- Air temperatures at or below 28 degrees Fahrenheit can be fatal to corn, even if the corn has not yet emerged.
- Frost injury, or injury from air temperatures above 28 degrees, can be extremely variable, even with the same frost event.
- Waiting a few days, and then carefully examining affected corn plants, is the best way to be sure about possible outcomes of frost-freeze injury.
- Certain management practices, like herbicide application, may need to be delayed when frost-freeze injury occurs.
Air temperatures at or below 28 degrees Fahrenheit for just a few hours can kill corn outright, even when the growing point of the corn is below ground. Air temperatures above 28 degrees have a variable effect on young corn. Frost injury to corn can actually occur at temperatures well above freezing. The above photo is a frosted corn plant showing signs of recovery.
When we have light frost occurrence, it typically shows up in bottom ground and lower parts of fields where the cold air has a chance to collect. It may also show up in fields protected by trees, since a light breeze can otherwise stir the air and prevent the cold air from collecting in one spot. Occasionally, frost injury is worse on high ground because this ground has better drainage and will lose stored heat faster than the moister, lower ground. During very still, clear nights, exposed crop plants can rapidly give up their heat to the upper atmosphere, resulting in freeze injury to the plant tissue, even when recorded air temperatures are several degrees above freezing.
We often see tillage or soil moisture effects with frost. Light tillage or cultivation tends to open the soil to rapid heat loss. Dry soils also tend to rapidly give up stored heat. A thick layer of crop residue on the soil surface will insulate the soil. This sometimes prevents radiant heat from protecting the young corn as air temperatures drop. It is important to note that the net effect of variable terrain and soil conditions on the young corn can drastically different, depending on the time and duration of the cold temperatures. Injury patterns in fields sometimes leave us scratching our heads until detective work helps explain what happened.
Frost injury on young corn plants typically has very little effect on yield. The growing point remains protected underground through growth stage V4. At V5 the growing point is right at ground level and by V6 it is above ground. Even after the growing point lifts above ground, it remains somewhat protected in the center of the rolled leaves. The growing point can be inspected by splitting the corn plant with a razor blade. A healthy growing point will appear firm. Dark, or water-soaked tissue is a sign that the growing point may have been damaged and the field should be monitored closely for a couple of days to assess the level of damage.
Frost injury is usually evident within two days or less following the frost event. Damaged leaf tissue will initially appear pale, or water soaked, and flaccid. The damaged tissue will subsequently turn brown, or straw-colored. Assuming that the frost was not of duration or intensity to kill the growing point, new leaves will emerge through the damage. Mowing damaged corn has not been shown to accelerate the recovery from frost injury and is not advised. In fact, mowing may introduce pathogens to freshly cut leaf tissue.
Postemergence applications of herbicides may need to be delayed following a frost. Injury to the crop may make it sensitive to the herbicide, while injury to weeds may make them tolerant to the herbicide. Affected fields should be evaluated carefully until the crop resumes normal growth and at least one new leaf collar (corn) is visible.