How cold is too cold for Michigan fruit crops?

Extremely cold temperatures in January have the potential to damage Michigan fruit crops during winter dormancy. Peaches, blueberries and wine grapes are the most cold tender.

It is not unusual to have plants damaged by winter cold. Winter injury normally occurs from three possible causes.

  1. It got too cold early, before the plants were sufficiently hardened to handle the cold.
  2. It got cold late in the winter after growth began and plants had lost the ability to acclimate to increasing cold.
  3. Severe winter cold – these events are usually the result of a mid-winter warming event followed by severe cold.

Most of the perennial plants grown in Michigan can easily handle a Michigan winter. When the days began to shorten in August and September, the plants began preparing for winter. As we got the first frosts of the winter, these plants got their second cue that winter was coming (see the Michigan State University Extension article, “Fall color show and winter dormancy in woody plants”). In the winter many, plants enter a form of dormancy called endo-dormancy, meaning that something inside the plant inhibits its growth. The plant monitors the time the temperatures are above freezing to monitor the passage of the winter.

Most Michigan plants need about 1,000 hours of chilling. The best temperatures are those between 35 and 45 degrees Fahrenheit. Once chilling is complete, the plant can grow when warm temperatures return. This chilling requirement prevents the plant from growing during winter warm spells (see the MSU Extension article “Winter dormancy and chilling in woody plants”).

During dormancy we often talk about cold hardiness. Cold hardiness is the plant’s ability to withstand subfreezing temperatures. The plant does this by controlling when and where the water freezes in the plant. I think of a plant cell as a soggy cardboard box with a balloon inside. The wet cardboard box is the cell wall made of cellulose that 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 the cell solution lower the freezing point. Proteins and membranes are changed to withstand colder temperatures.

As the temperature falls below freezing, water begins to freeze between the cells. As the temperature continues to fall, more and more water will migrate outside the cell and freeze. This ice outside the cell causes no harm and as temperatures continue to fall, more water moves outside the cell and freezes. This concentrates the solution inside the cell and lowers the freezing point even more. If the cell freezes, it will be killed. If enough cells freeze, we will see damage in the plant as growth begins 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 (see the MSU Extension article “Winter cold hardiness in Michigan fruit crops”). My rule of thumb is that most of our fruit crops can handle 0 F during the winter as their minimal cold hardiness. With colder temperatures I expect our plants to handle more cold. Our cold tender plants such as peaches, blueberries and wine grapes can withstand temperatures down to -10 F before we see injury to the fruit buds and the stems can handle temperature down to -25 F in peaches.

There is a lot of variation between grape varieties. Many of the European wine grapes suffer injury as the temperatures fall to zero and below -10 F, but the hardiest grapes can withstand -30 F. This -10 F threshold is only for cold tender – a relative term – plants such as peaches, wine grapes and blueberries.

More cold hardy are cherries and European plums that can withstand -20 to -25 F. Apples and pears should be able to go to -25 F with little damage and I would expect damage when temperature fell below -30 to -35 F.

If the high or low temperatures drop more than 50 F, I worry, 70 F I really worry. This means we had a lot of free water in the plant, or as the growers would say the sap is up. All this water can freeze quickly and this prevents the orderly controlled freezing, which allows the plant to withstand real cold temperatures. These temperatures are for healthy plants that are acclimated to the cold.

If the weather has been cold – below freezing – for several days, I don’t worry unless the temperatures drop to -10 F, then I start to worry about peaches, blueberries and wine grapes. If the temperature has been above freezing recently, the plants have lost cold hardiness. The plants will have lost all their acclimation to cold weather and will be back to the 0 F damage threshold.

There is actually a fourth cause of winter injury and that is when the plants were weakened going into the winter. It takes a lot of metabolic energy to change the constituents of the cell to get ready for winter. Plants that have been stressed by insect or disease pests, heavy crops or poor growing conditions such as drought do not have the ability to harden off to withstand cold levels as low as healthy plants. Work done by MSU’s Stan Howell here in Michigan over 40 years ago indicated that weak plants were slower to develop cold hardiness, did not acclimate to as low a level as healthy plants and developed later in the spring. This means that stressed plants will be much more susceptible to winter cold injury than healthy plants in the fall and winter.

2013 was a stressful year for many Michigan fruit crops. After the disastrous spring of 2012, most fruit crops had little if any crop in 2012 (see the MSU Extension article “With a backward spring, Mother Nature pitches a change-up after a fastball”). These rested trees, vines and bushes entered 2013 with a lot of vigor and many fruit buds. Many Michigan fruits had bumper crops in 2013. Plants that were stressed by heavy crops in 2013 along with drought or other problems will be much more likely to suffer from the cold in January 2014.

There is no quick way to assess winter injury during the depths of winter. You need to collect shoots and carefully dissect the buds and look for damage. Later in the year after the plant’s chilling requirements have been met, you can collect shoots and force them looking for injured buds. See the MSU Extension article “Forcing cuttings to determine the end of dormancy in fruits and other plants.”

For more information, see the related MSU Extension articles

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