Evergreen trees and shrubs provide a lot of interest in our landscapes during Minnesota's long winters. Boxwood and rhododendrons are examples of the few broadleaved evergreens grown in Minnesota. Most of our evergreen species are narrow-leaved conifers - pine, spruce, arborvitae, juniper, hemlock, and yew - that have needles or scales for foliage. Unfortunately all of these species can be injured by winter burn and the winter of 2013-14 is proving to be no exception. Winter burn injury has been observed on many evergreen trees and shrubs. As we move from late winter into early spring and temperatures continue to increase, more of this damage will probably appear and existing damage will become even more dramatic.
Symptoms of winter burn are browning (Photo 1) or bleaching (Photo 2) of foliage, particularly on the south, southwest, and windward sides of plants. Evergreen foliage buried under snow is usually protected from damage while plant parts above the snowline are often damaged (Photo 3). In severe cases, the entire plant may turn brown and die. Sometimes symptoms occur immediately as damage occurs, but often symptoms appear or worsen as temperatures rise in late winter and early spring.
Winter burn results from water loss in plants during winter.
During the growing season, water is absorbed and pumped from soil into the roots of plants. From there, streams of water are suctioned up into a plant. Some of this water is used for plant growth and reproduction. But the vast majority is lost during transpiration as water is released back into the atmosphere through small openings called stomates on the lower surface of leaves. This lost water is quickly replaced as roots continue to absorb and pump water from soil into plants.
As plants acclimate and prepare for winter, deciduous trees and shrubs drop their leaves with their thousands or millions of stomates. But because evergreen woody plants retain their foliage, transpiration and loss of water from stomates continue during winter. This is especially true on sunny and windy winter days when higher temperatures and wind speeds increase transpiration rates. Because roots in frozen soil have no ability to replace this water, winter burn occurs as leaves desiccate, die, and turn brown. In more severe cases, buds, stem tissue, or entire plants may die.
Wait until spring before deciding how to care for your winter burned plants. If leaves are dead but buds and stem tissue near dead foliage are still alive, new plant foliage will regrow to replace winter burned foliage. On the other hand, if buds and stem tips were damaged, branches should be pruned back to ¼" above a bud in the live portion of the plant. In severe cases the entire plant may have died and plant removal and replacement will be necessary.
To prevent winter burn in the future:
- Keep evergreens properly watered throughout the entire growing season until ground freezes. Initial soil freeze in Minnesota typically occurs in late November and early December .
- Maintain a 3-4" layer of organic mulch around evergreens to help retain soil moisture throughout the growing season. As this mulch breaks down, it will also improve your soil's moisture holding capacity.
- Protect plants in highly exposed sites during winter with burlap (Photo 4), lathing, snow fencing, or other materials to prevent too much exposure to sun or wind.
- During future planting, properly place evergreens in the landscape by planting them on the eastern or northeast sides of a building. Avoid planting evergreens on the south or southwest sides of buildings or in any site with high exposure to winter sun and wind.
- Plant evergreens in spring and water them consistently throughout the growing season so that they can use the entire growing season to establish well and expand their root system and water-absorbing capacity.