To understand how to water a tree, it is necessary to understand how a tree utilizes that water. A tree has two basic sections: the canopy, which consists of the trunk, branches, and leaves; and the root system, which contains several different types of roots and is, of course, located beneath the soil. The root system can be as large as or larger than the canopy of the tree, and in our desert environment, usually extends beyond the drip line of the tree.
Since we are applying water to the root system, let's look at that. The roots that absorb water for the tree to use are called water absorption roots. They are located between one and three feet under the surface of the soil. On a mature tree they begin about four feet from the trunk and extend out to the end of the roots. These absorption roots are very small, about the size of a human hair and are attached to larger roots. As well as absorbing water, these roots take up nutrients that have been dissolved in the water. Another job these absorption roots have is to take up oxygen from the soil to help the tree grow.
When the soil is saturated with water, the water displaces the air in the soil and thus the oxygen. To water our tree properly we need to balance the amount of water that the tree requires with the amount of oxygen that the tree requires. Our trees like deep (1-3 feet), infrequent (2-4 weeks) watering. The deep watering will move the water to the proper roots. The infrequent watering will allow the soil to replenish the oxygen supply between waterings.
From this information we can see that we should apply water to our trees away from the trunk, and that we must allow oxygen to reenter the soil after we water. Draw an imaginary circle around your tree 1/4 larger than the diameter of the tree's canopy. Now, draw another imaginary line 1/3 of the distance to the first line. The area between these two lines is the area you want to water.
A very common mistake many people make in watering their trees is to lay a hose at the trunk and let the water trickle. When we continually water the trunk of a tree we encourage the roots to grow in a ball around the trunk instead of growing out into a supporting network. Another common mistake is to water the tree every day or two and keep the soil too wet.
Some people have the misconception that once a tree is established it doesn't need as much water. In fact, the bigger a tree is, the more water it requires to maintain itself. Another popular myth is that roots will "go looking" for water. In fact, roots will not grow in dry soil.
Another problem I'd like to talk about here is the use of landscape plastic under trees. I personally think that this plastic is way over used in our landscapes. Plastic under trees causes a multitude of problems that may not show up for a number of years, and are difficult to correct in later years. The plastic prevents a lot of needed water from reaching the water absorption roots by letting it run off before it reaches the soil. The water that does condense on the underside of the plastic encourages shallow roots to form between the plastic and the soil. These roots then do very little to help support roots to form between the plastic and the soil. These roots then do very little to help support the tree, either physically or nutritionally. Plastic prevents a lot of needed oxygen from getting into the soil under the tree. During our hot summers there is a tremendous buildup of heat under the plastic which raises the soil temperature which can harm the tree. And lastly, it is a really nasty job to remove it once it is there and covered with gravel. Now that we know the "why" of watering, next month's Part II will talk about the "how".
**Cashman owns and operates Weed Control Company, Rt. 2, Box 125E, Hereford, AZ 85615.