Issue: January 19, 1998


Late winter landscape irrigation in New Mexico

Question:

When should I begin watering my trees and lawn? This has been a wet winter and I don't want to waste water. Answer:

This winter has indeed been wetter than usual for most of New Mexico. This has been the gift of El Niño. However, we are entering the months when precipitation usually declines and evaporation begins to increase. If the precipitation continues with good heavy snows or soaking rains, you can wait. It is more likely that we will have light rains which do not provide enough moisture to moisten the soil deeply, and as temperatures increase the water will evaporate from the soil much more quickly. In addition, we are going into our windy season. The winds can quickly extract moisture from the soil.

A shovel pushed into the soil can help you determine the status of the soil moisture. If the shovel enters the soil to its full depth easily and as you lift the shovel you notice moist soil at the end of the shovel, you can delay irrigation. Most perennials and trees utilize water to a depth of two-to-three feet in the soil. If our soils have received considerable moisture through the winter, the surface dries first. So, moisture in the top foot indicates that there is probably moisture below. This is not the case in the summer when the soil is dry to a considerable depth. Our irrigations or summer precipitation moistens from the surface downward, but may not moisten the soil to any great depth unless we take special care to provide enough water.

In winter when there is subsoil moisture, as you notice that the soil begins drying to the depth of a shovel, begin irrigating at least once a month. In sandy soils, irrigate once every two weeks. As the weather warms and the precipitation becomes a distant memory, increase the irrigation frequency to your summer standard. In most of New Mexico, we can water trees and perennials once every ten days to two weeks. Turf will need more frequent irrigation, depending on which type of turfgrass you are growing. Bluegrass and fescue can be watered every 2 to 5 days, depending on the water holding capacity of the soil. Bermudagrass, buffalo grass, and blue gramma can be watered much less often.

It is important to maintain moisture for trees late in the winter and early in the summer because they are becoming active underground, needing water at that time. When you see the buds swelling on the twigs, the trees are producing small hair roots and root hairs. These are very small, numerous, and essential for the uptake of moisture and nutrients for the tree. If the soil is dry as these roots are produced, the roots die and slough off. The tree is then unable to begin growth in a healthy manner in the spring. Remember, trees often have absorbing roots to at least three feet in depth in the soil, so that is the depth to which you should moisten the soil as you irrigate. A rough rule of thumb states that one inch of applied moisture will moisten sand to a depth of three feet, silt to a depth of one foot, and clay to a depth of one-half foot. Clay is much harder to moisten, but holds more water and needs irrigation less frequently.