With the forecast in Southern Alberta calling for frost, I thought I would weigh in some of my experiences on frost and what to expect.
Another factor that affects a plants ability to withstand frost are the climactic conditions before and after frost. If the conditions leading up to the frost are consistently cool then the plants will be more tolerant to the below freezing temperatures. For example canola that has not been exposed to cooler temperatures and the temperature was a rapid drop then, -3 degrees celcius would cause damage to the plant. If canola has been experiencing cooler temperatures then even a drop to close to -9 degrees celcius has been shown to be tolerated. If after a frost the temperatures rapidly increase then this can also cause more significant damage to your crop as the rapid increase in temperature can cause frozen cells to burst whereas a gradual temperature increase would allow them to simply thaw out. The bigger a crop is generally the more frost tolerant it gets (1 leaf canola is more susceptible to frost than 4 leaf canola). Time below freezing is another factor that must be considered, if the time below 0 is only a couple hours you shouldn’t see the kind of damage you will see as when the temperature is that low for 8 hours for example. To take things even further, low spots in fields tend to have higher frost damage as frost "settles" in the lower areas. One more factor to consider is soil texture/type. A sandy soil in the brown soil zone with less than adequate moisture and a clay soil in the black zone with good moisture will affect how much damage there will be. The black soil with moisture will tend to hold more heat than the sandy brown soil will, potentially protecting the growing point. On top of this, if a field is covered in straw or has more "trash", then there may be more damage. Straw does not act as an insulator for small plants, rather it hinders the soils ability to absorb sunlight and warm up.
One thing that a cereal crop may be subjected to due to cold temperatures is temperature banding. Temperature banding is due to a difference between air and soil temperature. The difference can cause cause yellow, chlorotic stripes on your cereal crops. Typically this is not yield damaging and the bands eventually go back to green in 10 days depending on further weather conditions.
Identifying frost damaged canola can be difficult, but generally speaking you must look at the growing point. If the growing point is green and healthy looking then it probably hasn’t been hurt to badly by frost. If the growing point is black, brown or purple then you may need to look into reseeding or other options. Another sign in canola can be curled or cupping margins (outside of cotyledon). Excessively dark plants or excessively yellow plants can also be a sign. Remember, you cannot see damage the following day typically. It is best to wait atleast 2-4 days before scouting to evaluate damage.
Temperatures Causing Damage or Death to Seedlings
-3 to -5.5
-2.5 to -4
-3 to -4.5
0 to -1
-4 to -5
-4 to -5
-4 to -5.5
Note: Temps can vary based on prior and after temps, stage, soil texture and time below 0