June 20, 2008

In Dry Weather, Large Trees Need Watering Once a Week


The end of last summer was dry. We finally called it a drought. And the impact of that dry period is still yet to be felt in its entirety.

Of course, all summers may not be that dry, but most are dry. And in spite of what you may have heard (or believed) to the contrary, our large trees do need supplementary watering in the dry months.

But how do you do it? Maybe you're thinking you're OK because you have an irrigation system. But even if you do, some extra effort is probably going to be required if you're going to water your large trees correctly.

First, the big trees that live here in our clay soil don't like being sprinkled for 20 minutes every other morning like they were zoysia or some exotic bedding plant. Their roots are a little deeper. Not much deeper, but deeper.

That means they prefer a slow soak for 2 to 3 hours per area, but only once a week in those weeks that we don't get at least an inch of rainfall.

Watering too frequently and too shallowly never lets the soil dry out and breathe. And constantly wet soil can rot roots.

Soaker hoses are usually better at watering large trees correctly than are sprinklers, because their application rate is slower. I've seen soaker hoses in two styles: The flat kind that creates hundreds of fine little mist streams about a foot high and the round black porous ones that ooze water all along their length.

Whichever you use, the application rate needs to be slow, because heavy fine-pored clay takes up water slowly and also because water applied rapidly to any slope will only run to the gutter. If your driveway is wet, slow down the application of the water.

Visualize your tree as the center of a circle and the limb tips as the perimeter of that circle. Apply water in the middle of that circle and work outward to a bit past the perimeter, which is often referred to as the "dripline."

You should try to keep the trunks of your large trees as dry as possible, not only because there are fewer non-woody feeder roots there but also because continuing excessive moisture around the base of a tree can be implicated in the development of irreversible root rot and mechanical destabilization.

This is especially important for trees that are surrounded by dense plantings of close dense plants or mature growths of ivy or vines. If possible, move the plants and get rid of the ivy on and around the lower trunks.

And if you have an irrigation system in place and if one or more of those heads soak your tree (or trees) constantly around its base, check with your service company to see how that can be stopped or at least minimized.

Certified arborist Fred Morgan of Cordova has owned and operated Morgan Tree Service since 1974, and is involved in arboricultural consulting, diagnostics and problem solving. His column appears in Home & Garden once a month. Get more tree info or contact him at morgantreeservice.com.

Originally published by FRED MORGAN .

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