Reducing Water for Trees
Water Your Trees and Not Your Grass
Austin has hot summers, and its water is expensive relative to other large cities in Texas, especially when summer irrigation is at its peak. Several smaller Central Texas cities, as well as private water companies, have water costs even higher than Austin. In addition, the region has been afflicted by mandatory water restrictions due to droughts in the last several years.
When faced with hard decisions, it is the better course to water trees and ignore turf. Turf can often go dormant and be successfully revived later. Even if grass dies, it can be reestablished much more quickly and less expensively than mature trees that not only add beauty and monetary value to a home and neighborhood, but also provide summer shade to lower energy bills.
The best things you can do to get trees to survive through a drought and reduce water use in the process are using correct watering techniques, applying mulch, and adding nutrients to the soil to prevent stress and retain moisture.
Best Water…Not Most Water
1. The Right Equipment – If you use a sprinkler, employ one with a fixed coverage that will saturate a defined area. Other options include soaker hoses, bladder bags (for small trees), and buckets with holes in them that allow water to slowly seep into the ground. Some of these options may be exempt from mandatory watering days in various cities. Check with your local utility for relevant requirements.
2. The Right Roots – Many people make the mistake of irrigating at the base of a tree trunk. This is not where an established tree’s feeder roots are located, and is largely a waste of water. The roots that take in most of the water lie 6 to 8 inches below the ground and concentrate near the “drip line” formed at the edge of the canopy where the most water is shed after rainfall.
3. The Right Place – The priority zone is inside of the dripline starting 1/3 of the distance between the trunk and the dripline. Feeder roots generally extend to about 3 feet outside of the canopy.
Whatever equipment is chosen, it is important to water evenly. If using a soaker hose, coil it evenly over the watering area. If using buckets, place in a pattern that will drip over the intended zone.
4. The Right Depth – Trees are not grass. The water needs to saturate the soil to reach the 6 to 8 inch depth where the roots are. It is best to water in the same place for 40 to 90 minutes at a time. Inserting a screwdriver into wet soil can give an estimate of moisture saturation after watering.
If you have mulch within the root zone, make sure the moisture seeps through it to the required depth. If mulch is too thick or crusty for good saturation, break it up.
5. The Right Amount – Trees are often over watered. The chart below is a good guide for directing the proper amount. This chart may not be accurate for trees native to rainier climates or river basins (e.g., cypress).
|Tree Size||Water Needs|
|Diameter (inches)||Gallons per Week|
6. The Right Time – Many cities in Central Texas mandate that irrigation can only be applied at certain times of day to reduce evaporative losses. This is particularly true for sprinkler systems. In Austin, outdoor watering is prohibited from 10 AM to 7 PM except with a hand-held hose and certain types of soaking options. (See #1 above.) Irrigation is also confined to certain days to reduce peak demand on the system. Again, check with your local water utility.
Watering for the first 3 years is vital to getting a new tree’s root system established. Once this time period occurs, it is more likely to survive without assistance. Of course, trees survive in the wilderness without any irrigation assistance. However, in times of drought, there are consequences. In 2011, the hottest Texas summer in recorded history was responsible for killing as many as 10% of the state’s trees.
Compost & Mulch
Compost is decayed organic matter that provides nutrients, water-holding capacity, and a home for beneficial bacteria and insects that encourage tree and plant growth. Mulch is generally undecayed organic matter, such as chipped tree bark and wood or lawn clippings, that is spread under a plant to reduce evaporative soil moisture losses. Mulch also provides roots insulation from extreme temperatures, a home for beneficial bacteria and insects, and nutrients as it decays over time. Other benefits include the discouragement of weeds and grass that compete with trees for resources; and in landscaping, mulch rings protect trees from lawn equipment.
In an optimal situation, you should add both compost and mulch to trees. A quarter inch of compost covered with 2 to 4 inches of mulch will add nutrients immediately while offering the other benefits of mulch. If the cost makes simultaneous installation difficult, laying mulch by itself is still important. Be aware of several things:
1. Cover the Feeder Roots – Many mulch installations are “islands” around a tree trunk surrounded by stones, and the circle surrounding the tree is only about 3 feet in diameter. This will help small trees whose roots are not well established better than larger trees. The farther mulch is laid toward the dripline, the more effective it will be. Small mulch islands still have some of the benefits, but they are not as effective for reducing watering needs.
2. Don’t Cover the Trunk – Many people lay mulch right up to the base of the trunk. This can create a breeding ground for fungus and disease when it is wet. Instead, leave a buffer of 4-5 inches between the trunk and the mulch.
3. Don’t “Volcano” the Mulch – Many people lay a rising cone of mulch up to the trunk of the tree. Not only can this cause fungus and disease, but it is an ineffective use of this material.
City of Austin Arborist
Texas Forest Service (Austin Division)