Winter desiccation, or drying out, is the most common winter injury of container grown evergreen nursery crops. If broadleaved evergreens are not watered adequately, they often turn bronze and their shoots later die. When temperatures remain below freezing for an extended period of time, the root ball can freeze completely in containers making water unavailable to the roots. To remedy the problem, irrigate adequately during the winter. When cold, sunny, or windy conditions are forecast, irrigate before the arrival of the weather front.
Irrigation increases plant turgor and helps the plant move water through cell membranes.
Also, if containers are irrigated to their capacity, an additional thermal resistance to freezing is provided. Plant roots are not as hardy as shoots. The soil provides insulation during low ambient air temperatures. Roots in unprotected containers are much more vulnerable to freezing temperatures. David J. Beattie, editor.