Trees in winter

Every Minnesota winter is a little bit different – but in general they can produce stress and sometimes even injury – not necessarily to us but to our trees and woody shrubs. Cold, sun, wind, ice, salt and animals can all contribute to winter damage in our landscape, but there are some things we can do protect our plants.

The cold – low temperatures – is most threatening to plants which are not hardy to zone 4, but even those which are hardy to zone 4 or even zone 3 can be threatened by abnormally low temps, especially if they go into the winter stressed or there is a lack of sufficient snow cover. Trees and shrubs can be stressed from drought, which is why it is very important to continue to water them throughout the fall until the ground freezes or the snow falls. Roots are less hardy than stems and branches; they can be killed at temperatures below zero and even up to 10° above. Fortunately, the soil cools down slower than the air so the ground will be warmer. Moist soil holds more heat than dry soil, another reason to keep watering in the fall. Insulating the roots is also important. Snow is an excellent insulator, but since we can never predict how much snowfall we will have in any given winter it is important to mulch. New or young trees should have 6-8 inches applied at the base.

Believe it or not the winter sun can do great damage to trees and shrubs. Sun reflecting off the snow can cause sunscald which appears as cracked, fried or sunken bark on the south or southwest side of a tree or woody shrub. The sun heats up the bark and encourages cambial growth. When the sun is blocked by a cloud, building or other structure the bark temperature drops very quickly killing the active tissue. Thin barked trees such as maples, crabapples or honey locusts are most susceptible to sunscald, as are trees that have been recently trimmed or moved from a shady to a sunny spot. Older trees have thicker bark and are less vulnerable. To protect a tree from sunscald the trunk should be wrapped with a tree wrap or a light colored material in late fall until early spring. Young trees should be wrapped for two winters; thin skinned trees will benefit from being wrapped for up to five years. If a tree does suffer from sunscald you can aid the healing and recovery. Cut out the dead bark back to live tissue, rounding off any sharp edges of the wound. There is no need to use any kind of wound dressing but you should wrap the tree in subsequent winters. You may want to use a fungicide on the wound to protect against fungal infection.

Winter browning and bleaching of evergreens is often caused by the wind and sun. Winds can cause excessive loss of water vapor through the foliage while the roots are frozen and unable to replace the moisture. The result is the withering and browning of the foliage. Bright sunny days can also cause foliar damage when it stimulates cellar activity. When the sun goes down and the temperature falls quickly the result is the foliage is damaged or killed. The sun can also destroy the chlorophyll in the foliage resulting in bleaching. To protect evergreens from winter damage from the wind and sun, avoid planting on the south or southwest facing sides of your home or other structures. For evergreens already planted in vulnerable spots you can protect them by propping pine boughs or Christmas tree branches against them on the south, southwest or windward sides. If boughs are not available you can build a barrier of burlap to surround the tree. Keeping evergreens properly watered throughout the growing season will also help to protect evergreens from winter damage. If they are injured, wait until mid-spring to prune. The brown foliage is most likely dead and should be pruned back to living tissue, but buds which are more cold hardy will most likely grow and fill in where the damaged tissue is removed. After pruning ,fertilize and water well.

Other winter threats are ice and snow accumulating on branches and bending and breaking them. Salt used to melt ice on streets and pavements can be injure roots and if it is absorbed by a tree or shrub in runoff can cause foliar damage. Avoid planting in areas that are likely to be exposed to salt. And fluctuations in temperatures can cause heaving of the crown of a plant. This can be avoided with 4-6 inches of mulch applied in the fall.

Most of the plants, trees and shrubs we use in our landscapes are hardy in our climate, but even then they can all benefit from a little protection from our Minnesota winters.