Wheat Spring Freeze Injury
Spring freeze injury occurs when low temperatures coincide with sensitive plant growth stages. Injury can cover large areas of the state or only a few fields or parts of fields. It is most severe along rivers, valleys, and depressions in fields where cold air settles. The risk of spring freeze injury is greater when wheat initiates spring growth early. This may be due to higher than average temperatures or inadequate moisture. If a freeze occurs, wheat has a greater chance of being damaged because it is further advanced.
Susceptibility to freezing temperatures steadily increases as maturity of wheat advances during spring. Some varietal difference in resistance to spring freeze injury has been reported, but it is mostly caused by differences in plant growth stages when freezes occur. There is little or no difference in susceptibility among wheat varieties at the same growth stage and, therefore, little opportunity to increase freezing resistance in improved varieties. Wheat that has had good growing conditions, optimum fertility, particularly nitrogen, and is actively growing is sensitive to freeze injury because of its lush growth and high moisture content. Drought stress tends to harden plants to cold and decreases their water content and the severity of freeze injury. Ample soil moisture, cool temperatures, and high soil fertility slow plant maturity, however, so that injury may be less severe than when plants have less favorable growing conditions and are at a more advanced growth stage when freezing occurs.
During fall, winter wheat goes through a complex process of cold hardening that increases its resistance to cold during winter. Wheat quickly loses its cold hardiness when growth resumes in the spring. Little resistance to freezing is present at that time. Cold temperatures that cause injury to winter wheat after hardening in the fall and dehardening in the spring. Wheat is most sensitive to freeze injury during reproductive growth, which begins with pollination during late boot or heading stages. Temperatures that are only slightly below freezing can severely injure wheat at these reproductive stages and greatly reducegrain yields. The degree of injury to wheat from spring freezes is influenced by the duration of the low temperatures as well as the low temperature reached. Prolonged exposure to freezing causes much more injury than brief exposure to the same temperature. Less injury can be expected from shorter exposure times. Injury might occur at somewhat higher temperatures from longer exposure times.
The many factors that influence spring freeze injury to wheat — plant growth stage, plant moisture content, duration of exposure, wind, and precipitation — make it difficult to predict the extent of injury.