Tree irrigation

Inline drip irrigation tubing with emitters every 18 inches, wrapped around a tree in a spiraling pattern and extending out to the drip line of the tree, can support the growth and development of a healthy root system.

Drip irrigation is attractive because it offers many benefits, including the potential to reduce water use outdoors, but successfully switching older trees from a flood irrigation system to drip can be difficult.

Drip irrigation was a new technology in the 1980s and many people were eager to adopt it into their yards. It worked very well in vegetable gardens and around shrubs but when large trees long adapted to receiving water from flood or sprinkler irrigation systems were forced onto a drip system, their health often declined to the point where they either died or were taken out because they looked so bad.

Yes, water conservation continues to be an important topic of discussion here in the desert Southwest and I know some may be thinking about installing a drip irrigation system to minimize the amount of water used in their yards. Don’t get me wrong. That is a good thing, but when switching from flood irrigation systems to drip irrigation, it is important to remember that large trees have a root structure that is larger and more widespread than smaller plants. If you are going to make the transition, it will be important to develop a plan that addresses this issue so that the tree will have a chance to survive the change.

What are the rules that govern this type of major adjustment? First of all, it is important to remember that in the desert, proper irrigation is essential to maintain the good health of any landscape or garden plant. This is particularly true for trees and shrubs.

While some years may receive more precipitation than others, there will never be sufficient rainfall that will arrive at the exact right time and in the correct amounts to meet the needs of most plants. With the exception of some of the more hardy native plants, like the saguaro, there are few that can survive any length of time without a dose of water now and then. Even a saguaro transplanted into a yard, however, will need an occasional drink once in a while.

The second rule to remember is that the depth to groundwater in most areas of Pinal County will be greater than the depth that trees can send their roots. In western Pinal County, the depth to water can be in the hundreds or even thousands of feet. As we become more reliant on groundwater pumping in the future, the depth to water may significantly increase. In most cases, the roots of surface-dwelling plants just aren’t going to get their roots deep enough into the soil to harvest water sufficient for their needs. We need to provide water to the plants through irrigation.

Third, the roots of all plants, and especially trees, tend to follow water. They grow into moist ground and avoid soil that is consistently dry. In flood or sprinkler irrigation systems, where the water is applied to the soil over a wide area, it is probable that the roots may extend far beyond the outer edge of the tree, or the drip line as we call it. In fact, where flood irrigation has been the standard over some time in adjacent properties, the roots may even extend over several yards. We are not talking about a 3-foot space. We are talking about neighborhood yards: my yard, my neighbor’s yard and maybe even the neighbor’s yard on the other side. Roots can grow a long distance from the trunk, if there is water to encourage their growth.

Fourth, we need to remember that all plants, including trees, abide by the balance of energy rule. That is, there must be as much energy in the bottom of the plant, the root system, as there is in the top of the plant. If energy begins to drop in one part of the tree, the other part will similarly decrease. If I short a tree water and some of the roots do not get irrigated, they will die back and the amount of energy in the root system decreases. When roots begin dying, we see a similar response in the top of the tree as it tries to match the roots. If enough of the roots die for lack of water, the entire tree, roots and top, could die. It may not be immediate, but the decline will continue until the plant can no longer sustain itself.

The last rule to remember is the Rule of One, Two, Three. This rule simply says that turf and bedding plants will have a root system located in the top 1 foot, or 12 inches, of soil. Shrubs have a root system that extends down 2 feet, and most trees send roots to a depth of 3 feet. For best plant health, all of the roots within the appropriate zone must receive water, every time.

In a flood irrigation system, where much water is available in the soil, many roots will develop throughout the wetted area. When we switch the irrigation delivery system to a point source, that is, water dripped onto one spot of soil, we tend to create dry areas between the wetted zones. Additionally, the water must run a bit of time to reach to the appropriate depth. If not, some of the plant roots receive little or no water and begin to die. According to the Balance of Energy Rule, the decline in root volume will correspond to a similar decline in the top of the tree. If the damage becomes too great, the entire tree may die. More likely, the tree will be removed sometime along the way as it becomes unsightly.

How do we avoid this? The solution is to keep wetting the entire root system after the switch to drip. This means that there must be enough emitters to wet the entire area underneath the tree to satisfy all of the needs of the existing roots. In addition, it also means that the system has to be run long enough to allow water to penetrate to the desired depth. This may be a challenge using 1 gallon per hour emitters dripping onto a small spot of ground, but some people have found that tubing with inline emitters works very well when the tubing is wound in a spiral fashion from the trunk out to the edge of the drip line of the tree. It is simply a matter then of setting the system to run long enough for the water to sink to the bottom of the root zone, and for the wet zones on the surface of the soil to touch each other.

A long soil probe is invaluable in measuring how deep the water is penetrating during an irrigation. I like to use a long screwdriver but soil probes 3 feet in length are available. A probe will sink through moist soil easily but will stop short when it reaches dry ground. Upon removing the probe, the length from the tip of the probe to the spot on the probe that was even with the surface of the soil can be measured, and this will tell the depth of water penetration.

Still, doing it right in your yard may not be enough. When whole neighborhoods have been irrigated with large amounts of water over a period of time, it is possible that the roots of your tree may be over in your neighbor’s yard. If they do not have an irrigation system that addresses the needs of the roots of your tree, problems may still arise. Good will and cooperation would be essential in this situation.

It can be a tricky business, but attention to detail can help alleviate tree problems during the transition from flood to drip. It takes careful planning, proper management of the system and a weather eye out for any changes in the tree. Signs of tree decline caught early can sometimes be headed off by quick attention to the problem.

Before attempting to convert a large flood-irrigated tree or shrub to a drip irrigation system, in order to have any hopes of success, it will be important to carefully consider each plant, its environment and the likely placement of its roots before making the conversion.

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If you have questions, you can reach one of our Master Gardener volunteers every day except Saturday and Sunday between 9 a.m. and noon by calling the Master Gardener hotline at 520-374-6263. You can also call the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande at 520-836-5221, ext. 204 and leave a message.

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