We are getting to that point where the first frost of the fall could come up any day, but just because the temperatures dipped doesn’t mean that your crop is a write off. There are a number of factors that come into play from crop type to length below freezing.
Quick explanation on what a freeze event does to a plant; When freezing occurs in the plant it causes the cells to expand causing them to burst or rupture.
The first factor that comes into play is the humidity. Cooler air will hold less water than warmer air. When temperatures drop to below where the relative humidity (RH) reaches 100% (dew point), the air becomes oversaturated and condensation occurs. When water changes from liquid to ice it will give off heat. As the dew on the plant is begins to freeze it gives off heat which can help keep the plant tissue above freezing point. So as water is freezing on the outer parts of the plant, the temperatures remain at about 0 degrees C until all the dew has froze. At this point there is no longer protection for the plant which is when we run into the issue of time below freezing and how it can affect the plant. If it begins to warm up right after this point, damage should be minimal, if temperatures continue to drop or stay around that point then there is potential for damage to occur.
The next thing to take into account is that the liquid within plants does not necessarily have the same freezing point as water. There are sugars, proteins, and a number of other solutes occuring in the plant. These other substances within the plant can take the freezing point anywhere from 2 to even 5 degrees (or more) lower than it would be without them. This shows us that a healthier plant which may have a higher degree brix (sugar) or higher protein content etc may in fact also be hardier than a less healthy, diseased plant.
Temperatures and stress leading up to the frost event can also be a driving factor in how much damage you will see from a frost. If temperatures have been cooler leading up you may see a tougher plant that is less susceptible to light frosts. Other stresses can also cause plants to shake things up a bit with hormones and photosynthate distribution etc. that allow a plant to tolerate lower temperatures, as we all know plants are very efficient at adapting to stress. The speed at which temperatures drop can be a factor as well, the faster they drop bigger the concern for damage.
Moisture content of the crop is another big concern, if your crop is still sitting at 50% moisture then it is a much higher risk than a crop sitting 30% moisture (close to swathing stage in other words).
Each crop has a different susceptibility, much like seedling frost tolerances vary by crop.
Canola is susceptible at a temperature of about -2 to -3 if it is sitting at a higher moisture content like I commented on above. Frost damaged canola dries down very rapidly locking in green seed count. If you have a canola crop ready to swath you may escape damage and if you have a canola crop that is below 25% moisture you should be relatively safe from damage. To avoid losses from a frost in canola you can swath prior to a frost event, ideally 48-72 hours to escape damage. This can be effective even at 0% seed colour change to avoid some of the damages. The other comment I have on canola is even a light frost of -1 degree C can have an impact on the enzyme that helps clear chlorophyll which may also cause green seed to get locked in. Remember to inspect fields after a frost, ideally 48-72 hours to look for frost damage. If it is a severe frost and you see significant damage, swathing immediately is recommended.
Again moisture content comes into play with cereals, if at a milk stage they are more susceptible to frost than a soft dough stage. For example a slight dip below 0 at milk stage may cause losses (shrivelled seed), but at soft or later dough stages they can tolerate upwards of -5 degrees. Cereals that have gotten a frost can be significantly impacted when it comes to germination, so be weary about using frost damaged kernels as seed. Cereals may take a week or so before you can truly evaluate damage. Wheat tends to be slightly more tolerant to fall frosts than barley.
Obviously, there are other crops, but I’m going to leave it there. If you have any questions about anything or on other crops feel free to ask.
Source: Sask Ag