How plants survive a Michigan winter
Most perennials grown in Michigan can easily handle a Michigan winter.
The winter of 2013-2014 will be remembered for its cold weather. December’s cold and snow started early and soon after the New Year we had real cold down to almost -20 F. You may remember the winter of 1993-94 was similar with a cold snap in January of 1994 where temperatures dropped to -25. It was so cold the pickle brine began to freeze in the pickle vats in Van Buren County that year. Livestock and Dairy farmers know that these are hard times for their livestock and they worry about feed, water and shelter for their animals. But how do plants manage in conditions when the temperatures are below zero and the wind chill is much colder?
Most perennials grown in Michigan can easily handle a Michigan winter. When the days shorten in September they began preparing for winter. Michigan State University Extension explains, with the first frosts these plants get their second cue that winter was coming and enter a form of dormancy call Endo Dormancy. Endo Dormancy means that something in the plant prevents growth. During winter the plant monitors temperatures above freezing to monitor the passage of the winter. Most Michigan plants need about 1000 hours of chilling. The best temperatures are those between 35 and 45 F. Once chilling is completed the plant can grow when warm temperatures return. This chilling requirement prevents the plant from growing during winter warm spells. Temperatures below freezing do not influence dormancy but do control cold hardiness.
Cold hardiness is the plants ability to withstand subfreezing temperatures. The plant does this by controlling when and where water freezes in the plant. Think of a plant cell as a soggy cardboard box with a balloon inside. The wet cardboard box is the cell wall, which gives the cell and the plant its shape. The balloon inside the box is the living cell. The constituents in the cell changed during the fall and early winter to lower the freezing point. Increased sugars and salts in solution lower the freezing point and proteins and membranes are changed to withstand colder temperatures. As the temperature falls below freezing, water begins to freeze between the cells. This ice outside the cell causes no harm and as the temperatures continues to fall more water moves outside the cell and freezes. This concentrates the solution inside the cell and lowers the freezing point. If the cell freezes it will be killed. If enough cells freeze we will see damage in the spring or collapse of the plant in the summer when it is stressed.
When the plants are dormant in the winter, they gain and lose cold hardiness depending on the weather. If the temperatures are below freezing, plants can actually acclimate to the cold and gain cold hardiness. A rule of thumb is that most of our fruit crops can handle 0F during the winter as their minimal cold hardiness. With colder temperatures, the amount of water frozen outside the cell increases and our cold tender plants such as peaches, blueberries and wine grapes can withstand temperatures down to -10F before we see injury to the fruit buds. Stems can handle temperature down to -25 in peaches. There is a lot of variation between grape varieties. Many of the European wine grapes suffer injury as the temperatures fall below -10 but the hardiest grapes can withstand -30F.
When temperatures fall below zero it's time to worry. If the weather has been cold, below freezing, for several days don’t worry unless the temps drop to -10F. Once the tempeture drops below zero start to worry about peaches, blueberries and wine grapes. If the temperature has been above freezing recently the plants have lost cold hardiness. A rule of thumb is 2 days above freezing, with the night time low above freezing, and the plants will lost their acclimation to cold and be back to the 0F damage threshold. The 0F threshold is only for cold tender (a relative term) plants such as peaches, wine grapes and blueberries. More cold hardy cherries and European plums that can withstand -20 F. Apples and pears should be able to go to -25 with little damage and you could expect damage when temperature fell below -30 F. These temperatures are for the fruit buds. Stems can handle colder temperatures.
If the temperature have been above freezing for several days and quickly drops more than 50 degrees, it's reasonable to worry. This means we had a lot of free water in the plant. All this water can freeze quickly preventing the orderly controlled freezing, which allows the plant to withstand real cold temperatures.