Frost damage can occur when air temperatures are in the mid-30s on calm nights, says Jeff Coulter, University of Minnesota extension agronomist.
The lack of wind allows the transfer of heat from air near the ground to the air above, resulting in colder temperatures near the soil surface.
"In general, frost damage tends to be worse in low areas where cold dense air settles, near field edges where vegetation reduces the potential for heat transfer from the soil to the air above, and in fields where high levels of surface residue coverage limit heat transfer from soil," he says.
Symptoms of frost-damage to corn are initially dark and water-soaked leaves, which later dry and turn brown.
"Since the growing point of the corn plant remains below the soil until the fifth to sixth leaf-collar stage frost prior to this stage typically does not kill the plant unless temperatures are low enough to freeze the upper part of the soil where the growing point is located," he says.
Because frost-damaged corn plants generally show new leaf growth a few days after the frost if their growing point was not damaged, delay assess damaged fields for 3 to 5 days after the frost.
To determine whether frost-damaged corn will survive, dig up plants and split stems to examine the growing point and the tissue directly above the growing point. Healthy growing points will be firm and white to yellow in color. If the growing point or plant tissue within 0.5 inches above the growing point is damaged, it will be watery and orange to brown in color, and the plant will not likely recover.
In general, crop recovery tends to be greatest when frost occurs before the third leaf-collar stage or when only a limited amount of leaf area is damaged after the third leaf-collar stage, since recovery is influenced in part by the amount of energy reserves in the seed and leaf area for growth," he says.
Source: U of M Extension Communications