How to Water Mature Trees During Hot, Dry Weather

Summer Watering for Mature Trees

High temperatures and dry conditions are hard on trees, and if severe enough can cause roots to begin dying back. Drought stress also causes trees to be more susceptible to insect and disease problems, such as borers and canker diseases, which can cause tree death.

Young or newly planted trees and shrubs are most susceptible to drought injury, since they have not yet established an extensive root system.

Leaf scorch is a common symptom of drought stress in trees. It’s seen as browning on leaf edges of deciduous trees or the tips of evergreen needles. Under very dry conditions, leaf scorch will advance to yellow/brown leaves falling from the tree. However, even trees that don't exhibit leaf scorch can be experiencing drought stress.

Assessing Soil Moisture - Screwdriver Method 

The standard guideline is that trees should receive one inch of water per week, either through rain or irrigation. But clay soil, which dominates eastern Nebraska, can store a lot of water after heavy rain. So, how can you tell when the soil is dry enough to need watering? Use a simple, long-bladed screwdriver.

Test moist soil by pushing the blade in to a depth of 8-10 inches, noting the resistance. Then try an area of dry soil; it will be much harder to push the probe into dry ground. Once you get a feel for the typical resistance of dry soil, use the screwdriver to determine when soil is drying out and water is needed.

How Deep are Tree Roots? 

Research shows that a majority of water-absorbing tree roots are in the top 24 inches of soil. If this layer of soil is dry, so is the tree. Deep, infrequent irrigation (1 or less per week) is best for tree growth. Focus on watering the entire area beneath the tree’s dripline – the area from the trunk to the outer edge of the widest branches.

A mistake homeowners with in-ground irrigation systems often make is watering too often. Frequent, light applications of water (3 or more per week) is a very common cause of tree death.

Irrigating Clay Soil – Soak & Cycle Method 

Proper watering for clay soil is difficult because water soaks in slowly, and water applied too quickly runs off. To avoid runoff, apply water slowly using the Soak & Cycle method.

Allow the irrigation system to cycle through all the zones once, using a run time for each short enough, so no runoff occurs. When done, cycle through the zones a second time - in the same day – to push the water deeper into the soil. If necessary, you can even cycle through the zones a third time. Then don’t water again until the screwdriver test indicates the soil is getting dry (usually 1-3 weeks). One deep irrigation is much better for tree root growth than multiple shallow irrigations.

If using a hose and sprinkler, let it run in a low pattern in one area until the top 8-12 inches of soil is moistened, then move it as needed to water the entire area underneath the tree's canopy. A deep soaking every 2 weeks is adequate for most trees in unirrigated landscapes.