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Published June 09, 2010, 08:35 AM

Growing Green: Water your trees

Planting has been very challenging this season.

By: Robin Trott, Extension Educator, The Osakis Review

Planting has been very challenging this season. The excessive warmth of Memorial Day weekend, the relatively low humidity, and the windy weather has damaged many young transplants. A spot of well placed water (or sustained sprinkler action) has retrieved many plants from the brink of a parched death, but the worst drought damage has been to our trees. I have seen many maples, ashes and apple trees suffering from leaf scorch this week, which is a direct symptom of hot, dry, windy weather and inadequate watering. Symptoms of drought stress can be sudden or may occur over time. Deciduous leaves may develop scorch, brown outside edges or browning between veins. Evergreen needles may turn yellow, red, brown or purple. In sustained drought, leaves may be smaller than normal, drop prematurely or remain attached to the tree even though brown. Although drought stress may not kill a tree outright, it could set it up for more serious secondary insect and disease infestations in following years.

Trees lose an incredible amount of moisture through the “pores” (stomata) in their leaves in a process called transpiration. When it is hot and dry, trees “cool off” through this process. Nutrients and moisture are pumped to the leaves, the stomata open up, and the tree is cooled. On a hot, windy day, a mature tree can transpire 50 gallons of water! All your trees (even large ones) need to be watered to maintain healthy growth.

The best way to water a tree is with a soaker hose or drip irrigation. Lay out the hose circling from 3 feet from the trunk to the drip line. Run the hose until the tree has received 10 gallons or water/inch diameter of the tree trunk. Watering your lawn does not count as tree watering, even if your sprinklers run under the tree. Trees need deep, slow watering once a week to encourage deep root growth. The best time to water your trees is during the evening hours (or overnight for soaker hoses.) During these cooler hours, trees have the chance to replenish their water resources without being stressed by the heat of day, and results in less moisture loss from evaporation. Begin watering trees as soon as the ground is frost free in the spring, and continue until the ground freezes solid in the early winter. This consistent deep watering will give your trees the best chance to survive the stresses of hot summer days and windy frigid winter nights.

Use organic mulch under your tree to maintain soil moisture. Four inches of bark, cocoa bean mulch or grass clippings spread under your tree but not against the trunk will keep the roots cool and the soil moist. Do not use river rock for mulch under trees, as rocks will absorb the heat of the day and radiate this heat into the soil beneath them. Remember, cool, moist soil is your ultimate goal.

For more information about watering trees, please visit our website at: http://www.extension.umn.edu/Drought/components/wateringtrees.html,

or contact me at the Douglas County Extension Office at 320-762-3890.

Until next time, happy gardening!

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