According to the USDA Soil Survey of Craven County, 5 of 10 years will have temperatures at or below 32° F past April 2^nd but only 2 of 10 years will have temperatures this low after April 14^th. There is only a 10% probability of having temperatures at or below 28° F past April 6^th. Thus, the most probable last frost/freeze event is likely to occur about mid-April in most years. These periodic cold temperatures may cause significant leaf tissue damage to corn but may or may not cause significant yield loss. Several key factors need to be considered when evaluating any potential damage to corn from frost or freeze events.

A key temperature to monitor is 28° F. Temperatures below this point may cause death to seedling corn plants. Typically, within this area of Eastern North Carolina, temperatures may drop near this point but seldom reach this point when corn is planted. However, colder air is dense and will move downward. Thus, if temperature actually does hover around this critical mark, damage may be found in field depressions.

Secondly, other factors effect the actual temperature within a field other than the air temperature alone. Freshly tilled soil losses heat rapidly, thus plants in fields recently cultivated will suffer greater damage than those in no-till production. Even factors such as a soil’s cover residue, moisture or weed population may directly influence the microclimate around a tender corn plant keeping the air warmer than surrounding areas with no cover residue or low moisture. Thus, variance of potential damage should be expected.

The soil also serves as a buffer against rapid changes in air temperature. Since the growing point of corn remains below ground until corn reaches about the 5-6^th leaf stage, the ground provides great insulation from such rapid decline in temperature. As such, germinating seeds are seldom affected by rapid changes in air temperature. Likewise, severe leaf damage may occur from just a few short hours of low temperatures but no significant long-term damage results because the soil insulates and protects the growing point of the corn plant. Warmer temperatures following the frost/freeze event usually lead to rapid recovery of plant.

Conversely, frost/freezing temperatures that remain for several hours or consecutive nights of such low temperatures may reduce yield or cause death to the plants. Evaluation of potential damage from very low temperatures should not be done until 3-5 days after this (these) frost/freeze events. Preferably, evaluations should follow several days of temperatures above 70°F. Recovering plants will show signs of new growth emerging from the soil but badly injured of dead plants will show dark gray to brownish color at the growing point.

To evaluate whether or not this damage has occurred, examine the growing point of the corn plant. Begin by digging up several of the plants and cut the stem of the plant longitudinally. Since the growing point remains below the soil line look for an areas of cloudy growth about 0.5-0.75 inches below the soil line. This growing point is the meristematic area of the corn plant where leaves and the tassel are initiated. If this area is brown or gray, some damage has occurred. If this area is black, the plant is dead. Suggested images for comparison are listed below.

• Growing Point Images
• Growing Point Images and Frost Damage 
• Images of frost damage to corn and corn’s growing point 

Worst Case Scenarios

Injured Growing Point –   Some slight delay in growth may result when the growing point is injured but the plant not killed by frost/freeze. Cloudy, cool or cold days following the frost/freeze event may plant recovery even slower. Under these circumstances, monitoring for disease is warranted. Some of the plant population may simply be loss. However, unless there are large areas within the field with dead plants or there are numerous areas of plants missing in 4 feet sections (or larger), yield is usually not significantly reduced.

Severe Plant Death – Assuming the worst case where the growing point indicates many of the plants are dead and that the field was originally planted on April 1^st, one must now examine whether or not to replant. Research data suggest that unless there are large areas with no corn population or if there are many areas with no corn plant within 4 feet of row consistently throughout the field (no corn in adjacent rows), then it is seldom worthy of replanting. There are several reasons for this. First, additional seed and inputs require a higher yield to justify the added expenses of replanting. Seldom will later planted corn yield higher than earlier planted corn. Thus, unless there is very low probability of recovering initial expense from the damaged field, it is seldom worthwhile to replant. Secondly, it’s rarely as bad as we think. Even low corn populations normally yield higher than anticipated when planted timely. Simply put, the penalty for later planting is steep! Generally, unless the corn plant population is less than about 18,000 plants per acre, replanting is not economically feasible.

Two resources that provide some insight as to whether or not to plant are below. The first provides some general guidelines based upon loss assumptions and data. The second provides a detailed estimation and calculation of actual costs of replanting based upon projected yield and new expenditures.

Replanting Decisions for Corn

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Email the Author: mike_carroll@ncsu.edu