Over the past couple of mornings, we have seen temperatures dip below -4oC at various Farmers Edge Field-Centric Weather stations in western Canada. We shouldn’t be surprised as the average first killing frost occurs in the second week of September for most of the prairies. Many crops will have missed most of the damage but some will still be in the susceptible stage. Here is some advice if you were unfortunate enough to be in the susceptible category.
Canola
If your canola is in the mid to late pod fill stage, stay calm and carefully evaluate the extent of the damage. Firstly, if your crop was reaching maturity and was below 20% moisture, chances are you will escape with minimal damage. You may see an increase in green seeds but hopefully, with some timely wet – dry cycles, this can be eliminated. The critical temperature for canola with moisture higher than 20% is generally considered to be -50C. This isn’t a hard and fast number as factors such as duration of this temperature, wind and relative humidity will all have an impact. Observations may include pods turning white or mottled. This seems to be hybrid specific and may not correlate to actual damage. However, under some conditions, Alternaria seems to affect these hybrids following a frost more than others. Under heavy frost, immature pods on late branches may turn to a dark olive green color and eventually to black. Immature seeds within pods may turn translucent and appear water filled. These seeds will dry up. The best time to evaluate damage is 3-4 days after the frost event.
Canola seeds showing heavy damage after 3 days
Mottled Canola Pod due to Frost
Canola Pod showing Heavy Frost Damage
Frost Damage resulting in Pod Mottling
Alternaria on frost damaged canola pods
Cereals
Cereals generally withstand frost better than canola. However, if the cereal is in the milk stage when temperatures dip below -20C, shriveled kernels can be expected. Frost tolerance increases from the milk through the soft dough stage to the hard dough stage. In wheat, an exposure to frost below -30C can result in bran frost. Below -40C may result in shriveled kernels and reduced germination. Generally, wheat is the most tolerant cereal to frost, followed by barley and then oats.
Flax
Flax can be quite susceptible to frost damage when the seeds are immature. Green seeds may be heavily damaged resulting in significant yield loss.
Soybeans
Soybeans are very sensitive to any temperatures below 00C. Frost damage within a soybean field may vary considerably, depending on microclimate effects, landscape position in the field, canopy density, and other factors. Generally, a thicker plant canopies formed by narrow rows and/or high plant populations tend to hold the soil heat better and protect the lower portion of the plants and pods to some extent.
If only a light frost occurs, damage may be confined to the upper leaves in the canopy. After a waiting period, damaged leaves will appear wilted and dried but usually remain on the plant. Undamaged leaves should still appear green and healthy. Some maturity delay (several days) may be expected on damaged plants, and small pods near the top of the plant may abort or fail to fill normally.
If a more severe freeze occurs, leaves in the lower canopy may also be damaged as well as the stems and pods. Frost-damaged stems turn dark green to brown. Beans that were still green and soft at the time of the freeze will shrivel, reducing soybean yield, quality and drying rate. If the soybeans had reached physiological maturity (R7) prior to the frost event, these yellow beans should dry normally, and quality should not be affected.
