How to evaluate frost damage on various crops

Temperatures dipped below -4 C at various weather station locations in Western Canada last week.

We shouldn’t be surprised because 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.

If your canola is in the mid-to-late pod-fill stage, stay calm and carefully evaluate the extent of the damage. Chances are you will escape with minimal damage if your crop was reaching maturity and below 20 percent moisture.

You may see an increase in green seeds, but hopefully, with some timely wet and dry cycles, this can be eliminated.

The critical temperature for canola with moisture higher than 20 percent is generally considered to be -5 C. This isn’t a hard-and-fast number because factors such as duration of temperature, wind and relative humidity will all play a role.

Observations in the field might 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 dark olive green and eventually black. Immature seeds within pods may turn translucent and appear water filled. These seeds will dry up.

The best time to evaluate damage is three to four days after the frost event.

Cereals generally withstand frost better than canola, but shrivelled kernels can be expected if the cereal is in the milk stage when temperatures dip below -2 C.

Frost tolerance increases from the milk through the soft dough stage to the hard dough stage.

In wheat, an exposure to frost below -3 C can result in bran frost, while below -4 C might result in shrivelled kernels and reduced germination for seed crops. Wheat is generally the most tolerant cereal to frost, followed by barley and oats.

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 are very sensitive to any temperatures below 0 C.

Frost damage within a soybean field may vary considerably, depending on microclimate effects, landscape position in the field and canopy density.

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. Damaged leaves will eventually appear wilted and dried but usually remain on the plant. Undamaged leaves should still appear green and healthy.

A maturity delay of several days might be expected on damaged plants, and small pods near the top of the plant could abort or fail to fill normally. A more severe freeze may also damage leaves in the lower canopy and 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) before the frost event, these yellow beans should dry normally and quality should not be affected.

Thom Weir is an agronomist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.

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