Chad Lee and James Herbek, University of Kentucky  

Almost 40% of the corn in Kentucky has been planted according to the April 12 Crop Weather Report from the Kentucky Agricultural Statistics Service. The five-year average for corn planting at this time is only 13%. Much of this corn was planted before the spring frost free dates. The early planting of corn increases the likelihood that some of this corn will emerge and be exposed to freezing temperatures. 

Factors Required for Damage

The severity of damage to a corn plant from frost primarily depends on temperature and growth stage of the plant. Length of exposure to freezing temperatures is another factor. A couple hours of freezing temperatures typically is required for damage to plant tissue. Temperatures between 32 and 28 degrees F typically have little effect on corn. Frost damage is usually limited to above ground plant parts (leaf tissue). Corn easily recovers from this type of damage early in its development. Temperatures at 28 degrees F or less for a few hours can be lethal to the plant. The growing point of a young corn plant can be injured or killed at these temperatures.  

Living plant cells contains water. Temperatures below 28 degrees F cause ice crystals to form in the plant cells. As the ice crystals form, they expand and puncture cell walls. As the temperatures rise during daylight, the ice crystals melt back to a liquid. The punctured cells lose water and nutrients and die. Frost damaged plant tissue often has a water-soaked appearance. This appearance is due to the loss of liquid from the punctured cells.

Freezing temperatures can destroy plant tissue but only have a minor impact on final yields. The growing point of corn plants are typically below the soil surface from emergence until about the V5 growth stage in most hybrids. The V5 growth stage is defined as a corn plant with five leaves that have visible collars. At this stage, the sixth leaf is visible in the whorl, but its collar is not yet visible. The soil and the plant typically protect growing point from a couple hours of freezing temperatures. Corn plants at this stage of growth can experience high levels of leaf damage and still yield comparably to non-injured corn.

The growing point of corn will move above ground at slightly different stages of growth in different hybrids. In general, the growing point is usually above the soil surface by the V6 growth stage. Temperatures of 28 degrees F or less for a couple hours will kill V6 corn and may severely injure younger corn plants. Although the growing point is reasonably protected by the soil from emergence through the V4-V5 growth stage, it can be injured or killed even if it is still below the soil surface if temperatures below 28 degrees F occur for more than a few hours.

Managing Frost-Damaged Corn

You need to wait three to five days after the frost event to accurately access the extent of damage and determine if the plant are capable of recovery. By five days, new growth should be evident in the whorl of the corn plant. If new growth is not evident, then corn plant is likely dead. Warm days after a frost event will benefit recovery, while cooler temperatures will delay recovery. Wet conditions after frost damage can induce pathogenic infections of the dead, moist plant tissue and inhibit recovery.

At times, the frost damage leaves will fuse together near the whorl. This fusing can impede the new growth of leaves. Some farmers have tried mowing the corn plants to resolve the problem of fusing. Mowing provides inconsistent results and is not recommended.

If 55 to 70% of the leaves are defoliated on V4 corn, but new growth is observed, then nothing should be done to these plants. In most cases, the damaged corn will yield as well as non-damaged corn. If 100% of the leaves are defoliated, then wait to see if recovery is beginning by five days after the frost. If recovery is not evident, then replanting is probably a good option.

Estimated stand of surviving plants is the most important item to measure about 5 days after the frost event. Frost damage is likely uneven across the field, so multiple stand counts should be made in the field. Both injured and non-injured areas of the field should be counted. Count the number of surviving plants within a row. Use Table 1 to determine how long of a row to count to estimate plant stand. Compare the number obtained in Table 1 to the population numbers in Table 2 to help determine maximum yield. The information in Table 2 was obtained and adapted from the National Corn Handbook, NCH-30, "Guidelines for Making Corn Replanting Decisions” and is Table 5 in ID-139, “A Comprehensive Guide to Corn Management in Kentucky”.

Table 2 should be viewed as a general guide rather than an absolute rule. Most of the data in the table is averaged across the Midwest and may need adjustment for your particular area. For example, mid- to late-April plantings in western Kentucky are closer to optimum than plantings after May 10. The optimum date range shifts to early to mid-May for central and eastern Kentucky. Populations above 25,000 plants per acre should provide yields comparable to stands at 25,000 plants per acre based on Kentucky research. Therefore, populations above 25,000 plants per acre will not need to be replanted.

The average corn stand and the date of replanting will both be factors in determining if replanting corn is an option. Other factors involved in a replanting decision include the cost of operation for removing the surviving corn stand and replanting. Seed costs and availability of suitable hybrid seed are other factors to consider. If replanting will occur after June 5 in Kentucky, then an earlier maturing hybrid should be selected. In addition, corn yields will drop about 1% per day when corn is planted after May 10 – 15.

If you have questions regarding the condition of your corn crop, contact your county extension office.