Winter Injury To Ornamental Plants

Although rare in the Houston area, the winter season can be particularly injurious to ornamental trees and shrubs, particularly if they have been stressed by poor growing conditions or are planted north of their hardiness zone.  Boxwood, camellia, crape myrtle, forsythia, Southern magnolia, mahonia, American holly, pyracantha, and rhododendron are commonly injured in the winter.  Winter injury is a catch-all for various kinds of injury which show up after the winter.  Most so-called winter injury results from low temperatures, winter drying, or sunscald.

Damage caused by low temperatures can occur in early fall before leaf drop, in spring soon after leaf buds open, or in winter when dormant or semi-dormant plant tissue is subjected to abnormally low temperatures or wide temperature fluctuations.  Because most ornamentals appear to be leafing out early this year, the chance for damage this spring from a late freeze is a likely possibility.  In the event of a late freeze, small, susceptible plants can be covered with Christmas lights and freeze cloth to protect them until temperatures warm up.

Broad and narrowleaf evergreens lose moisture even during the winter.  If the soil is very dry, this moisture cannot be replaced and various parts of the tree or shrub, such as foliage, buds or twigs, are damaged.  Symptoms of winter drying are browning of the margins of the broadleaf evergreens and the tips of the narrowleaf evergreens.  Because of the heavy rains this winter, this will probably not be a problem this year.

Winter sunscald is damage to the trunk where bark or cambium is killed.  High temperatures on a sunny, bright winter day, followed by low temperatures after sunset, can lead to this sort of injury.  In this instance, it is not simply the cold, but the rapid change in temperature, which destroys plant tissue.  Winter sunscald is more often seen on thin-barked and transplanted trees and, of course, favors the south and west sides of the tree.

Several techniques can be implemented to improve the appearance and health of the injured plant and to increase the chances for survival.  Prune out dead and dying tissue after the leaves emerge in the spring.  Help invigorate the plant through fertilization with a slow release fertilizer and proper watering.  Because stressed plants are more susceptible to insect and disease problems, control insects and diseases to prevent further plant stress.  It is important to realize that there may be a delay of injury symptoms until several weeks after leaf and twig growth or until a water shortage and/or high temperatures occur this summer.