By Jim Lakin, M.D., Master Gardener and Brynne Eisele
We’ve all heard that there are only two seasons in Minnesota—Winter and Getting Ready for Winter (or is it Road Repair?). In any event, Minnesota’s harsh winter can cause severe damage to landscape plants. The big three threats are sun, cold and critters. Happily, there are a number of things you can do this fall to protect your trees and shrubs from the Terrible Trio.
On a clear winter day, the sun can heat up the bark of a tree or the needles of an evergreen to the point where the cells will come out of dormancy. When the sun sets or goes behind a cloud cover, the temperature in the cells rapidly drops, killing the exposed plant tissue. This “sun scald” is most likely to occur on the bark of young or thin-skinned trees such as cherry, crabapple, honey locust, linden, maple, mountain ash or plum. To prevent sunscald, wrap the trunk of susceptible trees with a white plastic tree guard available at most nurseries and on-line. Stay away from brown paper tree wrap or black tree guards as they will absorb heat rather than reflect it. You should wrap newly planted trees for at least two winters and thin-barked species for five or more winters. Apply the tree guards in the fall and remove in the spring after the last frost.
Evergreens are susceptible to similar injury, especially with early cold weather in the fall or a spring cold snap after new growth has begun. Although all evergreens are susceptible to this “winter browning”, yew, hemlock, and arborvitae as well as new transplants are especially so. Several measures can reduce winter evergreen injury.
When planting susceptible species, consider placing them in areas protected from wind and winter sun such as the north or northwest side of buildings. If a plant is exposed, a barrier of burlap on the windward (usually the south or southwest) side can help. As with deciduous trees, watering in the late fall before a hard frost can make a big difference. Don’t prune after August as this may stimulate growth that a frost will “nip in the bud.” Commercial anti-desiccants and anti-transpirant sprays have not been found to be helpful.
Cold weather can kill trees and shrubs if conditions are right. The cold accomplishes this be freezing the root system which is much more cold-sensitive than the stems of branches. What normally protects the root system of a tree or evergreen is the relative warmth of the surrounding earth. The soil cools less rapidly than air. Any insulation from a snow-cover or mulch will further increase soil and root warmth. Thus, to reduce root injury cover newly planted trees and shrubs with 3 to 4 inches of shredded wood mulch. You should pull the mulch away from the trunk about 6 inches to create a “doughnut” not a “volcano”! Watering is also critical. Moist soil retains heat much better than dry soil. If the fall has been dry water heavily before the ground freezes. This will reduce frost penetration. Before you mulch, check new plantings for cracks in the soil. These act as cold air conduits. Fill them in with soil.
If you have a manageable number of trees and shrubs to protect, the best defense is to surround the plant or trunk with ¼ inch mesh hardware cloth 6 inches away from the plant. This will prevent animals from reaching through to do their damage. Keep in mind the anticipated snow line as it’s important to protect 1-2 feet above the snow line with the hardware cloth to prevent animals from nibbling partway up the plant. It is also important to bury the bottom of the hardware cloth 2-3 inches below the soil surface to reduce the chance of animals tunneling underneath. For smaller, less established trees you may be able to use plastic white tree guards. If you have large number of trees and shrubs to protect, consider applying an organic repellant.
Reducing the desirable habitat will help decrease protective cover and nesting locations. This can be done by cutting grass and other vegetation short in a 2 foot radius around young trees and shrubs. It also helps to remove brush piles and add fencing to other hiding places such as under decks.
While hungry animals in winter may gnaw on your trees and shrubs no matter the protective measures you take, these methods can significantly reduce the amount and severity of damage.
Deer, however, are a different story. They often adapt to repellents and a change of product is frequently necessary to discourage them. In late winter when they are starving, even that won’t stop them. Thus, a physical barrier is often the only way to manage deer. That can be a tough proposition though. Fences should be at least 8 feet high and of sturdy construction. However, a hungry deer if given a running start can clear a 12-foot barrier. You might try an electric fence such as used for cattle. Deer populations have been exploding in Minnesota as we have eliminated their natural predators. Maybe a few wolves here and there aren’t so bad.
For more information on this complex subject, check out the University of Minnesota Extension: https://extension.umn.edu/planting-and-growing-guides/protecting-trees-and-shrubs-winter