Even though Memorial Day weekend, the unofficial start to summer, is only two weeks away, parts of northern Iowa saw temperatures dipping to the freezing mark or just below this past weekend. Harry Hillaker, state climatologist with the Iowa Department of Agriculture says a light freeze was reported over parts of northwest Iowa on Saturday morning May 14 with a 30 degree F temperature at Sheldon. Freezing weather was more widespread on Sunday morning May 15, especially across the northeast part of Iowa where Cresco, Decorah and Elkader all reported 27 degree minimums.

Frost bite showing up on emerging corn in some Iowa areas

Farmers are reporting frost damage to leaves on emerged corn seedlings in some fields in north-central Iowa, too. “In general, the symptoms of frost damage tend to be worse in low areas of fields where the cold dense air settles, and near field edges,” says Max Henson, farming near Goldfield. To determine whether frost-damaged corn will survive, he says he will scout these field areas each day but will wait four days to see if the plants whose leaves were nipped will start to recover. He’ll then split the plants to see if the growing point has been injured.

Some corn and soybean seedlings were nipped by frost
So how did the young corn and soybean plants survive in the hardest hit areas? “Some corn seedlings were emerged and Jack Frost nipped the leaves,” says Brian Lang, Iowa State University Extension field agronomist in northeast Iowa. Temperature is the key for both corn and soybeans that have recently emerged or are just emerging.

Temperature below 28 degrees  for a couple of hours will be lethal to growing tissue that has emerged, says Lang. Temperatures between 28 degrees and 32 degrees  will result in definite frost injury to aboveground tissue. And temperatures between 32 degrees and 50 degrees will result in poor vigor and growth.

Wait 3 or 4 days after frost occurs, to see if plants will recover
“Regardless of temperatures, the last couple of days have put planted corn and soybeans at greater risk for pathogen infection,” says Lang. “Make sure to get out into your fields over the next couple of days to assess stands. For frost-injured crops, wait 48 to 96 hours following the last frost or cold temperature occurrence to be able to fully assess living versus damaged or injured tissues on the plants.”

Yellow corn will respond to the sun yesterday and today, he adds. Water-soaked corn likely won't grow, but if the growing point is still viable (the cold temperature did not penetrate to growing point) then the plants will have to slough off the dead tissue. A dead growing point is unlikely unless temperatures got below 28 degrees for several hours.

How will this weekend’s frost event affect soybean seedlings?
Soybeans at the VE to VC growth stage, that is up to unifoliate leaf stage, are fairly resilient to cold temperatures, says Lang. “You can expect some frost damage to cotyledons and hypocotyl but survival is likely unless there were several hours of temperatures below 28 degrees. The apical meristem and/or auxiliary buds are not expected to be killed at temperatures down to 28 degrees for only a couple hours. However, the trifoliate leaves are susceptible to temperatures below 32 degrees.”

Regardless of whether or not the corn or soybean seedlings have emerged or not, patience will be the key. Assess the stands after three to five days, he advises. If above- ground vegetation is dead, look below ground to check for viable growing tissue.

Weekend frost also nipped fields in northwest Iowa
Cold temperatures below 32 degrees May 15 and 16 also nipped areas of northwest Iowa. Also, parts of northwest Iowa have received a lot of rain the past few weeks, leaving low-lying fields and areas of fields flooded.

“Our region has a real range of conditions with corn planting 65% to 75% complete in the northwest part of my area,” says Paul Kassel, ISU Extension field agronomist at Spencer. “There is maybe 15% of the soybean crop planted in the northwest part of my area. But the corn and soybean planting progress is 95% plus complete in the northeast part of my area.”

FROSTED: This frost injured cornfield near Decorah in northeast Iowa needs to be watched closely the next 3 to 5 days to see if plants recover.—Photo by Brian Lang, ISU Extension agronomist.

Some corn replant decisions are easier than others 
Questions he is getting from farmers today are replant decisions and questions about the effects of the frost damage to corn. “Corn replant decisions are based on the yield potential of your current stand versus the potential yield of the replant crop,” explains Kassel. “A stand of 20,000 plants per acre today from a late April planting date will yield about 89% of normal. A replant date of May 20 will produce a corn crop that yields about 87% of normal. Therefore, you should keep this stand and not replant it.”

If only it was that easy. The areas of fields that need replanted defy logic for the most part, says Kassel. “Sure there are drowned out areas that will be easy to replant—when it eventually dries out. However, many areas within fields that will need to be replanted are kind of hit and miss.”

DON’T RUSH TO REPLANT: Agronomists advise that you give injured corn plants a few days to see if they will recover from frost, before you decided to rip up a stand and replant. After several days, dig up some plants, split open the stems and examine the growing point in the stem.

He adds, “Ideally, we would like to see those areas worked up so you eliminate the original stand and therefore eliminate the competition from the original stand—when you replant the corn. However, many of these areas of fields are not real conducive to reworking since they are irregular in size and difficult to get to without running over some good crop.”

The good news is replanting corn in May is usually low risk
So no there are no easy answers. The good news is that replanting corn in May is usually low risk. “Also, hybrid maturity does not have to be changed very much,” says Kassel. “Something in the 100-day hybrid maturity range will work well for planting or replanting corn in late May.”

YELLOW CORN LEAVES: These young, seedling corn plants were stuck by frost May 15-16 in northwest Iowa, where the temperature dipped to below 32 degrees.

The frost of this past weekend was an unwelcome addition to the other crop challenges farmers are wrestling with this spring. Initially, the frost damage of May 15-16 does not look too severe. “However, many times it takes a while for the damage to show up. Therefore, the best advice is to be patient when evaluating corn damage from frost,” says Kassel.