Conventional wisdom states that winter injury on evergreens, often called desiccation injury, occurs when foliage transpires water on sunny, windy days during the winter when temperatures are above freezing. If the soil is frozen, transpired water cannot be replaced and the tree or shrub suffers desiccation stress (foliage browning and tissue death). Certainly, this kind of winter desiccation can occur in early spring when the foliage is actively losing water and the soil is still frozen, however, researchers have demonstrated that water loss alone in midwinter does not cause foliar browning. Instead, recent studies suggest a complex combination of environmental stresses cause foliar symptoms associated with winter injury on evergreens.
Winter browning results from the interaction of injurious minimum temperatures, frequent freeze/thaw cycles, and rapid cooling and thawing rates. These conditions are more apt to be found on portions of the plant that intercept the most sunlight during the winter months, and explains why "browning" or "burning" is usually noticed on the south or southwest sides of evergreens. Therefore, the key to reducing this type of injury is to minimize foliar temperature extremes for injury-prone plants. Planting sensitive species in sheltered locations, providing tough companion plants like spruce to shield tender plants, or screening plants with burlap or snow fence can help prevent unsightly foliar browning. Landscape managers and homeowners should avoid the temptation to prune browned areas from evergreen trees and shrubs in early spring since these branches may still have viable buds that will produce new foliage when growth resumes.
This article originally appeared in the March 15, 1996 issue, p. 25.
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