From the May 21st NDSU Crop and Pest Report:
During the middle of May 2015, early morning frost occurred in many parts of North Dakota. The temperature at which freezing injury may take place in canola varies with the growth stage of the plant, soil moisture content, and the length of time the temperature is below freezing. Early seeded canola, after several days of near freezing temperatures, may undergo a gradual hardening process that will allow the plants to withstand freezing temperatures without serious damage.
Frost damage occurs to the small canola plant when ice crystals form within the leaf or stem tissue or when the plant actually freezes, which will cause cell walls to rupture. A severe drop in temperature which only lasts a very short time, may not damage canola plants; while a light frost of several degrees below freezing that lasts all night may cause severe damage.
Canola seedlings will usually recover from a light spring frost that does not damage the growing point of the plant. If a heavy frost does blacken the leaves, no action should be taken for at least 4 to 7 days. The extent of injury can be determined in a week or less following the frost. If there is any green color at the growing point in the center of the frozen leaf rosette, the plant will recover and yields will most likely be higher than if the field is re-seeded.
In evaluating frosted seedling fields, consider the percentage of plants killed and the percentage recovered. The surviving plants should be evenly distributed in a field. Even if two-thirds of the seedlings in a reasonable stand are frost killed, the field will usually produce more when left than if re-seeded. The surviving plants will take advantage of the reduced competition for light, moisture and nutrients, and grow larger, producing more branches, pods and seeds per pod, thereby compensating for the lost plants. The surviving plants may require five to eight days longer to mature; but a re-seeded crop will require an even longer period to reach maturity.
Story by: Hans Kandel – Extension Agronomist Broadleaf Crops