by Dan Carney
According to the Department of Water Resources, California usually receives 75 percent of its annual precipitation between November and March, and in many parts of California landscape plants need little or no winter watering during this time. When rain doesn’t show up, plants become stressed and need an occasional drink to survive. If you’re trying to decide how to meet your conservation goals, consider letting lawns and other less-essential landscape plants go dry and focus on keeping the trees healthy.
Trees bring incredible values to the community that increase every year. Literally, money grows on trees in the form of water, soil, and energy conservation; habitat value; air quality improvements; carbon sequestration; and quality of life. It’s hard to find a better economic deal anywhere worth the gallons of water required to keep trees alive.
Mature trees, with access to groundwater and broad extensive root systems, usually are better adapted to periodic droughts—100-year oaks have weathered some tough times—and may actually be harmed by supplemental irrigation. Please consult your local UC Cooperative Extension office, certified arborist, landscape professional, or MMWD if you have specific questions about your trees.
For most newly planted and newly established trees, here is a simplified method you can follow to ensure they get enough water to survive the drought: Rule of thumb—10 gallons per week per inch of trunk diameter.
♦ Using a garden hose:
Newly planted trees: Place the hose on the ground near the root ball. Turn on the faucet to medium flow. Give the tree 10 gallons of water for every inch of tree trunk diameter (measured 6” above the ground). Since newly planted trees have limited root systems, they may need to be watered 2-3 times each week. For example, a 2-inch diameter tree will get a total of 20 gallons of water each week. If the hose flows at 5 gallons per minute, water for 1-2 minutes several times a week. (Use the second-hand on your watch and a bucket to measure gallons per minute.)
Newly established trees (1-5 years old): Place the hose on the ground as near as possible to the outside edge of the branches (the drip line). Follow the same watering guideline (10 gallons per inch of trunk diameter), but measure the trunk diameter at chest height. Because established trees have a more extensive root system they can be watered less frequently (about once every 7-10 days) depending on the soil type and time of year. For example, using a hose flowing at 5 gallons per minute, a 5-inch diameter tree would be watered for 10 minutes (50 gallons) every 7-10 days.
♦ Using drip emitter tubing:
Newly planted trees: Place the drip emitter tubing directly on top of the soil and in a circle near the root ball. A typical drip emitter hose with one-gallon-per-hour emitters spaced one foot apart would need to operate for about 40 minutes 2-3 times a week.
Newly established trees (1-5 years old): Place the drip emitter tubing directly on top of the soil and in a circle near the outside edge of the branches (the drip line). For a tree that was planted 1-5 year ago, run the drip emitter tubing for 90 minutes every 7-10 days.
Important note: If water starts to runoff or pond, reduce the flow, move the hose to a new location around the tree every few minutes, or water for fewer minutes at a time so that all water infiltrates completely into the soil. Finally, apply a generous 4-inch deep layer of organic mulch under the tree (keeping the mulch 6 inches away from the trunk) to preserve the moisture in the soil.
Taking the time to correctly water trees helps preserve scarce water resources and maintains a healthy environment for everyone.