We are quickly approaching winter and in this upper Midwest we will experience some of the lowest temperatures in the USA. One undesirable side effect of winter is dead plants in our landscape come spring.
A plant’s ability to survive winter is normally attributed to the low temperature generally experienced at the site(see USDA Plant Hardiness Zone Map) and the lowest temperature that plant is considered to be hardy to. However, there are two other factors to consider when contemplating a plant’s cold hardiness.
First is a plant’s potential to acclimate, meaning its ability to increase tolerance to freezing in response to shorter days and increasingly cold, non-freezing fall temperatures.
The second is de-acclimation — a loss of tolerance to freezing temperatures due to warming temperatures, longer days and/or metabolic activity and growth. Typically de-acclimation occurs in midwinter when plants are exposed to above freezing temperatures for a duration as short as 48 hours. Moreover, when exposed to repeated cycles of above freezing temperatures, plants do not regain their maximum level of acclimation or cold hardiness. This means that while we may look forward to that January thaw and other warm-ups during deep winter, some plants cannot thrive through these times like we can.
Weather data over the last six decades shows a steady increase in the occurrence of 48 or more hours of above freezing temperature (de-acclimation) events during the winter, while absolute winter low temperatures based on the USDA Plant Hardiness Zone Map are rarely reached. Clearly, in this modern era, plants appear to have a higher likelihood of being damaged by cold due to de-acclimation than to the absolute winter cold temperature.
This article is from “The Arboretum” magazine by Dr. Stan C. Hokanson from the University of Minnesota. I did little addition or editing but think it is an important topic as we face another winter and unknown weather. The Master Gardeners often get inquiries about why this plant or tree died over the winter and I hope this information gives our readers some insight into a possible cause.
Be sure your gardens, trees, shrubs and perennials are mulched after the ground freezes. Water those same plants well up to freeze-up, giving them the benefit of good care to get through the winter whatever it brings.
Click on “Yard and Garden at the University of Minnesota Extension website www.extension.umn.edu for gardening information. Local Master Gardeners will respond to your questions via voice mail. Call 218-444-7916, leaving your name, number and question. Our Facebook page may also be of help to you: https://www.facebook.com/Beltramicountymastergardeners/.