Trees and Drought

Mike Davison, 2020

We often refer to certain species of trees as drought resistant. These trees have amazing ways of dealing with long periods without rain. Some techniques are common among these trees and some are unusual and clever. However, all trees while in their drought-protection dormant state, are much more susceptible to disease, infestations, and mechanical damage than usual. In California during or after a major drought, we have seen vast damages to our forests from bark beetles, fire, and wind.

Urban trees have these issues as well if proper irrigation is ignored, and they are left to fight drought on their own. Our larger trees in our urban forests are especially susceptible to drought and their needs are easily overlooked. In the last major drought in California, when we stopped watering our lawns as conservation guidelines were adopted, many people forgot or did not know that our trees were also not getting the water they needed. When we look at our landscape to be managed as long-term assets, replacing lawns is easily done after a drought. Trees are not, and special care is needed.

In Serrano Park during the 2011-2016 California drought , a significant number of our large eucalyptus trees dried out and went into self preservation mode. Limb failures increased, and long, tall stress fractures can be seen in many trunks. The trees became more susceptible to boring beetles and rot. Many of our trees were eventually removed, and some still may be facing removal from lingering issues from that drought.

An urban forest management plan must include a good drought plan. A good irrigation installation should allow trees to be watered on a separate zone from grass and flowers. Trees should be mulched to their drip diameter and even wider for conifers. They should be deep-watered using drip lines, bubblers, or deep drip watering stakes. Sprinklers should be carefully designed so as not to over-water tree roots. The drought resistant trees we plant in Southern California are not compatible with typical lawn and flowerbed watering. The roots must dry out between watering cycles to prevent root rot and to promote deep growth.

Using reclaimed water can further reduce our potable water usage during a drought and save costs. Locally, we have never had usage limits on reclaimed water, even during droughts. However, reclaimed water can carry some issues with high levels of salt and minerals that may require extra processing or soil corrections.

With a well designed irrigation system and a good plan, trees can be kept safe through a drought.