Air temperatures at or below 28 °F can be fatal to corn and soybeans, depending on their duration and crop growth stage.
Waiting a few days after the freeze or frost event to examine affected plants, is the best way to assess the extent of damage and make management decisions.
Certain management practices such as herbicide applications, may need to be delayed when frost-freeze injury occurs.
Air temperatures at or below 28 °F for just a few hours can kill corn outright, even when the growing point is below ground, while air temperatures above 28 °F have a variable effect on young corn. For soybeans, when the air temperature drops below 28-32 °F, plants can experience some degree of frost damage. If the freeze period lasts for several hours and damage occurs below the cotyledons, soybeans can die. When light frost occurs, 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 still clear nights, exposed 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 be drastically different, depending on the time and duration of the cold temperatures.
The plant growth stage at the time of freezing is also important in determining the extent of the damage. Minor 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. In soybeans the most vulnerable growth stage is the crook stage. If soybeans have not yet emerged at the time of freeze, no damage will occur. However, if a frost occurs while soybean crooks are exposed and the tissue below the cotyledons (hypocotyl) freezes, the result is likely plant death.
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