In general, the larger a tree, the more drought-tolerant it will be. This is because a well-established tree has a massive root system, often extending four or five times the diameter of the drip line. Also, if the soil is not too dense, tree roots can grow many feet deep, tapping into subsoil moisture. However, once the subsoil moisture has been depleted, trees lose this advantage and must compete with other plants for moisture near the surface. Sparse foliage, dieback or mortality of many large trees this spring is evidence that the drought is already taking its toll, before summer has even begun. Winter precipitation this year only wet the soil to a depth of 35 to 40 inches. While it is normally not necessary to irrigate established trees, it may be advisable to do some judicious watering to ensure survival of particularly valuable trees.
First, it must be understood that it will take a tremendous volume of water to totally replenish the soil moisture throughout the entire root zone of a large tree. If, for example, we assume that 8 inches will be required, then a single 30-foot-tall oak tree with a root system 100 feet in diameter would require 39,000 gallons of water. Using a sprinkler irrigation system attached to a garden hose delivering 5 gallons per minute, it would take five days of continuous irrigation to water this one tree. Obviously, it will not be possible to totally replenish the moisture reserve in the entire root zone of a large tree with irrigation. Fortunately, however, wetting even a small portion of a tree's root zone will greatly reduce drought stress.
Lawn irrigation typically has little impact on large trees. Most homeowners irrigate frequently for a short period of time. While this maintains enough moisture in the upper 3 inches of soil to keep the turf green, it provides little water to tree roots. Unless the turf is shaded, tree roots can not compete with the dense mat of turf roots near the soil surface. Maintaining a large mulched area under a tree or growing a less competitive ground cover, such as ivy, allows trees to utilize irrigation much more effectively.
An effective method to reduce drought stress for a large tree is to use a soaker hose of some type. For example, a 50-foot length of the type of hose that seeps from small holes can be attached at both ends to a Y-adapter on the end of a garden hose. This provides a double soaker with reasonably uniform delivery over its 25-foot length. Laying this within the drip line, under a tree and running it for about one hour will wet a band about 2 feet wide with approximately 200 gallons of water. This should be equivalent to about a 6-inch rain over 5 percent of the root zone of a large tree. Repeating this procedure three or four times in different locations during drought conditions should significantly reduce drought stress without using an inordinate amount of water. Another technique that may help is to divert water from downspouts so that, during precipitation events, some of the diverted water soaks into tree root zones rather than running into storm sewers. Deep irrigation of shrub beds will also help nearby trees.
Updated 6/19/06
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