Impact of Frost on Soybeans
There is a risk of frost for the mornings of September 11-12 in Manitoba. This would be normal for a light (0°C) frost but early for a killing frost (-2.2°C). The average date of a killing frost in Manitoba is Sept 17-21 for the southwest and Sept 22-26 for eastern/central parts. Temperatures < 0°C for an extended period of time will cause damage to top growth and temperatures < -2.2°C will kill soybean plants entirely.
Other factors can also impact the degree of frost:
- canopy thickness (narrow, thick rows will maintain heat longer)
- soil moisture (moisture in the soil will maintain heat)
- cloud cover (cloudy is better)
- wind speed (windy is better)
It is a good idea to assess your field prior to a potential frost to understand the staging and maturity of your beans beforehand and the potential impact.
What will happen to my beans if we get a frost?
A light frost (0 to –1°C) may kill top leaf growth but will not penetrate the canopy. Soybean plants should continue to mature but will take longer and there may be some green seed in pods where leaves were killed. Frost killed leaves will remain attached to the plant. A hard frost (< –1°C) will cause damage to green stems, pods and seeds, reducing yield and quality and may kill the entire plant. When entire plants are killed, seed fill stops because photosynthate can no longer be translocated. Beans that were green and immature (R-5 to R-6) will shrivel and remain green. This is worst case scenario. Beans that were green-yellow (R-6.5) will have a mixture of green seed that will not mature and yellow seed that will mature. Mature, yellow beans (R-7) will continue to dry down slowly with minimal yield and quality loss.