Plants with well developed crowns before freezeup are most desirable. However, plants that enter the winter with two to three leaves are usually not seriously disadvantaged. A translocatable substance that promotes cold acclimation is not produced when winter cereal plants are exposed to acclimating temperatures. Consequently, the cold hardiness level of different plant parts, such as leaves, crowns and roots, is dependent upon the temperature to which each part has been exposed. Because the crown contains tissues that are necessary for plant survival, it is the soil temperature at crown depth that determines critical cold acclimation rates.
Plant growth slows considerably at temperatures that promote cold acclimation.
Rate of cold acclimation during this period is dependent upon crown temperatures. Dehardening rate is approximately three times faster than the rate of acclimation and it is dependent upon the temperature to which the crown is exposed. Winter cereals normally do not realize their maximum cold hardiness potential until after freezeup in the late fall. In contrast, prolonged exposure of fully acclimated plants to winter temperatures above freezing results in a gradual loss of cold hardiness. The warmer the crown temperature during the winter, the shorter the period that maximum levels of cold hardiness can be maintained and, once started, the more rapid the rate of decline in cold hardiness.