If only trees could talk and tell us when they are thirsty. Wilted leaves are an obvious sign that a tree requires water. That usually happens on newly planted and young trees.
Established trees have an extensive root system, which may access the underground water table, a neighbor’s yard where the manicured lawn is irrigated five days a week or a leaking irrigation or sewer line. When an established tree is subject to drought conditions, there may be leaf drop, stunted growth, dead branches or exudates (bleeding) on the limbs or trunk.
Diagnosing tree problems can be challenging because you must determine if the symptoms are caused by an insect, disease, soil condition, culturing practice or a combination of factors. Trees that are dying make miraculous recoveries just from proper watering. During the drought Sacramento experienced from 2012 to 2016, trees died because residents stopped watering their lawns due to restrictions. However, watering once a month could have preserved a lot of trees.
Conversely, trees also die or warrant removal due to overwatering. The challenge is determining what is the right dose for the tree. You want the soil to dry out between waterings. Saturated soils over an extended period deprive the roots of oxygen and predispose them to disease pathogens such as armillaria and phytophthora.
Understanding your soil condition is one key to giving the tree the right amount of water. Soil conditions in the neighborhood can vary from block to block. Clay soils do not drain as well as silt loam, as is evident in William Curtis Park during heavy rains. There will be standing water in the south area of the park while at the north end you can walk on the turf.
Hardpan is a layer of soil largely impervious to water and which roots cannot penetrate. It can be a few inches to many feet below the surface. There are shallow hardpan layers north of Second Avenue between 24th and 28th streets. In other areas of the neighborhood, the hardpan is at a 3-foot depth. With hardpan you must be careful not to overwater or groundwater can accumulate above the water table and the hardpan.
Tree species and depth of rooting are other determining factors to consider when watering. Native oaks and drought-tolerant species like Chinese pistache or cork oak, once established, do not require regular summer watering. However, if these trees are in a turf area or have received regular watering, some changes are required.
Make sure sprinklers are not spraying the base of the trunk. Create a turf-free area at least three feet all the way around the base of the tree and install mulch. Reduce the amount of water by 10-20% over the summer. Monitor the tree’s health and continue to reduce the amount of summer watering.
Native oaks and drought-tolerant species may need a summer watering, if there is an extended drought. I have seen native oaks die because a new homeowner wanted to correct the prior owner’s mistake and completely cut off the summer watering.
Established non-native trees require deep watering during hot summers. Due to the sizeable canopy, the tree accesses all the moisture at the lower depths from the prior winter rains. The goal is moist soil 8 to 24 inches below the surface. Once this area has adequate moisture, then the weekly watering done for the lawn or landscape will prevent the subsurface soil from rapidly drying out.
Deep watering can be accomplished using a soaker hose, garden hose sprinkler or automatic irrigation system. The soaker hose needs to be on three to four hours before moving it unless there is runoff. Using a garden hose sprinkler, water until there is runoff, then move the sprinkler to a different spot and keep moving it until you have covered all the open soil area. With an automatic irrigation system, manually turn on and track the time to see when runoff occurs. Then wait an hour or so and run the cycle for the specified time.
With all three methods, you need to dig down to measure the depth of the watering. The soil should be moist and sticky, not dry and crumbly. Remember, roots can extend beyond the drip line, which is the outer tips of the branches. Therefore, deep water the entire yard.
Monthly deep watering in June, July, and August should be sufficient for established trees. Due to the meager rainfall this past winter, trees would especially benefit from the additional water.
Adding a 4- to 6-inch layer of wood chip mulch under the dripline of the tree is one of the best things to improve tree health. A layer of mulch conserves water, improves the soil microorganisms and fertility and helps prevent weeds.
Spend a little time understanding your tree’s water needs and you will have beautiful trees to enjoy.