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With the forecast for cold temperatures and the risk or incidences of frost, many producers may be concerned about damage on their canola that may be out of the ground. The temperature at which frost injury occurs varies with; growth stage, moisture content and the length of time the temperature remains below freezing.
Low temperatures injure plants primarily by inducing ice formation between or within cells. The water around the cell freezes first at about 0 degrees, while water within the cell contains dissolved substances that allow it to take lower temperatures without freezing. The length of time the freezing occurs is also important. A severe drop in temperature lasting a short time may not damage seedlings, but a light frost lasting all night can cause severe damage. Rapidly growing canola seedlings are more susceptible to frost and exposure to warm weather can cause cold hardened plants to lose frost tolerance. Canola at the cotyledon stage is more susceptible to frost than a three-to-four leaf plant. Canola seedlings will usually recover from a light spring frost if the growing point of the plant is not damaged. There may be discoloration of leaves or wilting depending on the severity of the frost.
The extent of the damage should be determined by waiting several days following the frost and inspecting the plants. Time is required to determine the extent of the damage and whether or not the growing point is dead. If there is any green color at the growing point in the center of the plant then the plant will recover and yields will be higher than if the field is worked and reseeded. This was observed last season as many of the Canola acres in Western Canada that were froze and not reseeded ended up yielding as good or better than the reseeded fields. In 2004 a Canola Council study of reseeding canola showed a 7.4 bushel loss in the reseeded compared to leaving the frosted crop. This obviously depends on the severity of the frost event and the extent of damage on seedlings. Under good growing conditions, green re-growth from the growing point should occur in four to five days. Under poor growing conditions this may take up to 10 days.
Before making the decision to reseed some factors should be considered. You should consider the percentage of plants killed by the frost, the percentage recovered, the weed population, and the time of year. To evaluate frost damage the field should be walked to calculate the percentage of the field that has adequate plant recovery. A good rule of thumb is if in 80% of the field there is 2 to 4 plants per ft2 then you probably have a higher yield potential than one that is reseeded.