Dakota Gardener: Winter is life and death for plants

With the recent snowfall, winter is on my mind. The hard freeze killed our annual bedding plants and vegetables. Our hardy perennial flowers are very much alive but dormant for the season. How do our plants survive winter?

Our perennial flowers survive the winter underground. Aboveground leaves freeze and die. However, the plant's crown is protected and insulated by the soil. The soil just inches below the surface remains much warmer than the air temperature. In spring, the plant's crown produces new shoots and eventually flowers during the growing season.

It is not unusual to lose hardy perennials over the winter. Several factors may contribute to a plant's death. If we have little to no snow cover, the soil temperature may drop to the point where the dormant crown cannot survive. Snow cover is our friend in the north country.

Many gardeners protect their perennials by applying a layer of leaves over borderline hardy plants to insulate them from a snow drought. I leave the stems and foliage of my perennial flowers intact in the fall to trap the snow. To prevent rotting in spring, mulch and plant debris should be removed before growth starts.

Another factor that can kill perennial flowers is by having "wet feet" over the winter. This happens during a wet fall when the soil does not drain before winter comes. While plants do not have feet, their roots can rot in cold, waterlogged soils. Susceptible plants include Delphinium, Echinacea, Dianthus and Heuchera. Plants in heavy, clay soils are more susceptible to this kind of winter kill than in well-drained soils.

Many of us carefully select plants based on the U.S. Department of Agriculture (USDA) Hardiness Zone Map. Dakota Gardener readers are generally in zones 3b, 4a and 4b. If we are buying recommended plants for our region, why do some of them still die over the winter?

The USDA Hardiness Zone Map is an imperfect system based upon a 30-year average of each winter's coldest temperatures. For example, the southern two-thirds of North Dakota is in Zone 4a which has an average coldest winter temperature that falls between minus 25 to minus 30 degrees Fahrenheit. Have we seen temperatures fall below minus 30 degrees F? Yes, both Fargo and Bismarck reached minus 33 degrees F in 2019. Poor Hettinger, North Dakota, reached minus 45 degrees F on January 1, 2018.

So far, we've only talked about our perennial flowers with underground crowns. Our ornamental shrubs have aboveground branches that are exposed to the elements above the snow. In addition to being susceptible to winter's coldest temperatures, we need to think about the dangers of variable weather in fall and spring. For example, if we have a long period of unseasonably warm temperatures in April, the shrubs may leaf out. If followed by a late spring frost, the shrubs may experience damage.

With all kinds of extreme weather, it is a miracle that the vast majority of plants survive and even thrive in the north-central U.S. Let's be grateful that plant breeders are always releasing tough new plants to brighten northern landscapes.