Helping Your Trees Survive Drought
by Sally Bruns, City and County of Denver Forestry Division

Colorado is undergoing the worst drought period it has seen in many years. We need to prioritize our water needs and conserve, using water as efficiently as possible. When choosing between watering a lawn and a large tree, keep in mind that a lawn can be replaced in a season by sodding, while a large tree will take decades to replace. Trees add a significant value to a home landscape.

Drought stress can make trees more susceptible to pest and disease problems. Birch borers will actively seek out birch trees under stress from lack of moisture, and conifers are vulnerable to beetle attacks. Mites can become a significant problem in dry periods.

Wilting is an early symptom of water stress. If the tree doesn’t recover by morning, it has reached its permanent wilting point. If it stays in this state for a prolonged period without receiving moisture, it will cause permanent damage, and possibly death. It can also cause trees to turn color and drop their leaves early, if drought is severe and prolonged. Some other symptoms are twig and branch dieback, which may not show up until one or two years later.

During this period, the key is to water the trees efficiently. As a general guideline, most landscape plants, including trees, require approximately 1-1½ ” of water per week. Newly planted trees will require special attention as they reestablish their root system, as drought stress can cause dieback. This will set the trees back and prolong the establishment period. In a drought situation, water not only the rootball of the tree, but also the soil surrounding it. Otherwise, the dry native soil will pull the moisture out of the rootball.

It’s best to apply the water in one thorough, deep soaking. The soil should be moist to a depth of 12-18”. Watch the watering in clay soils-it should be applied at a low rate. Clay cannot absorb water at a high rate and excess will runoff and be wasted, instead of going where the roots need it. Frequent, light waterings only promote the growth of shallow roots that are easily damaged by drought stress. Deep soakings encourage deeper root growth.

One of the most efficient methods is to use a soaker hose. Coil the hose around the tree base, out in a circular pattern. Start about 2-3 ft out from the trunk, and go out to approximately 5 ft beyond the tree’s dripline. Water for several hours.

A good method for watering new trees is the basin method. Build up a soil berm around the perimeter of the root ball. This will hold the water in while it drains into the soil. Fill the basin with water and allow it to drain. Repeat until the rootball and soil are thoroughly moistened-twice should be enough.

Adding a 3-4” layer of mulch around trees will be very beneficial. Organic mulches are the best choice, as they will add nutrients back into the soil as they decompose. Mulch will help conserve moisture, and cut down on weed and grass competition. This is especially important for new trees, but established ones will also benefit from having weeds and grass pulled away from them, and replaced with mulch. Keep the mulch a few inches away from the base of the trunk itself. The moisture against the trunk base (root crown) can cause disease problems, as well as keeping oxygen away.

Trees that are drought stressed should not be fertilized. Fertilizer can cause further damage to the sensitive absorbing roots. There is some evidence that applications of mycorrhizae fungi (a soil treatment available through tree services) can be beneficial by aiding the roots in absorption.

It is important that we keep in mind that Colorado is a semi-arid part of the country. Native species have developed their own strategies for dealing with low moisture. We will never have the abundant moisture that other portions of the US have. By using water efficiently, we can help our trees and landscapes to survive.