Growing Together: Lack of snow can be deadly to trees, perennials

Snow is an effective insulator, protecting roots that are more cold sensitive than above-ground branches. David Samson / The Forum

Two Norwegians met for coffee one day, and the first asked the other, “Why are you wearing only one overshoe?” The other replied, “Because the weatherman said there’s a 50 percent chance of snow today.”

We can laugh about snow or maybe cry about it, but snow’s importance in our yards and gardens is often understated. If snow is skimpy during winter’s first half, our trees, shrubs and perennials can be in big trouble if temperatures turn bitter in midwinter. An adequate layer of snow is a major aid in helping our adapted plants survive winter.

That’s why we select trees, shrubs and perennials that are rated winter-hardy in zones 3 and 4 across North Dakota and Minnesota. A plant winter-hardy in zone 3 can tolerate temperatures dropping between minus 30 and minus 40 degrees, while zone 4 plants are rated hardy to between minus 20 and minus 30 degrees.

If we select plant material adapted to our region’s hardiness zone, does that mean everything should survive winter if temperatures remain above these cold minimums? Not necessarily.

Lack of sufficient snow depth can cause winter injury to trees, shrubs and perennials. David Samson / The Forum


The roots of well-adapted trees and shrubs can’t tolerate the same depth of cold as their above-ground branches. Root systems are more cold-sensitive than branches, stems and trunks. While the above-ground branches of adapted plants can tolerate temperatures to minus 30 or minus 40 degrees, the roots will likely be injured or die if the soil temperature dips to between 0 and 10 degrees.

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Younger trees, shrubs and perennials are even more sensitive than established types. These plants survive because soil temperatures are normally higher than air temperatures. That’s why snow is vitally important to the survival of our adapted plants: it’s Mother Nature’s great soil insulator.

The ground is fairly warm in autumn after collecting heat all summer, and it slowly dissipates as air temperatures steadily drop and winter approaches. If there’s no snow to insulate the ground, heat, which rises, is quickly whisked up and out of the soil. It’s important that protective snow arrives before winter’s coldest temperatures attack exposed soil.

Snow is an effective insulator, having an average insulating value of R-1 per inch of snow depth, according to the National Snow and Ice Data Center. Twelve inches of snow provides a rating of R-12. In comparison, straw mulch has a value of R-1.45 per inch of depth. Researchers at Rutgers University found that a 9-inch layer of snow created a 42-degree difference between air temperature and ground surface temperature.

A generous layer of snow insulation has three important functions: It slows the loss of ground heat escaping upward out of the soil, it slows the penetration of cold temperatures downward into the ground and it prevents damaging effects of alternating freezing and thawing.

What do we do if snow is lacking and bitter temperatures are in the forecast and we don’t have straw or leaves available to insulate young trees, shrubs and perennials? Bags of wood mulch can often be found at retailers, and in the future, it might be wise for all of us to keep a few straw bales or bags of leaves on hand. The straw or leaves can then be reused in the summer vegetable garden.

If all else fails, and there’s at least a little snow, it can be scooped up and piled more deeply around the plants we wish to protect.

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Looking for gift ideas? Consider the 2019 Growing Together Gardening Calendar, a 12-month wall calendar graced with local gardening photos by Forum photographers and filled with nearly every "when-to" needed by gardeners tailored specifically for our Upper Midwest gardening region of North Dakota, South Dakota, Minnesota, Wisconsin and Montana. Order calendars online at https://www.inforum.com/2019gardencalendar.

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler’s Greenhouse in Fargo. Readers can reach him at forumgrowingtogether@hotmail.com.