How Cold is too Cold

How Cold is too Cold?

In the first week of January 2004, Southwest Michigan received a chilly blast of arctic air and temperatures dropped down into the single digits. This would have been a normal occurrence except the final weeks of December 2003 were pretty mild. Most perennial plants were dormant for the winter. Perennial plants get ready for winter after they receive two clues from Mother Nature. The first clue is the shortening of day length that takes place in September and October. The second clue is cold weather. This year we got cold hard frosts in the first week of October.

I actually went back and looked at the weather records for December of 2003. Our high temperatures were in the 40s and 30s and the lows were usually in the 20s. These are actually good conditions for perennial plants. The bad news is that on January 2 and 3, the high was 60 and the low was 41. This means that for two days the low temperatures were above freezing. On January 6, the high was 15 and the low was 4.

This sudden drop in temperature is what causes winter cold injury to plants. As a rule of thumb, I assume that most plants grown here in Michigan can withstand temperatures down to 10 or even zero in the winter without any preconditioning cold. Cold tender plants that can survive here should be able to handle 10 above zero. Those that we think of as cold hardy should be able to handle zero. If the cold event is preceded by cold weather most plants can handle temperatures below zero. Many of out trees, shrubs and vines can handle cold down to –30 if the drop down to that temperature is gradual and not sudden. If the cold snap is preceded by warm weather then I use the warm zero to 10 degrees figure. With the warm weather that preceded the January cold snap, I think that we will see some damage to cold tender plants and to young one to two-year-old plants.

Thirteen months ago we saw a similar cold event with a fairly rapid drop in temperature to lows around zero and below in the first week of December 2002. The following spring we saw damage to tender grapes and young fruit trees that were planted in the spring of 2002.

Come spring we will see damage to young trees and cold tender plants, especially those that had a hard time during the growing season so that they were weak going into the winter.

Posted: January 5, 2007