Early-planted corn faces a threat from unexpected spring frost events. Freezing temperatures can significantly damage young plants, potentially leading to substantial yield losses. Understanding the plant’s physiology and employing correct protective measures is necessary to mitigate this risk. Accurately assessing the damage and making informed recovery decisions determines the season’s final outcome.
Corn’s Vulnerable Growth Stages
The corn plant’s vulnerability to frost depends primarily on the location of its growing point, the area of rapidly dividing cells that generates new plant tissue. Up to the fifth leaf stage (V5), the growing point remains safely below the soil surface. This subterranean position provides natural insulation and protection from cold air and light frost. If frost occurs before V5, only the exposed leaf tissue is damaged, and the plant can recover by initiating new growth from the protected growing point.
The situation changes once the plant reaches the V6 stage, usually when it is 8 to 12 inches tall. At this point, the growing point begins migrating above the soil line, making the entire plant susceptible to frost. If temperatures drop below 28 degrees Fahrenheit for a prolonged period, the growing point can be injured or killed, even if still below ground. Damage at this later stage often results in plant death because the mechanism for new growth is compromised.
Immediate Strategies for Frost Protection
When a cold snap is forecasted, active protection measures can shield young corn plants from freezing injury. One effective technique is overhead sprinkler irrigation, which relies on the principle of latent heat. As water changes from liquid to solid ice, it releases heat energy, approximately 80 calories per gram of water frozen. This heat release maintains the temperature of the ice-coated plant tissue near 32 degrees Fahrenheit, preventing the internal plant temperature from dropping to a damaging level.
The irrigation system must start before the temperature reaches the freezing point, typically when the wet-bulb temperature drops to 34 degrees Fahrenheit. Continuous water application is necessary throughout the freezing event to ensure a constant layer of liquid water is freezing on the plant surfaces. If the application rate is insufficient, evaporative cooling can occur, drawing heat away from the plant and causing more severe damage than if no protection was attempted.
Irrigation should not stop until the air temperature rises above freezing and the ice begins to melt and detach from the plants. Managing soil conditions also contributes to frost mitigation. Moist soil absorbs and stores more solar energy during the day than dry soil, releasing this stored heat at night. This release slightly warms the air surrounding the seedlings, providing protection against light frost.
Air movement strategies combat radiation frost events, which occur on clear, calm nights leading to a temperature inversion. During an inversion, a layer of warmer air exists above colder air trapped near the ground surface. Wind machines or large fans are used to mix the layers, pulling the warmer air down to the crop canopy level. This mixing process can raise the temperature at the plant level by a few degrees, often preventing damage during a mild frost.
Diagnosing the Extent of Frost Damage
A quick assessment immediately following a frost event can be misleading, as the full extent of the damage is not instantly visible. It is necessary to wait three to five days after the freeze before attempting a final evaluation of plant survival. This waiting period allows surviving plants to initiate new growth and damaged tissue to clearly show signs of necrosis. Immediately after the event, leaves may appear water-soaked and quickly turn dark, black, or brown as cell membranes are destroyed.
The most reliable method for determining plant survival is to physically inspect the growing point itself. To do this, carefully dig up several affected plants from various field areas and longitudinally split the stalk with a sharp knife. The growing point is located at the base of the plant, often appearing as a small, cone-like structure.
A healthy, viable growing point will be firm and exhibit a bright white, creamy, or light yellow color. If the growing point has been killed, the tissue will appear mushy, soft, and discolored, often turning gray, brown, or black. Visual inspection of the whorl, the tightly rolled center leaves, is also important. The emergence of new, bright green tissue is a strong indicator of recovery, confirming the growing point is intact even if above-ground leaves are dead.
Post-Frost Management and Recovery Decisions
Once plant survival is confirmed, careful management is necessary to facilitate recovery. It is prudent to delay aggressive field activities, such as cultivation or the application of herbicides and fertilizer, for approximately one week. Surviving plants are already under stress, and adding further physical or chemical stress can impede their ability to regrow from the undamaged growing point.
The most difficult decision following severe stand loss is whether to replant the field. This is an economic choice, weighing the cost of new seed and planting operations against the potential yield reduction of the existing stand. A surviving stand often produces a higher yield than a replanted stand, as later planting dates inherently reduce the crop’s yield potential and increase the risk of a fall frost.
Replanting should be considered only when the stand count is significantly reduced, often below 18,000 to 20,000 uniform plants per acre in dryland situations. If replanting is chosen, it is most beneficial in areas where the stand loss is nearly complete. Inter-planting into a sparse but surviving stand creates competition between plants of different sizes and ages. The decision requires a calculation comparing the expected yield of the damaged stand with the expected yield of a late-planted, full stand, factoring in the cost of the replant operation.