Just off Larpenteur Avenue at the St. Paul campus of the University of Minnesota, test plots of corn show the late-planting consequences of the wet spring of 2019.

Corn planted on May 29, labeled with a white placard, is tall and tasseled, well on its way to producing mature ears. Corn planted two weeks later is shorter, with no tassels, and uncertain to produce a crop that can be harvested before the first killing frost this fall.

“We have a lot of corn in Minnesota like this,” said Dave Nicolai, a crops educator at the U, pointing at the shorter corn as a group of farmers huddled around an agronomist at the Extension’s Field School for Ag Professionals this week.

Rain in May and June kept farmers out of fields far longer than usual, forcing some to leave fields unplanted. Much of the corn that was planted is now behind schedule, stoking fears an early frost could devastate the crop.

A corn stalk needs about 60 days after it tassels to generate mature ears of corn. It needs still more time after that for the corn to dry out, said Jeff Coulter, a corn agronomist at the University of Minnesota Extension.

“It’s going to take most of September for the corn to reach maturity, and the corn that doesn’t even have tassels yet, that’s way behind,” Coulter said. “That’s going to take maybe even a little bit of October.”

Tom Haag, a farmer near Eden Valley southwest of St. Cloud, said he and his son planted about 330 acres of corn after June 1, so he’s a little worried about those fields, which are about two weeks behind schedule.

“If it were the middle of July we’d say we were right on target,” he said.

But now it’s early August, and the plants just started to get tassels. “They won’t mature until October 1st,” Haag said.

The average first freeze happens from early to mid-October in Minnesota, depending on latitude, but it can happen sooner. Climatologists at the U have run the probabilities, and the risk of an early freeze is not insignificant.

“There’s about a 10% chance of there being a frost by September 16th at Fari­bault,” said Luigi Romolo, climatologist with the Minnesota Department of Natural Resources.

Such a frost would cause widespread crop damage.

“The later you go, the higher the probability,” Romolo said. “The 20% probability occurs on the 21st. You don’t get to a 50% probability until September 29th.”

Faribault, in south-central Minnesota, is firmly in the Corn Belt and is surrounded by farmland where planting was delayed. The earliest freeze in recent years there was in 2012, on Sept. 25, according to the Midwestern Regional Climate Center at the University of Illinois.

Determining where farmers are at greatest risk is complicated by the fact that farmers planted corn at different dates —, even within the same county. And they planted different types of corn. Some planted corn that matures in 100 days, others planted corn that matures in 90 days.

“If we get a mid-September frost and there’s no tassels showing yet, the impact on yield could be quite substantial,” Coulter said.

Crop insurance would cover the majority of the loss for farmers, but the subsidized insurance program is already in line to pay U.S. farmers an estimated $3.6 billion for fields that were left unplanted because of the poor weather in May and June.

Coulter is most concerned, he said, about the costs to farmers of a wet harvest.

“Everybody’s going to have to deal with a wetter harvest,” he said. “Big hassle. A lot of cost. A lot of energy, fuel that goes into drying that grain.”

Because wet grain is heavier, farmers can’t haul as many bushels on a semitrailer truck, which drives up the cost of moving the harvest from the field to the grain elevator.

The concerns about soybeans aren’t nearly as great, said Seth Naeve, a soybean agronomist at the U, because soybeans accelerate their maturation as the weather cools.

“Soybeans will mature in response to the environment,” Naeve said.

A sudden frost after a period of warm weather might catch the plants off guard, he said, but generally soybeans handle the first frost well, and they don’t need to dry out in the fields the way corn does.