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Into the Garden: Harsh weather endangers trees, shrubs

CHARLESTON, W.Va. -- Weather has been a hot topic around our area this year -- hot, hot summer; snowy fall; warm early December.

Certainly cold winter months are still ahead of us. Bert T. Swanson, a professor in the Department of Horticultural Science at the University of Minnesota, and Richard Rideout, city forester for the city of Milwaukee, are living in an area that is truly familiar with harsh temperatures. Recently they wrote an article about different ways the cold weather can damage our landscape plants. Over the next few weeks, I'll share some of their tips for saving your trees and shrubs from winter damage.

Sun scald

"Sun scald is characterized by elongated, sunken, dried or cracked areas of dead bark, usually on the south or southwest side of a tree. On cold winter days, the sun can heat up bark to the point where cambial activity is stimulated. When the sun is blocked by a cloud, hill or building, bark temperature drops rapidly, killing the active tissue.

"Young trees, newly planted trees and thin-barked trees (cherry, crabapple, honey locust, linden, maple, mountain ash, plum) are most susceptible to sun scald. Trees that have been pruned to raise the lower branches, or transplanted from a shady to a sunny location are also sensitive because the lower trunk is no longer shaded. Older trees are less subject to sun scald because the thicker bark can insulate dormant tissue from the sun's heat ensuring the tissue will remain dormant and cold hardy."

The specialists suggest wrapping the trunk of the tree with a commercial tree wrap, plastic tree guards or any other light-colored material, keeping the bark at a more constant temperature. Put the wrap on in the fall and remove after the last frost. Newly planted trees should be wrapped for two years, and thin-barked species up to five winters.

Evergreen discoloration

"Browning or bleaching of evergreen foliage during winter occurs for four reasons:

"1. Winter sun and wind cause excessive transpiration (foliage water loss) while the roots are in frozen soil and unable to replace lost water. This results in desiccation and browning of the plant tissue.

"2. Bright sunny days during the winter also cause warming of the tissue above ambient temperature, which in turn initiates cellular activity. Then, when the sun is quickly shaded, foliage temperature drops to injurious levels and the foliage is injured or killed.

"3. During bright, cold winter days, chlorophyll in the foliage is destroyed (photo-oxidized) and is not re-synthesized when temperatures are below 28 degrees. This results in a bleaching of the foliage.

"4. Cold temperatures early in the fall before plants have hardened off completely or late spring after new growth has occurred can result in injury or death of this nonacclimated tissue.

"Foliar damage normally occurs on the south, southwest and windward sides of the plant, but in severe cases the whole plant may be affected. Yew, arborvitae and hemlock are most susceptible, but winter browning can affect all evergreens. New transplants or plants with succulent, late-season 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 exposed (windy, sunny) places. A second 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, 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.

"Keeping evergreens properly watered throughout the growing season and into the fall is another way to reduce winter injury. Never stress plants by under- or overwatering. Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up. Watering only in late fall does not help reduce injury.

"Anti-desiccant and anti-transpirant sprays are often recommended to prevent winter burn. Most studies, however, have shown them to be ineffective.

"If an evergreen has suffered winter injury, wait until mid-spring before pruning out injured foliage. Brown foliage is most likely dead and will not green up, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed. If the buds have not survived, prune dead branches back to living tissue. Fertilize injured plants in early spring and water them well throughout the season. Provide appropriate protection the following winter."

Reach Sara Busse at sjbusse@gmail.com.


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