Trees and Drought

Mary Helen Ferguson
NC Cooperative Extension Agent, Horticulture
TREES NC Board Member

You may have trees that are losing their leaves and seem to be dying. The dogwood is just an example of a tree that seems to be showing effects of the drought, although unusually high temperatures may be involved in this as well.

The good news is that established trees should have a good deal of energy stored in their roots, and they are likely to come back next year in spite of losing their leaves early this year. In fact, losing leaves is a way for trees to conserve water. On the other hand, if leaves turn brown at this time of year and do not fall, it may be a sign that the tree is dying or dead.

If you have trees that were planted within the past year, they need approximately 1 inch of water, applied over the root area, every 7 to 10 days (this is recommended whether or not there is a drought). For a plant that came in a container that was 1 foot wide (or had a root ball that was 1 foot wide, for ball-and-burlap trees), applying “1 inch” of water requires about ½ gallon. For trees planted last fall, you might consider the root area to be larger than the container in which the tree came, since the roots should have grown out into the soil by now. A little over 1 gallon of water is needed to apply 1 inch to a root area 1½ feet wide, and about 2 gallons are needed for a 2 foot wide root area.

According to information from University of Georgia, large trees should be watered during drought, as well. One to 3 inches per week is a suggested rate. Those in clayey soils may want to lean towards to lower end of this range, since clay holds water well, and excessive water can kill tree roots by crowding out oxygen. Apply water in the area starting 3 feet out from the trunk and ending at the drip line (where the tree limbs end). Within this space, you can choose an area totaling 1/3 of the area of the tree canopy in which to water. This may add up to a lot of water—about 5½ gallons is needed to apply 1 inch of water to 1 square yard of ground.

Remember to adjust the amount of water you apply by the amount of rain that you have received. For example, if you get a half-inch of rain in a given week, you can subtract that half-inch from the inch of water that you might otherwise apply. If you would like to know how much rain your yard has received, you can get a rain gauge. In fact, I have some rain gauges to give away, and you are welcome to come by our office and get one, while they last.

For trees, watering sometime between the late evening and the early morning is better than other times, because less water evaporates. However, avoid getting water on the leaves and, if feasible, the trunk. (If you are watering vegetables or turf, the guidelines are a little different—watering in the evening may allow water to remain long enough for disease to develop. Because of this, watering in the early morning is suggested, so that water can soak in without much evaporation, but plants can dry as temperatures get warmer and as the sun rises.)

In our clay soils, it is recommended that you apply the suggested amount of water within a relatively short period of time (maybe a half-day or a day), so that the water moves down in the soil rather than remaining in the top couple of inches. Watering “deeply and infrequently” rather than “shallowly and frequently” is important for good (deep) root growth for new plants and for getting to the roots of established trees. As you water, you may need to wait for some of the water to soak in before applying the rest so that water is not wasted due to runoff. In sandy soils, you may need to apply less water at one time, but water more frequently, since sandy soils do not hold water like clay soils do.

Having mulched areas under tree canopies are helpful for conserving water. For newly planted trees, two to four inches of mulch are suggested, but keep the mulch 1 to 2 inches away from the trunk. You may benefit from mulching beyond the drip line of a tree, since roots typically extend further than do the limbs.

While this article is primarily about trees, I want to let you know about a new web site from NC State. Here, you can get free customized watering recommendations for your lawn based on where you live (rainfall and other climate data is considered), what type of grass you have, and how you irrigate.