Winter injury can take many forms. It can be the result of desiccation and temperature fluctuations or it can result from damage due to salt, snow or ice.

Desiccation:

Desiccation (drying out) injury usually takes place mid-January through March, when the grounds is still frozen and plant roots are dormant. When a thaw warms the foliage, moisture drawn out of the leaves on sunny, windy winter days cannot be replaced because of the frozen roots.

Desiccation is easy to detect. When the leaves dry out, they may look scorched or yellow to dark brown at the tips, outer margins or between the mid-rib and outer edges. The symptoms sometimes do not become evident until early spring. Arborvitae and rhododendrons are especially affected by desiccation.

Frost damage on Spruce (Picea spp.). Photo courtesy of the Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources

Temperature:

Injury due to temperature fluctuations can result from a loss in hardiness or from sunscald.

As fall approaches, gradully lowering temperatures harden off (acclimatize) woody plants. This pattern reverses as spring approaches. If temperatures fall too quickly (more than six degrees/hour) and stay too low for a prolonged period of time, the plants may not have enough time to harden and can be injured. Early fall or late spring frosts cause the most damage. A mid- or late-winter thaw can cause the plant to lose hardiness and begin new growth which is injured or killed when the temperature drops again.

Temperature injury results in the browning or dieback of newly-emerged shoots or buds. Flower buds may be killed while there is no obvious injury to the rest of the plant. To see if your tree or shrub was damaged, try to force the flower buds in March or April. Cut branches and put them in warm water in the sun. If the buds do not swell and open, they were probably damaged during the winter.

Winter sunscald is a result of rapid changes in bark temperature. During sunny winter days, the sunny side of the bark heats up, then cools rapidly after sunset. Most sunscald happens during January through March. Dark- or thin-barked trees, like cherries, plums, peaches and crabapples, are the most susceptible. Sunscald injury shows up on the southwest side of trunks, and the bark may split or develop cankers.

Salt:

Salt in the soil during active growth can make water and essential nutrients difficult to absorb and predispose the plant to other kinds of stress. However, salt is usually leached out of the soil by spring rains and causes little damage unless there is a winter thaw and roots come out of dormancy.

Salt spray is much more of a problem, as the salt lands directly on the foliage of evergreens and causes rapid water loss or "burning."

Trees along roadside may develop a one-sided appearance with the tips or margins of the leaves or needles looking burned as a result of salt spray from snow plows landing on the leaves. Conifers may develop shorter tufts of needles when active growth resumes.

De-icing salt damage. Photo courtesy of Tim Tigner, Virginia Department of Forestry.

Snow and Ice:

Snow and ice accumulation on the plant can cause physical damage as the plant is very brittle when frozen. A result may be bent or broken stems or branches.

Control for All Winter Injury:

1. Use plants hardy to your area. Consult your nurseryman as to the zone you are in. Drought-resistant or native plants may suffer less winter damage.

2. Increase hardiness by slowing down growth. Do not fertilize after July 4th.

3. Apply 3 to 4 inches of mulch when growth has hardened (around the time deciduous trees lose their leaves). If the plant was previously mulched, remove the mulch gently (to make sure that the plant has not rooted out into the mulch) in October to allow the roots to cool down and dry off somewhat. Replace the mulch (3 to 4 inches only) when ground freezes.

4. Use an antidesiccant in late fall.

5. Provide windbreaks on the side of the prevailing winds for plants in exposed sites. Lath, snow fencing or burlap may be used.

6. Plant in sheltered locations, downwind side of less-sensitive evergreens or courtyards. Avoid southern exposures. Move in early spring or late fall when roots are dormant.

7. Plant at least 30 feet from sides of roads or provide barriers/screens to decrease damage from salt spray.

8. Use salt-tolerant species.

9. Shade plants in mid-February through March to prevent desiccation.

10. Use protective trunk wrappings on young or thin- or dark-barked trees to prevent sunscald.

11. Keep plant healthy. Control insects. Make sure plant receives adequate moisture throughout growing season, especially during drought. In dry autumns, water the plant heavily before the ground freezes. Never water when the ground is frozen. Fertilize in spring.

12. Corrective pruning of broken branches or stems should take place just before growth begins in the spring.

13. When snow or ice accumulates on plants, do not try to remove it. Let the sun gradually melt the snow and ice. The plant will recover slowly.

By Diana Vogel. Edited by Kathleen Mallon and David B. Wallace.