Corn (maize) is a tropical grass, making it naturally sensitive to cold temperatures. Understanding the precise temperature limits the plant can endure at various stages is important for successful planting decisions and protecting yield. While corn can survive brief exposures near 32°F (0°C), prolonged cold or a deep freeze causes permanent damage or death. The extent of the injury depends on the duration of the cold event and the plant’s specific stage of development.
Understanding Cold Stress: Chilling Versus Freezing
Cold damage to corn falls into two distinct categories: chilling injury and freezing injury. These occur at different temperature ranges and impact the plant in fundamentally different ways. Chilling injury occurs when temperatures are above freezing, generally between 32°F and 50°F (0°C to 10°C). Exposure to this range slows the plant’s metabolism, impairs water and nutrient uptake, and reduces photosynthesis efficiency. This stress can lead to stunted growth and a lack of vigor, especially in young seedlings.
Freezing injury is a physical and often lethal event that occurs at or below 32°F (0°C). When temperatures drop sufficiently, ice crystals form within the plant’s cells, rupturing cell membranes and leading to tissue death. Plant tissue usually does not freeze until 30°F to 28°F (-1°C to -2°C) due to natural solutes acting as antifreeze. Damage severity escalates rapidly; temperatures around 28°F (-2°C) can kill corn in only a few minutes of exposure.
Stage-Specific Temperature Thresholds for Corn
Early Stages (Planting/Germination)
The earliest stages of corn development are highly susceptible to cold exposure, particularly during the first 24 to 48 hours after planting. When the dry seed first absorbs water, a process called imbibition, soil water temperatures below 50°F (10°C) can cause imbibitional chilling injury. This rapid uptake of cold water interferes with cell membrane hydration, causing them to rupture. This leads to nonviable kernels or aborted shoot and root growth. Planting into cool soils followed by cold rain or snow melt increases the risk of this damage.
If the soil temperature dips below 50°F after imbibition, the risk of specific injury decreases because water uptake slows. However, consistently cold soils below 50°F significantly delay emergence, potentially keeping the seed in the ground for up to four weeks. The growing point remains protected below the soil surface, usually about three-quarters of an inch deep, until the V5 to V6 stage. This underground location buffers the plant against above-ground frost and brief freezing air temperatures.
Vegetative Stages (V1-V5)
During the early vegetative stages, corn is resilient to frost because the growing point remains safely below the soil line. Even if air temperatures at or slightly below 32°F (0°C) kill all above-ground leaf tissue, the plant can often recover. The temperature threshold for lethal damage is the soil temperature surrounding the growing point, not the air temperature. If soil temperatures at that depth fall to 28°F (-2°C) or lower for several hours, the growing point can be killed, resulting in plant death.
Reproductive/Maturity Stages (Tasseling onward)
Once the corn plant reaches the V5 to V6 stage, the growing point moves above the soil surface, increasing its vulnerability to cold. Cold exposure during the reproductive stages, especially around flowering (tasseling and silking), severely impacts yield. Cold temperatures can affect pollen viability, leading to poor kernel set.
In the fall, a killing frost (at or below 32°F) prematurely halts the grain-filling process. If this hard freeze occurs during the mid-dough stage (R4), yield loss can be substantial. If the plant is killed by frost at the dent stage (R5) or earlier, it leads to reduced grain quality and the forced formation of the black layer, signaling premature maturity.
Assessing Damage and Recovery Potential
Immediately following a cold event, the visible damage to corn leaves may look severe, often appearing water-soaked before turning brown or black. It is important to resist the urge to assess the damage too quickly, as initial appearance can be misleading. Waiting three to five days after the cold event is necessary to accurately determine the extent of the injury and the plant’s capacity for recovery. This waiting period allows new growth to become visible if the plant is still alive.
The most definitive way to check for survival is to inspect the growing point, especially in plants up to the V5 stage. This involves digging up a few plants and splitting the stalk lengthwise through the crown. A healthy growing point will be firm and white or pale yellow, indicating likely recovery. If the growing point is soft, mushy, or discolored (water-soaked brown or orange), the plant is dead and unlikely to recover. Signs of recovery include the emergence of new, healthy, lime-green leaves from the center whorl.