HORTICULTURE Winter Cold Hardiness in Woody Plants
Most of the plants that grow here in Michigan can handle our extreme winter temperatures. These plants can handle temperatures to 10F easily and can handle temperatures below zero if the weather has been cold for several days. Plants need two things to prepare for winter, short days and cold temperatures. The shortening days of summer and fall are the environmental clues that tell the plant that winter is approaching. The plant stops growing and begins to make changes that will allow it to withstand winter cold. The plant cells accumulate sugars and salts that act as antifreezes and the fats in the cell membranes change to polyunsaturated fats that stay liquid at lower temperatures. This makes the cell membrane is less likely to crack during freezing and thawing. Most of these preparations are finished by the time winter comes and our plants are dormant and cold hardy by Thanksgiving. At this point most perennial plants can withstand temperatures down to 10F with no damage. This is sometime called the base cold hardiness level.
Plants can gain more cold hardiness as temperatures go below freezing. Plants gain cold hardiness when the temperature stays below freezing. Some of this is due to changes in the plant cells and some is caused by water freezing outside the cells. Cell water moves outside the cell to freeze with the ice, which is already there. Eventually the plant reaches a point where it can accumulate no more cold hardiness. For most Native Michigan plants this around -25 F. Some of our ornamental plants and fruit crops can suffer injury at warmer temperatures. Peaches and Forsythia begin to lose flower buds at around -15 but the plants themselves can withstand -20 to -25 F. Wine grapes and tender perennials usually suffer damage from -10 to -15F. Wind chill is not a problem for plants. It is the actual air temperature that determines the plant temperature they can't get any colder than the air.
Long cold spells are not a problem as long as the temperature does not drop rapidly. Steady drops in temperature or long periods below freezing do not cause winter damage unless the temperature drops below -25 as it did in February of 1994. What causes most winter damage is rapid changes in temperature. The passage of two cold fronts one after another when the temperature drops rapidly into the below zero range may damage plants because they are unable to accumulate more cold hardiness before the temperatures become really cold. Another way we get winter injury is when we have a winter thaw and temperatures are above freezing for several days in a row. These warm temperatures cause the plant to lose cold hardiness that it has accumulated and predisposes it to injury due to a rapid drop in temperature down below its basic cold hardiness level.
