Summer tree care

August 16, 2007 11:00 pm

The hot days of summer can be tough on trees and most will need some extra water to make it through.

When water is in short supply, trees cannot function normally. A tree that is suffering from moisture stress may have wilting or curled leaves, yellow leaves and leaves that are brown along the edges. If the lack of water is severe, the tree will lose its leaves and will eventually die if the deficit is not corrected.

Trees absorb water from the soil through their roots. Contrary to popular belief, most tree roots are not located directly beneath the tree and few roots are found deep in the soil. As much as 90 percent of a tree' s absorbing roots are within the top 15 inches of soil. Since the tree's branches act like an umbrella, most of the absorbing roots are at the dripline, the edge of the branches. Mature trees may have roots extending 6 to 50 feet past the dripline.

Newly planted trees possess only a small percentage of their original roots and must be closely monitored and watered carefully. Young trees should be inspected at least once a week to determine if watering is necessary, and more often during hot, dry weather.

A good rule of thumb is to water newly planted trees three times a week, applying five gallons of water each time. To prevent wilting, young trees should be watered more often than older plants. After they become established, in one or two years, allow the ground to dry between watering. The trees will adapt to the stress and become more drought-tolerant.

During extended periods of drought, large established trees also need to be watered. A single, large tree can transpire more than 100 gallons of water on a typical summer day. Watering should occur when less than an inch of rain has fallen in a two-week period and temperatures have passed 85 degrees. If the soil is dry, water with a sprinkler or soaker hose to a depth of 18 inches. The most active water absorption area is at the drip line and beyond. This is where you should water.

Avoid watering close to the trunk, this can cause root rot. An inch of water will usually wet the ground 6 to 8 inches deep, depending on the soil type. For practical purposes, plan on providing an inch or two of water every two weeks. In order to saturate the soil, apply the amount in a single, slow application. Several light applications over the course of two weeks tend to force roots to the surface in search of water. These surface roots are prone to drying out when the soil surface starts drying. A single, heavy application encourages roots to grow deeper.

An inch-thick can, like a cat food can, may be placed near the dripline when sprinklers are used. As soon as the can is filled, empty it and allow it to be refilled once more.

Morning is the best time of day for watering when using a sprinkler. Soil soakers can be used, but they tend not to soak in as deep within an average time period. Soakers almost need to be left running overnight to deliver an adequate amount of water.

Caution: watering too frequently can also kill trees. If the soil is continually saturated the roots will suffocate. Always check the moisture status of the soil around your tree before watering. An easy way to check soil moisture is to use a metal rod. Choose a spot halfway between the trunk and the dripline. If you can't easily push the rod into the soil to a depth of 3 or 4 inches, it's time to water. You have watered enough when you can push the rod to a depth of 12 to 18 inches.

One more thing you can do to help trees through the summer is mulching. A 3- to 4-inch layer of mulch applied 1 foot out from the trunk to beyond the dripline will greatly reduce evaporation and keep the water in the ground for the tree to use. Organic mulches like bark or compost have the added advantages of keeping the ground cool and adding nutrients to the soil. Don't put plastic sheeting down under the mulch because it reduces oxygen in the soil and creates poor growing conditions for the tree.

If you have any questions about summer tree care, call the parks division at 962-1352.


Gustafson is the tree care educator for the City of La Grande Parks Division.