One of the major factors affecting canola production in Western Canada is the short frost-free period.
The frost-free period is the number of days between the last freezing temperature in the spring and the first frost in the fall. It varies considerably from one location to the next.
Significant variations usually occur on a local scale and extreme variations of the dates of spring and fall frost may occur from year to year.
Although frosts can occur in any month, it is usually those in late spring and early fall that are critical.
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A killing frost during seed development or seed maturation is especially damaging. The amount of frost injury will depend on moisture conditions, the rate at which thawing occurs, the growth stage of the plant, and the amount of cold temperature hardening the plant has experienced before the frost.
The temperature at which frost injury occurs varies. Frost cover or ice crystals on a plant do not necessarily mean the plant has been damaged.
Injury occurs between or within plant cells. Water that surrounds the plant cells freezes first at about 0 C, while the water within the cell contains dissolved substances that, depending on their nature and concentration, lower the freezing point of water several degrees.
As the water around the cells becomes ice, more water vapour moves out of the cell and into the spaces around the cell where it becomes ice. The reduced water content of the cells further depresses the freezing point of the cell water.
This could continue, up to a point, without damaging the cell, but if conditions are bad enough, ice crystals form within the cell, disrupt the cell membrane and injure the cell.
The duration of freezing temperatures is important. A severe drop in temperature that lasts only a short time may not damage canola plants, while a light frost of a few degrees that lasts all night may cause severe damage.
After several days of near freezing temperatures, fall-sown and early spring-seeded canola will undergo a gradual hardening process that will allow the plants to withstand freezing temperatures without serious damage.
It is likely that cold weather sets off a chain of plant gene activities that produce or degrade proteins that protect cells.
Hardened plants that emerge under cold conditions are slower growing, producing smaller cells that have a higher concentration of soluble substances more resistant to frost damage.
Studies at the University of Manitoba, the University of Saskatchewan and at Agriculture Canada’s Beaverlodge Research Centre have shown that fall-sown and early-seeded canola seedlings that had undergone hardening could withstand temperatures of -8 to -12 C.
This would also explain why volunteer canola and other weeds such as winter annuals like stinkweed have a high tolerance to cold temperatures in the spring.
Rapidly growing canola seedlings are more susceptible to frost damage than plants that are growing slowly under cold conditions, especially when there is ample moisture. Exposure to warm weather can cause cold hardened plants to lose frost tolerance and be killed by temperatures of only -3 to -4 C.
Canola at the cotyledon stage is more susceptible to frost damage than canola at the three- to four-leaf stage, which can usually withstand temperatures two degrees cooler.
Canola seedlings will usually recover from a light spring frost that does not damage the growing point of the plant.
A light frost that wilts the leaves but does not cause any browning will not injure the plants. There may be some discolouration of the leaves, usually a yellowing or whitening especially under drought conditions.
When frost blackens the cotyledons and leaves, no action should be taken for at least four to 10 days. The extent of killing can be determined only by waiting several days following the frost. Time is required to determine the extent of the damage and whether or not the growing point has been killed.
If there is any green colour at the growing point in the centre of the frozen leaf rosette, the plant will recover and yields will be higher than if the field is worked and reseeded.
In a 2004 study by the Canola Council of Canada, reseeded canola showed a 7.4 bushel per acre loss compared to leaving the frosted crop.
Under good growing conditions, green re-growth from the growing point should occur in four to five days. Under poor growing conditions affected by cool temperatures or poor moisture, this may take up to 10 days.
When checking frozen canola, consider the percentage of plants killed, the percentage recovered, the weed population and the time of year.
To evaluate a frost damaged field, walk a diagonal path across the field. Every 20 paces, evaluate all plants in an area of three sq. feet. Take notes of each sample. This should result in 50 to 100 samples.
Calculate the percentage of the field that has adequate plant recovery.
For example, if 80 percent of the field has a minimum of two to four recovering healthy plants per sq. foot and a light, controllable weed population, the field probably has a higher yield potential than one that is reseeded.
With a moderate weed population that cannot be controlled, the reseeding threshold would increase because the competition limits the ability of the crop to compensate. 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, compensating for the lost plants.
Surviving plants that have been damaged by frost will require longer to mature than normal, but a reseeded crop will require a longer frost-free period and will have a greater risk of frost damage in the fall.
Frost damage to seedlings in the spring has never been more than a minor problem across Western Canada.
