A Killing Freeze, a Winter’s Injury to a Minnesota Garden

A Bitter Winter! How does this Weather affect the plants in our Garden?

There are two types of killing freezes that affect overwintering plant material. The first is a ground killing freeze. The second is winter burn, typically affecting evergreen species.

We know from trying to dig in spring that frost moves into the ground. We also know that plants can survive these low temperatures as rated by their hardiness, we are considered zone 4 on the hardiness map. That hardiness rating is based on a ten year average of low, and also high temperatures. In zone 4, plant species should hardy from minus 20 to minus 30 degrees Fahrenheit.

Ground Killing Frost

A ground killing freeze occurs when temperatures either drop down below those minimal temperatures, or a freeze-thaw-freeze cycle occurs. Plants vary, but hardy varieties survive either by their solid root structure or a viable crown. Luckily for us, snow is a great insulator. According to the Department of Natural Resources, Minnesota receives an average yearly snowfall of 36 inches in the southwest to 70 inches or more near Lake Superior. It is that snowfall which insulates the abrupt temperature changes that may happen during the winter months and stabilizes temperature fluctuations, keeping the temps at or around freezing.

Winter Burn, or Winter Kill

A foliar freeze, also called winter burn, can damage foliage on evergreen species. Evergreens keep their foliage throughout the year, typically shedding the older needles gradually, depending upon species. During the winter months stems freeze up, shaded by trunks, branches, and foliage. On a sunny day, the sun can thaw out foliage and cause moisture loss. The tree or shrub cannot replenish the moisture loss due to the slow movement of moisture within the stem and leaf tissue dies. During the spring of the year as the evergreen comes back to life this dead tissue rears its ugly head and turns brown.

How to Protect Against a Killing Freeze

So what can you do? I never cover my perennials. I figure that if one does not make it, it allows me to plant a new variety that has just been released. If, on the other hand, you have that special plant you hate to lose, cover the area with a mulch (ie hay or straw). It will stabilize the temperature fluctuations and protect the ground from the wind and sun. Another trick I use is during fall cleanup. I cut my perennials back to about 6 inches. This allows the leftover stems to grab the fall leaves that are blowing around. As for evergreens, you can wrap them with a burlap, paying special attention to the south and northwest (the direction the wind comes).