COLUMBIA, Mo. – Dry soil conditions can significantly reduce the life span of valuable landscape trees.
“Because they are difficult and expensive to replace, your trees need attention during and after periods of drought,” says University of Missouri Extension state forestry specialist Hank Stelzer.
Most of a tree’s active roots are within the top few inches of soil and can extend well beyond the edge of the tree’s canopy, Stelzer says. Not only are they competing for precious water during a drought, but high air temperatures can actually bake the soil and severely damage or kill fragile, fine roots critical for water and nutrient uptake.
Perhaps most life-threatening to a tree suffering drought is invasion by borers and disease-causing organisms as the tree is recuperating and still in a weakened state, he says. Elms succumb more quickly to Dutch elm disease, and oaks are more susceptible to oak wilt, hypoxylon canker and hardwood borers. Pines are more likely to become infested by pine bark beetles during drought.
Some common symptoms of drought-stressed trees include wilted foliage, off-color leaves, leaf scorch, leaf drop and premature fall coloration. Closer inspection may reveal poorly formed buds.
Proper watering is critical. “Slow, deep watering every five to seven days during drought is ideal for mature trees here in the Midwest,” Stelzer says. “When temperatures climb above 95 degrees Fahrenheit, water every four to six days.”
A good way to water is to put a sprinkler beneath the tree canopy. Place an empty, shallow tuna can close by and run the sprinkler slowly until 2 inches of water has collected in the can. Be sure to water the entire root zone beneath the tree canopy. The best time to water is in the morning.
“If turf is underneath the canopy of the tree, more water will be needed because the turf will absorb much of the water that is applied to the surface,” he says. The goal is to get the water through the turf roots and down to the tree roots. Removing the turf around the base of the tree and replacing it with mulch can help eliminate competition for water between the turf and the tree.
“For young or newly planted trees, slow, deep watering every two to three days is a good gauge,” Stelzer says. “There are also a number of soaker products available to keep newly planted trees from drying out.”
Soil moisture check. “When watering any tree, remember that soil type and method of water delivery have a big impact on how successful the general recommendations might be,” he says. Trees planted on a slope may need some type of soaker hose or drip emitter, as applied water will run off. Sandy soils need shorter watering intervals, and clay soils should have longer intervals. Clay soils are hard to wet, and water will not infiltrate but puddle if applied too quickly. “The puddling of water may make one think sufficient water has been applied, but often only the top inch may be wet.”
The depth to which water has infiltrated the soil must be checked by hand, Stelzer says. “It is always advisable to physically check soil moisture by hand to a 1-foot depth instead of using watering intervals or relying upon automatic timers.”
Tree TLC. Proper tree care during drought includes watering, mulching and pest management. Applying organic mulch such as wood chips to a depth of 2 inches will help the soil retain moisture. But if the soil does not have any moisture, mulch will have little effect as there is no water to retain. Inorganic mulch like crushed granite might help the soil retain moisture but may not be as effective as organic mulch, and it can add to the heat stress as temperatures climb above 90 degrees.
Routine pruning is not recommended during severe drought, Stelzer says. Pruning can cause tree stress, which can make the trees even more prone to borer attack. Late-season fertilization is generally not recommended. Fast-release fertilizers like urea will utilize water first and make the effects of drought more severe. Slow-release fertilizers may even encourage new growth when the drought breaks and make it susceptible to an early frost. Planting or transplanting trees is usually not recommended during drought conditions.
— MU Extension