It's no secret that winter snow and ice can damage trees, but contending with these two cohorts of cold weather is just the tip of the iceberg when it comes to preparing trees for winter. Aside from snow and ice, winter sun, wind and, of course, temperatures can harm trees in a variety of ways.
To keep winter weather from damaging your trees, Bert T. Swanson, a professor in the University of Minnesota's Department of Horticultural Science, and Richard Rideout, the forester for the city of Milwaukee, Wisconsin, recommend taking the following steps to prevent seasonal damage to bark, branches, and roots.
Sun scald occurs when sunlight heats trees to the point of stimulating cambial activity and then the sunlight is blocked, so the bark temperature drops rapidly, killing the active tissue. Sun scald often is characterized by areas of elongated, sunken, dried, or cracked, dead bark; is usually found on the south or southwest side of a tree; and typically occurs on winter days when there is significant temperature fluctuation.
Swanson and Rideout warn that young trees, newly planted trees, thin-barked trees, trees that have been pruned to raise the lower branches, and trees transplanted from a shady location to a sunny location are most susceptible. Older trees, the researchers say, are less vulnerable because their thicker bark can insulate dormant tissue from the sun's heat, ensuring that the tissue will remain dormant and cold hardy.
To prevent sun scald, Swanson and Rideout advise wrapping tree trunks with a commercial tree wrap, plastic tree guards, or any other light-colored material that will reflect sunlight and keep the bark at a constant temperature. Newly planted trees should be wrapped for at least two winters, and thin-barked species should be wrapped for five winters or more.
Discoloration of Evergreen Foliage
Evergreen foliage that appears brown or bleached may occur during winter months as a result of excessive transpiration caused by winter sun and wind while the roots are in frozen soil and unable to replace lost water. Discoloration also may be caused by sun scald, the destruction of chlorophyll when temperatures are below 28°F, and the occurrence of cold temperatures early in the fall before plants have had a chance to harden.
Foliar damage normally occurs on the south, southwest, and windward sides of the evergreens, but in severe cases the whole plant may be affected. New transplants or plants with succulent, late-season growth are particularly sensitive.
To minimize winter injury to evergreens, Swanson and Rideout recommend the following:
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