Assessing spring frost damage is an art as well as a science, agrologists say.
Although frosts can come in any month, those that arrive shortly after seeding create difficult assessments and choices for producers.
Frost injury to plant tissue occurs when ice forms inside the tissues and threatens cellular structures.
At 0 C, water surrounding the cells will freeze and wick water vapour from cells.
Dissolved salt and other compounds help resist ice crystal formation inside cells by a few additional degrees below zero. When water is removed from the cells as the surrounding water freezes, the concentration of dissolved minerals rises inside the cells, further helping them resist freezing.
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But if the temperature becomes low enough, the cold will eventually overcome the process and the cellular fluids freeze. When this happens cell walls burst and the plant tissue dies.
Under the right conditions, temperatures as low as -12 C won’t harm canola seedlings, but the plants must first have become acclimatized to the cold.
Jim Bessel, a Canola Council of Canada agronomist in Saskatoon, said the plants can tolerate a lot of frost in the right year.
However, he said this spring’s unusually warm temperatures provided little help boosting cold tolerance.
Researchers across the West say they have observed early and fall-seeded canola becoming highly resistant to frost under the right conditions. The same principle applies to volunteer canola plants.
Maturity matters
However, a combination of warm temperatures and adequate moisture will cause rapid plant development, especially in later-seeded canola. Frost damage is common to cotyledon-staged canola that was seeded into soil warmer than 5 C and then grew rapidly, even by frosts of -2 C.
Three-leaf and four-leaf plants are more resistant and it could be this that results in higher yields in earlier seeded crops.
A light frost may slow a crop’s development, but unless it damages the plant’s growing point, it won’t likely kill the canola.
Pale colours and wilting will only slow growth and may cause disease and insect susceptibility, but they aren’t generally signs of dying plants. Black is the colour producers don’t want to see in the tissues.
Derwyn Hammond of the canola council in Brandon said farmers who find their crop hit this hard should still wait four to 10 days before making a reseeding decision.
After a four- to 10-day wait, farmers can walk the field in a diagonal line across the direction of seeding looking for an indication of potential
recovery.
If recovering plants inside one sq. metre or an 11 sq. foot circle number between 60 to 80 and weed competition is light, or if the canola is herbicide tolerant, then producers can expect a better crop by allowing the field to recover instead of reseeding.
Hammond said reseeding comes at a production price and often a field that has recovered, even from a devastating frost, will outyield a reseeded one.
Canola council studies in 2004 found that on average, 7.4 additional bushels were harvested from a frost damaged crop than from a reseeded one. This is a $38.85 per acre advantage even with depressed 2006 prices. And that doesn’t include the cost of reseeding.
New growth should begin at the growing point, where the first leaves part ways from the plant’s stem. This should occur within four days under average conditions or a week to 10 days under cold, dry ones.
When canola plant densities drop below 65 plants per sq. metre, producers should begin examining the crop for insect infestations, particularly flea beetles.
Murray Hartman of Alberta Agriculture said insects need plants to eat and the fewer the plants in the field, the more damage they will cause.
Defoliation rates of 25 percent, which is the insecticide application threshold, are reached much faster when there are fewer plants in the field.
As a result, crops should be scouted early and often.
