If your soils are dry enough, go ahead and plant corn as early as the ground is workable.
"We no longer use soil temperature as a guide for planting after April 1," points out University of Illinois agronomist Emerson Nafziger. "Once soil conditions are good for planting in April, there probably should be little hesitation in getting started."
There is, of course, a greater risk of frost damage when planting super early. Frost can develop on corn leaves even though the temperature may not drop below 32 degrees, says Purdue University corn specialist Bob Nielsen. That's due to heat loss to the atmosphere (radiation cooling) on clear, calm nights.
A corn plant's survival depends primarily on whether the growing point has been injured by the frost. It stays below ground until the plant has six leaves with visible collars, Nielsen points out. That's roughly knee high. If the crop freezes before that, it usually recovers from the damage.
But that's not a certainty. Disease can attack.
"Seedlings stressed by frost are more susceptible to pythium," says Larry Herrman, Pioneer agronomist at Princeton, IL. "Pythium, which is the prime killer of injured plants, thrives in cool, wet weather. Consequently, the plants' chances for recovery depend on temperature and moisture conditions following the frost.
"We saw lots of pythium last spring because of the prolonged wetness."
Frosted cornfields need to be left alone for three to five days after injury to give them time to start recovery, Nielsen notes.
"Damaged plants will turn greenish-black during the first 24 hours, then bleach to a straw color as they dry out," he says. "As the frosted leaf tissue in the whorl dries, the whorl often develops a 'knot.' That knot may restrict expansion of the undamaged whorl tissue."
However, says Nielsen, assuming there is no disease, knotted plants usually will recover as expanding whorl tissue breaks those knots.
Even if there is no frost damage, early planted corn should be closely monitored if there is any kind of stress, recommends Pioneer's Herrman.
"Dig plants and examine the roots and mesocotyl (the below-ground stem)," Herrman advises. "Rootless corn can occur if there is either hard, dry soil or erosion that washes soil from the roots and causes them to lose their anchor. Cultivation can help by throwing loose dirt at the base of the plants."
Also check for uniformity of stand, recommends Nielsen. Gaps and crowded plants can easily decrease yields by 5-8 bu/acre, he notes.
Try to determine if the problem is related to planter accuracy (doubles, gaps or both) or pests or weather (gaps). Correct what is correctable the following spring.