Tips for monitoring early season frost damage

JOHNSTON, Iowa — Wide temperature swings are a given each spring in the Midwest and often occur as those young corn and soybean plants are just getting established.

Paul Yoder, Pioneer field agronomist, gave some pointers on what to look for and when to be concerned or not concerned if temperatures start to push the freezing mark.

“Corn for the most part has either just been planted or to almost V2 stage. If temperatures get down to 32 degrees and below, what does that mean and what are some things to look out for?” Yoder said.

For the corn crop that’s just below the ground or just starting to spike, if the air temperature gets down to 32 degrees or frost layer, that crop for the most part should not be affected.

“The good thing is soil temperatures at the two-inch level is still warmer, so that will be a positive,” Yoder said.

“If corn is already up out of the ground when the temperature goes down to 32 degrees or frost layer and is close to V2, we have some things playing into our favor, as well, because the soil temperature is around warmer, so you’re going to have heat radiating from the soil surface to help warm that plant if the temperatures get down to freezing.

“The other thing in our favor is that the growing point on a corn plant does not get above the soil line until V5. So, we still have plenty of time and even if the top part growth of the corn plant is taken down from the frost we should still be able to have nice growth coming up, but it will be something that we’ll want to watch and pay attention to.”

He added if the top part of the corn plant has been frosted where the cells have ruptured and turns yellowish or brown and are laying flat on the ground, give that field at least five days.

“After five days, go back to the field and see if there’s any growth coming, which more than likely you will,” he said.

“The other good thing if the temperature is forecast to get around 33 and if it stays above 33 degrees, we should be fine. It’s when it gets 32 or below.”

Soybeans

At this point, some soybeans have been planted and beginning to emerge in some cases with the first trifoliate out of the ground making them susceptible to cold temperatures.

“Soybeans are a little bit tougher, which is a good thing, but for the most part they should be fine,” Yoder said.

“The growing point for a soybean is basically if you take the soybean and split it in half, the two halves that come up — the cotyledons — that is the growing point. The good thing about soybeans and those cotyledons is if you look at them closely, they’re thicker, they’re probably about 95% water.

“The other thing, too, is they’re full of fat and oil and that will help protect them if the temperatures drop down.

“For the most part, I think soybeans should be fine. Soybeans that are just starting to crust or get out of the ground where the neck — the hypocotyl — is exposed should be fine because the growing point or the cotyledons or the actual seed still probably still has its head in the soil.”

Similar to corn, the heat radiating off the soil surface from the warmer soil temperatures should help protect the plants.

“If your soybeans are a little bit farther along and we get a frost, you might see the tops get burned out. The good thing about soybeans is they will compensate, they will normally branch out and they’ll be fine,” Yoder said.

“If we do get a frost and the soybeans are looking kind of sick, give them at least five days and then go back to the field and evaluate.

“The fields that I want to go to first if we do have temperatures that get down to 32 degrees are the fields that have high residue or cover crops that were there last year.

“The good thing about high residue or no-till or cover crops is that it will keep the moisture in the ground, but it also keeps that soil cooler. Where conventional tillage will help bury the trash and also warm up the soil and that helps protect the plants if the temperature gets to 32 degrees.

“Poor soils like sands would be another field that I’d be looking at based on the fact that our research has shown that as the temperature fluctuates up and down, so does the soil profile even up to four inches. It pretty much follows what the air temperature does.”

He added that the temperature “edge of damage or killing” a corn and soybean plant needs is 28 degrees for at least two hours.

“If we would get into that 28 degrees for a minimum of two hours, then that could warrant some additional concerns,” Yoder said.