With nearly unprecedented drought conditions for May and June over most of Missouri, even well established trees and shrubs are in danger of permanent damage. Many trees are still recovering from similar conditions in July and August of 2011. In Columbia, I am beginning to see 20-year-old redbud trees browning in their tops. Given that trees are valuable assets to a landscape, doubling one’s water bill to keep them alive is a good investment.
An established tree may have a root system that occupies thousands of square feet to a depth of 18 inches. When the soil in that volume nears permanent wilting point, the tree may exhibit drought survival strategies like leaf scorch, leaf drop or dieback. If drought persists, extreme dieback or death may ensue. Fortunately, wetting the soil in a tiny fraction of the total root volume of a tree can greatly reduce drought stress. The challenge is to apply enough water to do some good in a soil volume where tree roots can access it.
One approach to tree rescue irrigation is to lay “leaky hose” type soaker hoses within the drip line. Soaker hoses are notoriously uneven in their distribution. To improve uniformity, make a “gender bender” (figure 1), consisting of an 8” piece of hose with two female ends. This allows both ends of the soaker hose to be attached to a garden hose using a Y-adaptor. Lay the loop within the dripline of the tree to be rescued and irrigate long enough to apply at least 100 gallons of water. Although the wetting pattern varies greatly with soil type, two inches of irrigation will generally wet the soil to a depth of about one foot. This is generally deep enough to access many tree roots. Assuming that a 50’foot soaker hose will wet a band of soil about 18” wide, it will cover an area of about 75 square feet. Since it takes 0.62 gallons of water per square foot of surface, it would take about 50 gallons of water to apply one inch. Most domestic water supplies deliver about 5 gallons per minute from the hose bib, so it would take about 20 minutes to apply 2 inches. Check regularly for runoff. Under “normal” conditions, a typical landscape soil may only be able to absorb ¼ inch per hour. However, under present conditions, irrigation water will flow freely into cracks in the soil. Keep the hose running during the day, moving it around to the trees showing the most severe drought stress symptoms. Consider using a timer so you will not forget to turn the water off when you are not tending the hose.
REVISED: September 29, 2015