Browning or bleaching of evergreen foliage during winter occurs for four reasons: 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. 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. During bright, cold winter days, chlorophyll in the foliage is destroyed (photo-oxidized) and is not resynthesized when temperatures are below 28 F. This results in a bleaching of the foliage. 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.
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.
Deciduous trees and shrubs can incur shoot dieback and bud death during the winter. Flower buds are more susceptible to injury than vegetative buds. A good example of this is forsythia, where plant stems and leaf buds are hardy, but flower buds are very susceptible to cold-temperature injury.
Little can be done to protect trees and shrubs from winter dieback. Plants that are marginally hardy should be planted in sheltered locations (microclimates). Plants in a vigorous growing condition late in the fall are most likely to suffer winter dieback, so avoid late summer pruning, fertilizing and overwatering. Fertilize in the spring on sandy soil or in the fall on heavy soil after the leaves have dropped.
Roots do not become dormant in the winter as quickly as stems, branches and buds, and roots are less hardy than stems. Roots of most trees and shrubs that grow in Minnesota are killed at temperatures at or below 0 to 10F.
These plants survive in Minnesota because soil temperatures normally are much higher than air temperatures and because soil cools down much more slowly than air temperature.
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