Current Drought Threatens Many Urban Trees

This may end up being the driest Winter on record here in Tucson. With only a trace of rain since December, even desert plants are beginning to show the withering effects of drought and heat. What is even more critical is the condition of many of our urban trees. Although they may look well, they are also beginning to suffer. If our dry conditions continue, many of these trees could die if not provided some water.

There is a tendency to think that large, established trees such as pine and eucalyptus don't need watering. This is not true! During our last drought in 1996, even large, established, desert-adapted trees were lost. These trees appeared normal for a time, but all the while water was being drawn out of the trunk, roots and branches. Finally, needles or leaves turned brown when no water remained to be consumed. In this final stage, no amount of water would have been enough to revive these drought stricken trees.

The only way to insure that trees are not lost to drought is to give them water before they become moisture depleted. If you haven't been watering your trees, do it now! Large trees should be given the first priority in watering. Pines, eucylptus, elm, ash, poplar, sycamore and oak are some of our largest growing species. Next in line for watering come the true desert types, such as palo verde, mesquite, desert willow, acacia, and ironwood. Just because they grow naturally in the desert doesn't mean they don't need water. During a normal winter they rely on 4 to 6 inches of rainfall. So far they have received none!

For now, one good soaking is all that is necessary. By Agood soaking@ I mean a long, slow watering of the roots. Most drip systems do not adequately water tree roots. This is because tree roots extend out well beyond the ends of the branches. A good soaking must include all the ground between the tree trunk and the branch ends. Watering out farther is even better.

A soaker hose is most effective in watering large trees. Coil the hose around the tree, beginning two or three feet from the trunk. Spiral the hose out maintaining a two foot spacing between the circling hose as you go. If you can't cover the entire area to be watered at one time, then water the tree roots in sections. When your done with one section of the roots, then begin watering another. Water should be applied until the top two feet of soil is moistened. This will take some time.

Periodically check when applying water to see how deep the moisture has penetrated. To do this, use a metal rod, such as a two foot long re-bar which can be purchased at hardware or building supply stores. When the soil is moistened, the rod can be pushed in to the depth of wetting. Try pushing the rod down in several locations. Because of rocks and other soil debris, the probe may strike one of these objects and give you a false reading. Try another spot.

Another way to water is to use the basin method. Build a small berm around the tree a few feet beyond the branch ends. This circular berm will hold the water in until it can be absorbed by the soil. Rapidly fill the basin with water and allow it to drain. Repeat this process until the soil has been moistened down two feet.

If drought conditions continue, provide another thorough soaking of your trees around the mid to later part of May. The hotter the weather, the sooner you will need to reapply water. Remember, these recommendations are for trees that are not being regularly irrigated. It also applies to medium and large trees that are not being thoroughly watered by drip irrigation. Less than 10 emitters on such trees means you better hand water using the previously stated recommendations.

Trees are the most important component of our urban environment, especially here in the desert. They provide cooling shade, oxygen to breath, filtration of air pollutants, wind buffering, soil improvement and beauty. They're also your most important and valuable home landscape asset. Keep that in mind when making a decision about what to water. Vegetables, flowers, ground covers and shrubs can be repaced at minimal cost. They also grow quickly. Not the case with trees!

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on March 28, 1999
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