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Harsh winter weather can potentially injure trees, shrubs

Boy am I glad that the warmer weather has returned, but I wonder how the recent extremely cold weather will affect our landscape plants.

Winter sun, wind and cold temperatures can bleach and desiccate evergreen foliage, damage bark and injure or kill branches, flower buds and roots. If you performed some of the recommendations below, you probably protected your landscape plants and minimized potential injury.

Cold temperatures can damage plants in several ways. Injury is more prevalent and more severe when low temperatures occur in early fall or late spring, when there is little or no snow cover during the winter or when low temperatures are of prolonged duration. Pronounced fluctuations in temperature can be extremely detrimental to plants throughout the fall, winter or spring.

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, such as cherry, crabapple, honey locust, linden, maple, mountain ash and 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.

Wrapping the trunk with a commercial tree wrap, plastic tree guards or any other light-colored material can prevent sun scald.

Put the wrap on in the fall and remove it in the spring after the last frost. Newly planted trees should be wrapped for at least two winters and thin-barked species up to five winters or more. To repair sun scald damage, cut the dead bark back to live tissue with a sharp knife, following the general shape of the wound, rounding off any sharp corners to facilitate healing. Wrap the trunk in subsequent winters to prevent further damage. Do not use a wound dressing. Spraying the area with a fungicide may help prevent fungal infection of the wound.

Winter discoloration of evergreens, 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 degrees. 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 non-acclimated 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 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. 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. 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. Many factors influence soil temperature. Moist soil holds more heat than dry soil, so frost penetration will be deeper and soil temperatures colder for sandy or dry (drought) soils. Snow cover and mulch act as insulators and keep soil temperatures higher. With newly planted trees, cracks in the planting hole backfill will allow cold air to penetrate into the root zone, reducing fall root growth or killing newly formed roots.

To encourage fall root growth and to reduce root injury, mulch new trees and shrubs with 6 to 8 inches of wood chips or straw.

Repeated freezing and thawing of soil in fall or spring causes soil to expand and contract, which can damage roots and heave shrubs and new plantings out of the ground. A 4- to 6-inch layer of mulch will prevent heaving by maintaining more constant soil temperatures.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by email at wlamont@psu.edu.

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