Are You on the Lookout for an Early Frost?

Sally Krueger

Sep 11, 2018

Fall has officially arrived, bringing with it cooler temps and harvest season. Depending on where you farm, you probably have a good guess as to when a frost or freeze could come sweeping in unexpectedly, but it’s hard to be certain when the season’s very first frost will hit your county.

The map below provides a look at the 10-year average for first frost dates across the country.

While these averages help tell a story about when a first frost is most likely based on past events, the truth is there’s always a chance that a fluke frost or freeze could come early.

Could your crops survive an early frost or freeze?

FBN network historical weather dataset

In both corn and beans, a killing frost occurs at 28 degrees Fahrenheit. Once a killing frost hits, plants can no longer move carbohydrates from the leaves, where photosynthesis occurs, to the grain. At this point, a plant’s full yield potential has been reached regardless of physiological maturity because the killing frost stops the plant from maturity any further.

Between 28-32 degrees F, which is a partial frost, the process of grain fill isn’t fully ended, but because a partial frost can damage the leaf area and upper stem, it becomes more difficult for plants to move carbohydrates in and through the damaged area, so growth and yield can suffer.

The amount of yield loss incurred during a true freezing event is directly proportional to the growth stage that your plants are in at the time.

What happens to crops that experience a frost or freeze?

There are a variety of factors that will impact the effects of a cold weather event on corn and soybeans. Delayed planting, extended cool or wet conditions, and fuller season variety choices can all impact how well your crop will do in the event of a frost.


Corn reaches full maturity at black layer (R6). Once it hits this stage, no additional dry matter will be accumulated. Since corn is roughly 30-35 percent moisture at maturity, corn will still need to dry down, but a frost at this stage should not have an impact on ultimate yield.

For corn that hasn’t quite reached physiological maturity, give it 5-7 days post-frost before you make an assessment. Late-season frost damage in corn shows up as water soaked leaves that eventually turn brown. If your corn is only partially damaged, give it a chance to finish out the season; if there’s still time before harvest, the weather conditions may improve and allow the crop to still finish out well.


In soybeans, like with corn, once full maturity has been reached, a frost won’t have much impact on final yield. But if your beans haven’t reached maturity when a frost hits, it’s important to know what growth stage they’re at in order to accurately assess any damage. At the beginning of the R6 growth stage in beans, dry matter accumulation is only at about 50 percent; but once soybeans begin to turn, about halfway through the R6 stage, they will have accumulated nearly 90 percent of their dry matter.

Soybeans that have been damaged by a frost may have reduced oil content along with a higher moisture level than indicated by a moisture meter. The beans may shrink to a smaller than normal size after drying and the field may experience a slower dry down overall.


Sally Krueger

Sep 11, 2018