Prairie grain and oilseed growers are advised to wait a day or more to assess their fields in the wake of frost damage which hit parts of the region over the weekend.
Maps generated by Weather Innovations Consulting for the WeatherFarm website show lows dropped into the -1 to -3 C range in much of eastern Saskatchewan and western Manitoba on Sunday (Sept. 15), while lows since midnight Monday (Sept. 16) dropped below 0 C around Winnipeg.
Evaluating crop damage, however, is difficult and should be done about 24 to 48 hours after the frost for initial symptoms -- and up to a week to 10 days to gauge the full extent of damage, according to Saskatchewan's agriculture ministry.
A "white appearance" to a crop is a good early indicator of some frost damage, the ministry said, noting "heavily damaged" crops would quickly show signs of frost injury such as discolouration, darkening and "water-soaked" appearance of fleshy tissue and pods.
Slightly damaged pods or heads, meanwhile, may show "very little" symptoms, but seeds within the heads may be damaged, the ministry said. Seed harvested from crops exposed to frost should be vigor-tested prior to using the seed for next year's crops.
More specifically, when observing standing canola the morning after a frost, growers will want to wait at least four to six hours after frost to allow the full extent of severe frost damage to become clear, according to the Canola Council of Canada.
"The crop may look undamaged that morning but by lunchtime, wilting, desiccation and pod splitting may begin. If you scout early and then not again, you may underestimate the damage and miss a chance to swath now to save some of the yield."
That said, light-to-moderate frost damage may take longer to show itself in canola, so canola growers will want to scout again after two to three days to reassess the field and make a decision, the council said.
"If most or all seed is mature and you planned to swath the day after a frost anyway, then don't bother waiting four to six hours. Just start swathing."
Pods of immature canola crops frozen at "lethal" (-5 C or lower) temperatures have been seen to turn black, while mild frost turns pods white or white-speckled, the council said.