A frost in mid-September isn’t unexpected, but the damage caused varies widely, depending on a number of factors. Just because the temperatures dip doesn’t mean that your immature crop is a write-off. Crop type, maturity and length of time the temp sat below freezing all play a role in the final damage done.
The first thing that affects frost tolerance is the humidity. Cooler air holds less water than warmer air. When temperatures drop to where the relative humidity reaches 100% (dew point), the air becomes over saturated and condensation occurs on plants. When water changes liquid to ice it will give off heat. As the dew on the plant begins to freeze it gives off heat which can help keep the plant tissue above freezing. 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. This is why speed at which temperature drop occurs and length of time below freezing is important.
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 other things that impact the freezing point. These other substances within the plant can take the freezing point 2 or more degrees lower than just water.
Canola is susceptible at a temperature of about -2 degrees C to -3 degrees C if it is sitting at a higher moisture content (45%, for example). 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. To avoid losses from a frost in canola you can swath prior to a frost event, though ideally you need a full 72 hours to minimize damage, and forecasts don’t always give us that much notice. This can be effective even at 0% seed colour change to avoid some of the damage. On canola, 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. Click here to view more on frost damage of canola from the Canola Council of Canada.
Moisture content also comes into play with cereals. Cereals at milk stage are more susceptible to frost than at the soft dough stage. A slight dip below 0 at milk stage may cause losses (shriveled seeds), but at soft or hard 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 leary about using frost damaged kernels as seed. Wheat tends to be slightly more tolerant to fall frosts than barley.
It’s also good to remember that, just like in the spring, frost damage can take time to show up. If you have a frost event go out and check for damage a few days after to assess the full damage.
Photo Courtesy the Canola Council of Canada.