Minnesota’s harsh climate is often responsible for severe damage to plants in the landscape. Winter sun, wind and cold temperatures can bleach evergreens, damage bark, injure or kill branches, flower buds and roots.
Browning or bleaching of evergreen foliage occurs in three ways. First, the winter sun and wind causes excessive transpiration (foliage water loss) while the roots in frozen soil are unable to replace lost water. This results in the browning of the plant tissue.
The second reason for bleaching is the bright sunny days during the winter causes warming of the tissue, which, in turn, causes cellular activity. Then, when the sun is quickly shaded, the foliage temperature drops to a level where the tree is injured or killed.
Another reason for browning or bleaching is during bright, cold winter days. Chlorophyll in the foliage is destroyed and is not re-synthesized if the temperature is below 28 degrees.
Damage normally occurs on the south, southwest, and windward side of the plant, but in severe cases the whole plant may be affected. Yew, arborvitae and hemlock are the most susceptible, but winter browning can affect all evergreens. New transplants or plants with new, late seasonal growth are particularly sensitive.
There are several ways to minimize winter injury to evergreens. The first is proper placement of evergreens in the landscape. Yew, hemlock, and arborvitae should not be planted on south or southwest sides of buildings or in highly windy or sunny places. Another way to reduce damage is to prop pine boughs or Christmas tree greens against or over evergreens to protect them from wind and sun and to catch more snow for natural protection.
Winter injury can often be prevented by constructing a barrier of burlap or similar material on the south, west and windward sides of the evergreen. If a plant has exhibited injury on all sides, surround it with a barrier but leave the top open to allow for some air, and light penetration.
Keeping evergreens properly watered throughout the growing season and into the fall is another way to reduce winter burn. Never stress the plant by under or over watering it. Decrease the watering slightly in September to encourage hardening off then water thoroughly in October until freeze-up. Watering only in the late fall does not help reduce injury.
Anti-desiccant and anti-transpirant sprays are often used to prevent winter burn. However, studies have not shown them to be effective for winter protection.
If an evergreen has suffered winter injury, wait until mid-spring before pruning out injured foliage. The brown foliage is most likely dead and will not green up, but the buds, which are more cold hardy than the foliage, will often grow and fill in the area where the brown foliage was removed. If the buds have not survived, prune off the dead branches. Fertilize winter-injured plants in early spring and water them well throughout the season.
— Jerry Vitalis is a Chisago County Master Gardener