With only six to eight inches of rain a year, even novice area gardeners know that adequate moisture is essential to successfully growing lawns, gardens and landscape plants.
However, not every gardener knows how to correctly water. The tendency is to water a little every day during the summer, such as 15 minutes once a day. That sometimes isn’t enough.
Only 5 percent of the water that plants absorb through their roots is used for growth. The majority (95 percent) is lost through transpiration, which is the loss of water vapor from the pores in leaf surfaces. High temperatures, wind and sun increase the rate of transpiration, increasing a plant’s need for water.
To be able to absorb from the soil, water in the location of the water-absorbing roots. As plants become established, the roots typically move out radially from the root ball. Water that once was applied near the trunk or base should be applied further in the root zone. For established shade trees, the root zone area extends from the drip line (the outermost reach of the branches) and beyond.
If a lawn is watered just 15 minutes every day, depending on the output of the irrigation system, adequate moisture probably is not reaching the roots. The major portion of the root systems landscape trees are found in the top 12 to 18 inches of soil. Shallow, daily watering during a hot summer may keep a lawn alive, but it probably is not providing enough water.
When plants suffer from drought, they wilt or show other signs of stress. Some respond to this stress with yellowing and dropping of leaves. Others develop a disorder called “leaf scorch,” where the edges of the leaves turn brown and crispy, or brown tissue develops between the leaf veins.
Certain plants may show the same symptoms even if there is enough moisture. The problem may be that the root system is inadequate to keep up with the demands put on the plant by transpiration. This can happen when a plant is transplanted in late spring or early summer and the roots have not had time to grow to support the top of the plant. This also happens if the root system is impaired by being planted too deep or by restricted roots. Some plants tend to show leaf scorch because they aren’t well adapted to a hot, dry climate.
If a plant looks stressed, check the soil moisture. Be sure to deep-water once a week. A mature shade tree with a spread of 30 feet should be receiving at least 350 gallons of water once a week during our hot and breezy weather.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.