Maintaining Tree Health Despite Drought

Maintaining Tree Health Despite Drought

As we make our way through summertime, our landscape plants endure the yearly cycle of reduced soil moisture at this time of year. In most years, the rainfall shortages have not been too significant to cause serious damage to trees, but I am still remembering the drought of 2011. That was the year when the entire State of Texas, according to the U.S. Drought Monitor, was colored bright-to-dark red in August, indicating extreme to exceptional drought. That year in most locations, we also had over 60 days of 100-degree heat. That year, Texas broke the record for the driest year on record, which was also the second warmest year on record.

Trees died or were severely affected by the lack of rainfall. Even in home landscapes where residents were still allowed to water their lawn and plants, trees were adversely affected. The effects of that drought lingered for many years, and you might still see trees struggling to recover their former glory.

Now, I am not predicting this weather reoccurring, and hopefully it will not. But, as I am writing this in mid-April, over half of Texas is under moderate- to exceptional-drought conditions. What I would like to present are some tips on helping our trees through a typical summer as they go through drier spells.

Before getting into watering trees, it would be helpful to take a look at the hidden, unseen world of tree roots, which support and supply the above-ground canopies we all appreciate and love. First, let’s dispel the myth that the below-ground root system of a tree is a mirror image to the branches. This idea partly stems (no pun intended) from the thought that the initial taproot continues indefinitely downward with lateral roots branching as it proceeds deeper into the soil. It is true that most trees initially produce a strong downward-growing root, but it may or may not continue deep into the soil.

Instead of being a mirror image of the above-ground trunk and branches, tree roots tend to stay in the upper few feet of the soil, growing where there are optimum levels of oxygen and moisture. Roots require oxygen to live, and oxygen decreases in concentration with depth. New root growth proceeds from the tips of each root, and close to that new growth is where most of the water is taken up into the roots to be transported upward into the rest of the tree. As new root growth encounters pockets of water and oxygen, roots will proliferate in that region to utilize those resources.

A frequent recommendation for watering and fertilizing trees is to do so in the area under the dripline of the tree. The reasoning for this is that there are few “feeder” roots near the trunk of a tree. The dripline is the area located directly under the outermost spread of the branches. While that recommendation is fine for relatively young trees, as trees grow, their roots continue to extend outward well beyond the dripline of a tree. A rule of thumb holds that the length of a tree’s roots extending from the trunk will be equal to the height of the tree. Actually, that distance can easily be one-and-a-half to two times the height of a tree.

Dr. Larry Stein, a horticulturist with Texas A&M AgriLife Extension, told me an anecdote that illustrates the extent of the spread of tree roots. A school class was visiting the pecan teaching-orchard located in the fertile soils of the Brazos River bottom. They had each child sit on the ground three feet apart, starting at the trunk of a mature pecan tree, and going out in a line. I’m not sure how many kids were involved, but a class of 20 would have put the last child 60 feet away from the trunk. The tree was shaken by a pecan shaker, a machine that grabs the trunk or major branches and vibrates vigorously to shake loose mature pecans for harvesting from the ground. The children were asked to raise their hands if they felt the ground shake beneath them. Every child did, all the way to the end of the line. The moral of this story is that the root system of a mature tree extends way beyond the dripline of a tree. Both watering and fertilizing a mature tree near its trunk do little to meet the need of the tree.

How much should you water? Roots move water upward through the tree’s vascular system, eventually reaching the canopy. That water becomes components of new cells and is important for the tree’s ability to photosynthesize and make food to support growth of both the above- and below-ground portions of a tree. But most of that water is lost through a process of transpiration, which is water escaping through leaves as water vapor. Transpiration is what drives the uptake of water and is important in keeping the tree well hydrated. When water in the soil is limited, as during a drought, transpiration is reduced. When insufficiently hydrated, the tree’s ability to produce new growth is also limited.

Some tree species will show signs of water stress by wilting. Other species have leaves which prematurely turn yellow and then drop off. Some species do not show signs of stress until water availability becomes extremely limited. Others, like dogwoods, have leaves that will turn brown and dry but still cling to their branches. What I’m saying is that it is difficult to tell simply by observation whether a tree is experiencing water deficits. They are not like the proverbial canary in a mine that quickly dies when oxygen falls to dangerous levels, alerting miners to the danger.

Just as stress can impact our own health, water stress impacts the health of our trees. Prolonged drought can have both short-term and long-term effects on our trees. During the heat of summertime, a mature tree loses a tremendous amount of moisture through transpiration. Soil moisture is also lost through evaporation as well as transpiration through turf-grass foliage. This lost soil moisture needs to be replaced, either through rainfall or irrigation, to keep our trees optimally healthy. Dr. Stein says their recommendation for pecan orchards is one inch of water per week in the early part of summer and two inches per week during August and September in order to keep the trees healthy enough to mature a crop of pecans. As homeowners, our concern during a drought is not producing a crop but maintaining our valuable shade trees in a healthy state.

Keep a diary record of how much rainfall your yard has received and when it occurred. Turf grass will require more frequent irrigation than trees to maintain its vigor. During a prolonged dry spell in the summer, valuable trees in our landscape should be watered deeply at least one or two times a month. The goal of this watering should be to wet the soil to a depth of about 8-to-12 inches.

An easy way to determine if the soil is dry at that depth is to push a long screwdriver into the soil. It will easily move through moist soil until it meets resistance where the soil is dry. If you can’t get it down to about eight inches, it is time to deeply water. It may take only three-quarters of an inch of water to penetrate that deep in sandy soils, whereas it might take one-and-a-half inches or more in a clay soil. Sandy soils will need to watered more frequently than clay soils, which have a greater water-holding capacity. Apply water slowly so that it has time to be absorbed into the soil without running off.

Young trees need more frequent watering since they will not yet have produced an extensive root system to utilize available water. Weekly watering for younger trees will help them survive a drought. Newly planted trees can be watered by hand with a hose and water wand. For newly planted and young trees, maintain a wide area free of grass and other plants, which compete for soil moisture. But do not leave that large circle bare; mulch with wood chips, pine needles or other organic materials to stop water from evaporating from the soil and to prevent weed seeds from germinating. But don’t pile the mulch up (volcano-like) around the trunk.

Finally, during a drought, do not fertilize trees or lawns where tree roots are located. Doing so could burn roots in dry soils. Also, you do not want to stimulate new growth, which places more water demand on the tree’s root system.

Here’s hoping this summer will not be extremely dry and that our trees, which provide valuable shade and cooling for our environment, will not suffer from a prolonged drought. Dead grass can be quickly replaced, but a 20-year-old tree takes 20 years to replace. Let it rain! 

By Keith Hansen
Smith County Horticulturist, Emeritus
Texas A&M AgriLife Extension Service / Owner, East Texas Gardening