Wrong way to water trees

I hired a man to remove my lawn and plant a xeriscape for me. He has covered the soil with black plastic to prevent weeds, except for a small area inside a ring of rocks around my two large ash trees. The trees have been here about 15 years and are important for shading our house in the afternoon. To provide for watering the trees, he did not put plastic in the rings around the trees. These rings are about six feet across. Then to be sure we could water the trees deeply, he place a piece of pipe into the ground about three-feet deep and about 18 inches from the trunk of each tree. He told us to be sure to water the trees by filling the pipe every two weeks. Is every two weeks often enough to water the trees?

Here in New Mexico, in all but the sandiest soils, irrigating an established tree every two weeks in the summer and once a month in the winter should be adequate. However, the frequency of the irrigation is not the important factor in your case. You should be concerned about where you apply the irrigation water.

The pipe in the ground will place the water below the level of the majority of the absorbing roots, those responsible for absorbing the water and mineral nutrients needed by the trees. Most people are surprised at the shallow depth of tree roots. The majority of the absorbing roots of trees are in the top six to twelve inches of the soil. Some absorbing roots will extend down to a depth of about three feet, but the pipes placed at the base of your trees start irrigating at three feet, below the level of the ash tree roots. While ash trees can have deeper roots, in our soils and dry climate there are few roots below the two- to-three-foot depth in the soil.

The second problem I see in what you described is the fact that you are watering at only one point for each tree. A tree has an extensive, spreading root system which is necessary to extract sufficient water and mineral nutrients from the soil. A wide spreading roots system is also necessary to support the tree in strong winds, which we experience here in New Mexico. To adequately supply irrigation water to a tree, it is necessary to provide water to most of this root system. That is, you must water a large area, not just one spot. So the pipe is a bad idea because of the depth, which is below most of the absorbing roots, and the location which waters only a small portion of the roots.

Another, and very critical, consideration is the fact that the active, absorbing portion of the root system is the system of very small, fine roots at the ends of the larger roots. Watering at the pipe, only a foot and a half from the base of the tree, applies the water to only the big, supporting roots (and below them at that), not to the fine, absorbing roots. The large roots do not absorb, and water at that location may increase the likelihood of termite, carpenter ant, or fungus damage to these roots. The black plastic which has been placed under the rocks in the landscape prevents rain and snow melt from entering the soil at the location of the critical, absorbing, root system. It also restricts the diffusion of oxygen, needed by roots, into the soil. A porous weed barrier doesn't cause this problem and is definitely preferable to the impermeable black plastic. However, in our windy, dusty environment, dust accumulates above any weed barrier fabric, even the porous fabrics, and weeds often begin growing in this dust when there is sufficient moisture. If the roots of these weeds penetrate the pores of the weed barrier, it is very difficult to remove them by pulling. In such a case, herbicides become the most feasible means of managing the weeds.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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