What to expect from frost on emerged corn and soybean: The cold temps on Friday night resulted in questions on what to expect regarding any potential frost damage to emerged corn and soybeans. For crop damage situations, waiting five to 10 days helps when assessing regrowth potential. The warm conditions of the past weekend following this freeze event can result in a quicker assessment of 3-5 days.

Plant tissues typically do not freeze when the air temperature around them is 32°F. The reason is because solutes are present in the membrane-bound cytoplasm (and also just outside of cell membranes) and they act like a very modest anti-freeze. Thus, plant tissue usually does not freeze until the tissue temperature reaches 30°F to 28°F. Source: Pearce, R.S. 2001. Plant Freezing and Damage. Annals of Botany 87:417-424.

Early season hail and/or freeze damage on corn can cause exposed leaves to be damaged. Fortunately, until V5-V6, the growing point is below the soil and the corn should recover. The damaged leaves will dry and, as regrowth pushes through, the damaged leaves should blow away in the wind. However, wrapping can occur, so fields should be monitored.

In assessing corn plants, dig up and then slice open plants to evaluate the growing point. If the growing point is white or cream in color and firm, the plant should survive. In contrast, if the growing point is brownish-colored and mushy, the plant will not likely survive.

For soybeans, cotyledons are actually somewhat frost tolerant since they are 95% water and fairly thick, so low temperatures are needed for more time to cause damage. However, soybeans that are just emerging with the hypocotyl hook exposed at or just above ground level, are the most at risk for damage. The hypocotyl hook is the area of the stem below the soybean cotyledon. Anything that impacts it will result in seedling death. Watch for plants that have soft, mushy, or pinched hypocotyls. Soybeans can survive the cotyledons being stripped and/or burnt off. Soybean has additional growing points at the axillary buds. If damage occurred below the cotyledons, the plant will die. If above the cotyledons, check for new growth at axillary buds.

What about soybeans where the unifoliate leaves are exposed? Unlike corn, once the cotyledons emerge, all of the potential growing points are above ground so damage could occur. However, the key word in this last statement is growing points, as in more than one. So, even if low temperatures damage the stem tip, there are still two more growing points where the cotyledons attach that the soybean could regrow from. This can set back the plant and cause two stems to form but the plant is still viable if the growing points are still viable.

 Picture showing the environmental variability associated with frost damage (Photo by Jenny Brhel).

Pictures taken in 2019 by Jenny Brhel. The soybeans in the left photo had cotyledons just at the soil surface at time of frost. They survived (indicated by thumbs up emoji). The upper right-hand photo shows a seedling with light scarring on the hypocotyl and cotyledons. However, the hypocotyl wasn’t pinched and you can see the plumule between the cotyledons is alive and healthy. The lower right-hand photo shows the hypocotyl was damaged on these seedlings causing pinching. Thus these seedlings didn’t survive (indicated by thumbs down emoji). Soybeans with the hypocotyl hook just at the crack of the soybean surface will have to be watched for how deeply impacted that hypocotyl hook is (if it pinches clear through like in the thumbs down photo).