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Colorado Plants Don't Like Sudden Temperature Swings

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture/plant pathology

"If you don't like the weather, wait awhile."

That's the advice of Colorado natives to newcomers. Few of us -- whether we've lived in the state a long time or have just arrived -- are prepared for the snowstorms that may hit Colorado in early fall. And, our plants are even less prepared.

Think back to the many days when the temperature one day was in the 80's, and in less than 24 hours, had dropped to below freezing. And then it snowed.

Such sudden temperature swings can harm plants, no matter what the season, but are especially harmful when they occur in the growing season. Under these conditions, trees and shrubs have not had a chance to "harden off." A plant's natural response to cooler temperatures and shorter day lengths is to move water and nutrients to the center of the tree (not to the roots), and leaves will begin to fall off. The process occurs slowly and during a normal year, the tree will be hardened off completely by the time the first frost hits sometime in October or November.

But, plants have not hardened off when an early freeze hits in September. They are still functioning and still several weeks away from dormancy.

What can we expect the results of a September cold spell and snowfall to be on trees and shrubs?

The lessons of a sudden freeze might be instructive. Pines are the first to show symptoms. A few weeks following the freeze, needles begin to turn brown, often appearing green with brown bands. In some cases, needles turn half green and half brown. The following spring, many deciduous trees and shrubs fail to leaf out. Buds have been killed outright by the temperature drop. Those trees that do have enough stored energy to leaf out often can't support this new growth, once the weather turns hot. As a result, branches that appear healthy suddenly wilt and die. Bark of trees crack and split, leading to long-term stress and decline.

Think of it this way: Mother Nature is here to remind us why many trees and shrubs weren't meant to survive in Colorado's climate. If left to its own rule, Mother Nature surely would turn our environment back into the treeless plain it was meant to be.

Photo: Judy Sedbrook.

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Date last revised: 01/05/2010