Trees in drought: Love and water deeply

VERSAILLES, Mo. — With more than 90 percent of Missouri under drought, browning lawns and wilting cornfields are common sights. But don’t forget about your trees.

A lawn can be replaced in months, but it takes decades to replace a large tree, says University of Missouri Extension natural resources specialist Joni Harper.

Trees are a valuable part of home landscapes, says Harper. They beautify surroundings and help save energy by providing shade.

Signs of stress include wilted foliage, off-color leaves, leaf scorch, leaf drop and early fall coloration.

Most of the tree’s active roots are within the top 12 inches of soil and extend beyond the tree canopy. Prolonged heat and drought can bake the soil. This can damage or kill fragile roots critical to water and nutrient uptake, says MU Extension state forester Hank Stelzer.

Harper recommends the “screwdriver test” to check soil moisture. In a press release, Harper said “Try to push the head of the screwdriver 6-8 inches into the ground around the tree root zone,” she says. “If it doesn’t push in easily, it is time to water.”

In “normal” weather, trees need about 1 inch of water per week during the growing season. In dry weather, Stelzer recommends watering deeply every 6-7 days. As a drought becomes more severe or when air temperatures rise above 95 F, step up watering to every 4-5 days, he says.

When watering mature trees with well-established root systems, water the entire area beneath the tree canopy. Water slowly so that water can saturate the soil. This is true for sandy and clay soils. “In sandy soils, water tends to head straight down,” says Harper. “Watering slowly allows for more lateral spread.” When clay soils are watered too quickly, water runs off the surface and will not reach tree roots

Use a soaker hose to help the water reach the ideal soil depth. Another option is a sprinkler. When using the sprinkler, place an empty tuna can close by and run the sprinkler slowly until 1 inch of water collects in the can. Again, be sure to water the entire root zone beneath the tree canopy.

Young and newly planted trees are most vulnerable to drought since they are still establishing their roots. Stelzer says, “Remember, during the first year after planting, most of a tree’s roots are still in the root ball, which can dry out very quickly, especially in hot, dry and windy conditions.”

One simple method is to use a 5-gallon bucket with a few nail holes in its sides near the bottom of the bucket. Set the bucket next to the trunk and let water flow into the soil. Water daily for the first week, then twice a week for the next month or so. Fill the bucket either late in the evening or first thing in the morning. This way, the tree is ready to face the heat of the day, says Stelzer.

Proper care of young trees during drought also includes mulching, Harper says. Apply organic mulch such as wood chips to a depth of 2 inches to retain soil moisture. Inorganic mulch such as crushed granite might help but may not be as effective. In addition, it can absorb solar radiation and add to the heat stress, especially when temperatures climb above 90 degrees.

More information:

“Irrigating Trees and Shrubs During Summer Drought” (MU Extension publication G6870), available for free download at https://extension.missouri.edu/g6879.

“MU Extension Advice: Irrigating Trees in a Drought” (video), https://youtu.be/VVxOeCwqu-I.

“Tree Watering Tips: Caring for Trees During Extreme Drought,” Texas A&M Forest Service, https://lee.agrilife.org/files/2020/07/Tree-Watering-Tips-Caring-for-Trees-during-Extreme-Drought.