Cold injury to Mahonia (70197 bytes)

Cold Injury to Woody Plants

By Carl Wilson, Extension Horticulturist, Denver County

Two periods of cold injury to woody plants are of interest to people in spring 2001. One is winter damage to some woody plants. The other is more recent spring damage along the Front Range due to late snowstorms and associated freezes.

The winter damage has puzzled people because many didn't believe the winter was that cold for that long. The damage may be attributed to temperatures that dipped the second week of November before plants were fully acclimatized for winter. On November 12, temperatures in Denver-Boulder dropped to 1 degree. In the western part of the state, Norwood experienced 1 degree temperatures while Hayden and Meeker dropped into negative numbers.

Winter damage symptoms seen this spring in western Colorado were split grape vines, spruce and juniper foliage burn and deciduous tree branch dieback. Mahonia seemed particularly hard hit as it was in the Denver Metro area, even in cases where it was well watered.

The sudden drop in temperature in November as well as fluctuating up and down temperatures through the winter are hard on woody plants. It would be ideal if temperatures would gradually get cold and then stay cold for the winter. This certainly was not the case in the Front Range.

The upslope spring storm and freeze along the Front Range on May 20 dropped temperatures from the mid-70's to the 30's in an hour and left several inches of snow. This late storm completely froze the new foliage of late emerging vines such as grape and woodbine. Tree foliage showed various degrees of damage depending on location and stage of leaf emergence. Newly emerged leaves are more tender than leaves that have been hardened with time.

As a general rule, deciduous woody plants that lost foliage will grow new leaves when affected by freezes, hail or severe insect damage in the first half of the growing season. Foliage lost in the second half of the summer is generally not replaced until the following year.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010