Drought stress can have long-term effect on trees, shrubs
By Sarah Browning
UNL Extension Horticulture Educator
Drought stress is often slower to appear in woody plants but can have long-term consequences. Drought-stressed trees are more susceptible to secondary attack by insect pests and disease problems that can cause tree death.
One common symptom of drought stress is leaf scorching, a uniform yellowing or browning of the edges of leaves on broadleaf plants or the tips of evergreen needles. However, even trees that don't exhibit leaf scorch can be experiencing drought stress, and once a tree is stressed it takes three to five years of normal moisture conditions before the tree recovers its full vigor. Healthy trees, receiving adequate water, are much more resistant to pest problems.
Water should always be placed where the roots are growing. Research shows more than 70 percent of tree roots are in the top 24 inches of soil, where the water, oxygen and nutrients they need for healthy growth are available. Water placed below 24 inches cannot be absorbed by the roots.
Avoid watering too often. Applying a light application of water daily or every other day results in a shallow layer of continually saturated soil. It also drives oxygen out of the underlying soil, resulting in severe root decline or death.
Water requirements for trees vary by species, size of tree and soil type. Of these, soil type is the greatest factor in watering frequency and amount. Sandy soils absorb water quickly and release it easily to plants, but the amount of water sandy soil can hold at any one time is low. Clay soils can hold a lot of moisture, but absorb water slowly.
When too much water is applied to sandy soil it drains quickly through the soil and isn't available to tree roots. Frequent watering in low amounts is the best way to water trees in sandy soil. Ideally, soil should be moist 18 inches into the ground for as long as the tree is growing. Watering every five to seven days may be necessary for trees in sandy soils to maintain adequate moisture in high temperatures and high winds.
Finally, make sure trees and shrubs have a 3- to 4-inch layer of wood chip mulch to hold moisture in the soil.