“Even well-established trees and shrubs in the landscape will come under severe stress during July and August.”
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It will take a lot more than a sprinkle to replenish the dangerously low subsoil moisture on which trees and shrubs depend, a University of Missouri horticulturist said.
“Sparse foliage, dieback or mortality of many large trees this spring is evidence that the extended drought is taking its toll before summer even begins,” said Chris Starbuck, state Extension specialist for woody ornamentals. “Even well-established trees and shrubs in the landscape will come under severe stress during July and August.”
His solution: “Judicious irrigation.”
To replenish totally the soil moisture in the root zone of a 30-foot oak tree, he calculated, would require about 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.”
Fortunately, however, wetting even a small portion of a tree’s root zone will greatly reduce drought stress, Starbuck said.
He recommended using a soaker hose attached at both ends to a Y-adapter on the end of a garden hose, making a large loop. Lay the loop under the tree, within the drip line, and run it for about an hour. It will wet a band about 2 feet wide with about 200 gallons of water. The effect is roughly equal to 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,” he said.
An even more economical, if less reliable method is to divert water from downspouts so that the diverted rainwater soaks into tree root zones rather than running into storm sewers, Starbuck said. Deep irrigation of shrub beds will also help nearby trees
Lawn irrigation typically has little impact on large trees, he said, because most homeowners irrigate frequently for a short period of time. It percolates deep enough to keep the turf green but provides little water to tree roots.
“Unless the turf is shaded, tree roots can’t compete with the dense mat of turf roots near the soil surface,” Starbuck said. 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.