Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, August 24, 1995 TAG: 9508240017
SECTION: CURRENT PAGE: NRV-6 EDITION: NEW RIVER VALLEY
SOURCE: JOE HUNNINGS
DATELINE: LENGTH: Medium
Water stress affects most of the physiological processes of plant growth. Under drought conditions, plants become more susceptible to disease and pest infestation.
As with people, weak, stressed trees are more vulnerable to problems. For example, pin oak decline affects trees stressed by drought and improper soil conditions; bark beetles and some wood-boring beetles occur in declining trees; hemlock wooly adelgid shows up on dry-site hemlocks; and spruce mite invades junipers, spruces and hemlocks grown in compacted, dry soil. These pests may push trees in a marginal state of health over the edge to death.
For most trees, the majority of the small, absorbing roots are concentrated in the upper six inches of soil, with roots extending laterally about 11/2 to four times the distance from the trunk of the tree to the drip line. This leaves only a small percentage of deeper anchorage roots.
For the most part, it is these surface roots that absorb water and nutrients needed by the tree. When the soil surface dries out, as happens during a drought, the fine surface roots become less absorbent and eventually die, but water still can be absorbed by roots at greater depths. Also, available water is affected by other factors, such as soil texture and structure. With the return of adequate moisture, a surface root system can re-establish its activity.
However, drought-stressed trees and shrubs may not return to their predrought status. Plants growing naturally on a site adapt to changing moisture levels more easily than recently transplanted materials. Also, established plantings withstand drought stress better than newer plantings. When trees are dug at a nursery, 90 percent of the root system usually is severed. Trees that have not had adequate time to re-establish strong root systems are at risk from drought.
Some species, such as white birch, oak, sugar maple, and magnolia, are very susceptible to drought stress. Stunted bud development and twig growth and leaf curling are symptoms of drought stress. Later signs include an overall slowing of growth and increased susceptibility to disease and insect attack.
During a drought, adequate moisture must be supplied to trees by irrigation. Strict recommendations are difficult, since variables like soil type and species of tree affect the need to irrigate. Drip irrigation, which saturates the top six inches of soil, is most desirable. Recently planted trees should be given priority, followed by drought-sensitive species and, finally, other plantings.
Another variable to consider in recently planted trees is whether your specimen was balled-and-burlapped or container-grown in pine bark. When planted, the soil surrounding the balled-and-burlapped tree retains more moisture around the roots. A container-grown tree in pine bark has a water reservoir of only one to two days, so it must be watered every day or two during a drought.
Mulching also aids in retaining moisture. Without sufficient water, any fertilizer that has been applied may become so concentrated around the roots that it becomes toxic to the tree.
A little extra care this fall can be the key to keeping your favorite trees healthy. If the weather has been dry, be sure to water. Don't be lulled by cool, fall breezes and assume all your plantings will survive without your help. This is especially true of evergreen trees and shrubs which will continue to lose moisture through their leaves or needles.
Joe Hunnings is the Virginia Cooperative Extension agent for agriculture in the Montgomery County Extension Office in Christiansburg. If you have questions, call him at 382-5790.
by CNB