Trees may need water in dry spell

Bark beetles damage water-stressed pines.
Bark beetles damage water-stressed pines. Jack Kelly Clark • University of California Statewide IPM Program

Q: I usually don’t water my trees in winter. I rely on Mother Nature, but it has not rained for a while and my soil looks dry. Should I water my trees now? How can I tell if they need water?

A: Established trees may benefit from supplemental irrigation if rain does not periodically wet the soil at least a foot deep. Inadequate soil moisture stresses trees and can make them more susceptible to damage from sun, wind and insect pests. Bark beetles readily attack and kill Monterey and other pines that lack water and do not have enough sap flow to repel the insects’ attack.

Trees, even dormant deciduous species, need water throughout the year. At least a portion of a tree’s root system should be in moist soil. Some roots may grow to a depth of 4 feet under favorable conditions, but most of a tree’s root system is in the top 2 feet of soil.

You can’t tell if soil in the root zone is moist just by looking at the ground. Surface soil can be dry while soil further down is moist, or surface soil can be wet while soil further down is dry. The best way to check soil moisture is by digging one or more small holes about 18 inches deep under a tree’s drip line. Examine soil from the hole and try to squeeze some loose soil together in the palm of your hand. Loamy or clay soil that forms a “ball” contains enough moisture for tree roots. Very sandy soil may not form a ball but should feel damp. If soil in the top 18 inches is dry, irrigation may be needed if substantial rain does not occur soon.

When you irrigate a tree, apply water under its entire canopy and wet the soil to a depth of 2 feet. This may require slowly applying 3 to 6 inches of water, depending on how much sand or clay your soil contains. After thorough irrigation, a tree will not need to be watered again for several weeks or months.



