SCHMIDT: Winter injury to conifers
Many evergreen trees are showing signs of winter injury across our region. “Winter injury” (also known as winterburn) is a generic term for unusual browning of needles, portions of trees, or entire trees that can occur throughout the winter months, but is often most noticeable in late winter/early spring. Trees such as arborvitae, spruce, juniper, cedars, pine and yew are susceptible to winter injury and the effects of the condition have been noticeable this spring in North Dakota
The incidence and severity of winter injury is often attributed to (1) moisture conditions in the fall prior to freeze up, and (2) the length of the winter. Evergreens transpire (give off) tiny amounts of moisture through their needles during the winter months. Water loss is accelerated when evergreens are exposed to warmer windy days in late winter/early spring as the trees slowly transition from dormancy. Excessive moisture loss during this time can cause browning of the needles as trees are unable to replace lost moisture because the ground remains frozen. Consequently, long winters tend to result in greater levels of winter burn. Similarly, dry fall conditions can result in winter injury as the trees have a water deficit going into the winter. Winter desiccation may be greater on trees improperly planted, stressed by insects, diseases, other environmental factors, fertilized at an improper time, lacking winter hardiness, or on trees adjacent to light colored surfaces that reflect the sun’s radiance (white siding, white rock mulch).
Unfortunately, the brown portion of winterburned conifers will not regain a healthy green color and dead needles will be shed and not regrow. However, if the buds at the tips of branches are not dried out, new growth may emerge this spring and eventually hide the damaged bare spots within a few years. It is recommended that homeowners assess the total injury to the tree after the new growth has occurred in early summer.
Predicting winter injury to trees is not easy and susceptibility varies among evergreens species and cultivars. Winter injury is more easily prevented than treated after damage has occurred. Trees should be watered weekly in late fall until freeze-up. Two gallons per inch of stem diameter is recommended, if the soil is dry at a depth of six inches. Conifers can be burlapped or shaded to reduce the chance of winterburn in late winter to spring thaw.
Schmidt is the NDSU Extension agent for Pierce County.
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