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How to Protect Trees from Winter Damage

From the November 2003 issue of The Forestry Source

Although heavy snow and ice storms cause damage by bending and breaking branches, winter sun, wind, and cold temperatures can also harm trees.

Courtesy of MN Department of Natural Resource

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

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:

  • 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.

  • Construct a barrier of burlap or similar material on the south, southwest, and windward sides of evergreens. 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.

  • Water throughout the growing season and into the fall. Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up. Watering only in late fall will not help reduce injury.

    Root Injury

    Because roots do not become dormant in the winter as quickly as stems, branches, and buds, and roots are less hardy than stems, it is important to prevent cold air from getting through cracks in the soil, where it can reduce fall root growth or kill newly formed roots. Often the best strategy to protect roots from cold temperatures is to encourage root growth in the fall by mulching new trees and shrubs with 68 inches of wood chips or straw. If the fall has been dry, water heavily before the ground freezes to reduce frost penetration.

    Snow and Ice Damage

    Heavy snow and ice storms cause damage by bending and breaking branches. Swanson and Rideout recommend wrapping relatively small trees together or tying the leaders with strips of carpet, strong cloth, or nylon stockings at approximately two-thirds of the way above the weak crotches. These wrappings must be removed in spring to prevent girdling and to allow free movement of the stem. Proper pruning, to eliminate multiple leaders and weak branch attachments, will reduce snow and ice damage. For trees with large, wide-spreading leaders or large, multistemmed trees, the main branches should be cabled together by a professional.

    Although winter damage is a common concern, Swanson and Rideout say that selecting the proper site at the time of planting and performing the necessary preventative maintenance at the appropriate time will significantly reduce or prevent serious injury to trees.

    Adapted from the University of Idaho Extension Service Publication, Protecting Trees and Shrubs Against Winter Damage, by Bert T. Swanson and Richard Rideout.

    For more information, contact Bert T. Swanson, professor emeritus, Department of Horticulture, University of MinnesotaCrookston, 35423 County Highway 46, Park Rapids, MN 56470; (218) 732-3579.

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