As temperatures rise this summer, many homeowners are cautious about the amount of water they use on their landscapes. It is important to know, however, that if water is cut back too severely, trees will be in danger. A large tree can use hundreds of gallons of water a day and will not fare well if water becomes sparse. Consider this information.
Determine moisture levels. Trees and shrubs have much deeper and more extensive root systems than turfgrass so they should be watered less frequently and for longer periods of time. The optimum time to water is just before water stress occurs. To determine sub-surface soil moisture, use a soil moisture probe, a screwdriver or long metal rod. The probe will easily penetrate moist soil but stops when it hits dry soil. The soil should be moist to a depth of 18-20 inches for trees and shrubs.
Soil moisture can also be determined by the “feel method.” Feel or squeeze a handful of soil. Since the top layer of the soil dries fastest, take the soil sample at a depth of about 4-8 inches. Sandy and loam soils are considered dry when the soil runs through your fingers and no stain remains. Clay soils are hard to break apart when they are dry. When moisture levels in the soil are optimum for the plant, you will be able to squeeze the soil to form a ball and a thin layer of moisture and stain will be left on your hands. No water should run out of the ball.
Estimate water amounts. Sandy soils absorb water the fastest at about 2 inches per hour. Loam soils absorb at three-quarters inch per hour, and clay soils have the slowest absorption rate at one-half inch per hour. Allowing water to penetrate deeper into the soil profile encourages deeper rooting and a more drought tolerant plant. Frequent, light irrigations will lead to plants with a shallow root system that are more prone to water stress. When using sprinkler systems, about one-half to 1 inch of water may be required weekly for shrubs and smaller trees of up to a 4-inch trunk diameter. For drip irrigation, about 5 to 50 gallons of water may be required, depending on plant size. (These amounts are higher in southern Utah and lower in northern Utah.) Large trees with a trunk greater than 4 inches in diameter may require hundreds of gallons of water per week. For newly planted trees and shrubs, water frequently until the root system is established.
Watch for stress. Wilting and leaf scorch are symptoms of water stress. However, over-watering of plants can create similar browning symptoms and even cause the leaves to drop. Roots need oxygen to survive and when the soil is over-saturated with water, there is little oxygen for the plant’s roots. Without a healthy root system, the plant is no longer able to absorb water and will show signs of water stress. Many gardeners assume these symptoms are from a lack of water and they will continue to add to the problem by over-watering, eventually causing the plant to die.
• Consider all factors when irrigating.To provide adequate water to all plants without over or under-watering, group plants with similar irrigation needs in one area of the yard and adjust the irrigation system to variations in the season. In the spring and fall, less water is lost from the plant and soil so less irrigation water is needed. Irrigate in cycles instead of applying all the water at once, which can cause run-off. Turn off automatic systems during and after rain. Water by hand if necessary. This can be an efficient way to water individual plants showing signs of water stress if applied slowly enough to be absorbed by the soil. Mulch and control weeds and grasses around the trunk of the trees and shrubs to reduce competition for water and nutrients. For a comprehensive listing of tree characteristics and selection criteria, see “Selecting and Planting Landscape Trees” at http://extension.usu.edupublica/natrpubs/nr460.pdf.

By: Mike Kuhns - Jul. 18, 2008