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Sometimes it's hard to realize that the cold snow can protect tender plants over a harsh winter. How can the cold snow warm up the plants from underneath?

We think about insulation in the context of homes, where it separates the cold outside air from the heat we generate indoors. It's true that neither the snow, nor the plants, nor even the soil are able to generate any heat.

But hot and cold are relative terms. An object that is warmer than its surroundings has more heat, even if the object and the surroundings are well below freezing. Insulation slows the transfer of this heat from the warmer object to the surroundings. Insulation prolongs the time it takes the higher temperature to cool down to the lower temperatures.

Most gardeners understand that it's extreme low temperature that damage tender plants. They are also aware that temperatures fluctuate over the course of the day, with the coldest temperatures generally reached overnight or just prior to dawn. Also, soil accumulates heat throughout the summer and fall. The earth itself retains heat well, and even if fully exposed to the temperatures of winter; it will take a matter of days or even weeks to cool down to the air temperature. If it is covered with snow or some other insulation, such as leaf matter or straw mulch, this cooling may be prolonged to months.

By late January when the coldest weather usually hits, the soil insulated by a deep snow cover may only have cooled to the equivalent air temperature experienced in November or early December. The tender roots of perennials and above ground stems of smaller shrubs may only feel as cold as the air temperature of late fall.

For the past few winters we have experienced a lack of snow. In fact, a couple of years ago the lack of snow caused a deep frost, which in turn froze many septic and mound systems. That was the year I learned that blueberries set their buds in the fall for next year's harvest in the fall.

Due to the lack of snow cover, the buds were damaged so the next summer I didn't get any blueberries.

At the Fruit Growers Conference, in St. Cloud I attended last February, I asked how they protected the blueberries at the Grand Rapids Experiment Station. They told me that even in years where there is adequate snow; they use large snow blowers to completely cover their berries. When we returned from St. Cloud I scraped up as much snow as I could find and put it on my blueberries. It must have helped, as my crop this past summer was good.

If the snow cover is sparse, it is good to put a shovel of fresh snow over the more tender perennials and small shrubs. Do this before the really cold weather hits. You can also achieve the same effect by mounding a thick leaf or straw mulch over the plants. However, even the best insulation can't protect against prolonged cold spells.

There are some down sides to a year of heavy snow. The weight of the snow can damage delicate shrubs, especially if the snow is heavy and wet. Also, frequent freeze-and-thaw cycles can turn fluffy snow cover into chunks of ice. A deep snow cover also may create a comfortable place for mice to make a winter home for winter. This may result in critters chewing off the bark for their winter meal.

Some plants, such as lilies, are susceptible to prolonged periods of wetness in spring as the snow melts. Therefore, it is important that there is good drainage around these plants.

Parts of this article are taken from an article by Jim Kohut and Stefan Fediuk in the November/December 2006 issue of Northern Gardener magazine.

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