summer period without rain, newly planted trees and shrubs should be deeply watered once a week.

Allowing the soil surface to dry out somewhat between waterings encourages major root development at greater depths where soil moisture is highest. Plants watered frequently but lightly are more apt to develop roots close to the surface, making them more vulnerable to wilting. This generally happens with automatic overhead sprinkler systems designed only to moisten the surface and run for a short period each night.

Remember that environmental conditions are the primary factor affecting plant watering needs. Consider these other factors:

– During cool seasons, less watering is necessary because evaporation is slow from the leaves and soil.

– Water use under clear blue skies can be twice as high as use under cloudy conditions.

– The best time to water is in the morning or evening when air temperatures are lower than they are at midday. In the evening, wet foliage can encourage fungus or mildew, making plants unsightly and jeopardizing their health. Be prepared to control diseases if you irrigate at night.

– All trees and shrubs need more frequent watering from planting time until becoming well-rooted, which may take two growing seasons. Once established, water-efficient plants can then be weaned to tolerate less-frequent watering. Proper weaning develops deep roots and makes the plants more drought enduring.

You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.