Frost Tolerance of Spring Seeded Canola
The temperature at which freezing injury occurs varies with the plant's stage of growth, moisture content and the length of time the temperature remains below freezing. Frost cover (ice crystals) on a plant does not necessarily mean the plant has been damaged. Damage occurs when ice crystals form within the plant or the plant actually freezes, causing rupture of cell walls or physical disruption of the cell contents. The time interval in which freezing temperatures occur is important. A severe drop in temperature which only lasts a very short time may not damage canola plants, while a light frost of a few degrees that lasts all night may cause severe damage. The amount of frost injury will depend on moisture conditions, rate at which thawing occurs, the growth stage of the plant, and the amount of cold temperature hardening the plant has experienced.
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 10 days. The extent of killing can be determined only 4 to 10 days 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 be higher than if the field is worked and re-seeded.
Early seeded canola, after several days of near freezing temperatures, will undergo a gradual hardening process that will allow the plants to withstand freezing temperatures without serious damage. A number of chemical changes occurs, resulting in a higher concentration of soluble substances in the cell sap. Studies at the University of Manitoba and at the Agriculture & Agri-Food Canada Research Station in Beaverlodge have shown that early seeded canola that had undergone hardening could withstand -8 to -9 degrees (about 17B F) C temperatures, while later sown canola which did not undergo hardening was killed by temperatures of only -3 to -4 degrees C (about 25BF).
In evaluating frost damaged seedling fields, a grower must consider the percentage of plants killed and the percentage recovered and the time of year. 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 will require five to eight days longer to mature; but, a re-seeded crop will require an even longer growing period. It should be noted that frost damage to seedlings in the spring has been only a minor problem in any one year.
Source. Canola Growers Manual of Canola Council of Canada