Tomatoes are warm-season plants native to tropical regions, making them particularly vulnerable to cold temperatures. Because they lack the natural cold-tolerance mechanisms of hardier vegetables, even brief exposure to a cold snap can cause significant damage or death. Providing nighttime protection is necessary for gardeners in spring and fall when unexpected drops in temperature are common. Understanding temperature thresholds and the biological impact of cold allows for effective protection.
The Critical Cold Threshold for Tomatoes
The decision to cover tomato plants depends on the forecasted overnight low temperature, as different ranges cause distinct types of stress. Gardeners should monitor forecasts and act whenever temperatures are predicted to dip below 50°F (10°C). Below 50°F, the plant’s physiological processes slow significantly, leading to stunted growth and reduced flowering.
Between 40°F and 50°F (4.5°C to 10°C), tomato plants experience chilling injury, which disrupts normal function and can cause leaf discoloration. Protection becomes necessary when temperatures are expected to fall below 40°F (4.5°C). Temperatures in this lower range cause severe tissue damage, and if the temperature reaches 32°F (0°C)—the freezing point—unprotected plants will likely be killed by frost. Young seedlings are the most susceptible, sometimes suffering damage below 55°F (13°C). Established plants may briefly survive a dip just above freezing if protected.
Mechanisms of Cold Damage
The physical damage to tomato plants occurs through two main mechanisms: chilling injury and freezing injury. Chilling injury happens at temperatures above freezing, typically between 32°F and 50°F (0°C and 10°C), and is characterized by the disruption of cell membrane integrity. This breakdown impairs nutrient uptake and reduces the efficiency of photosynthesis, leading to symptoms like wilting, lesions, and necrosis.
Freezing injury occurs when the temperature drops to the freezing point, causing far more destructive damage. Water inside the plant’s tissues freezes, forming ice crystals that physically rupture cell walls and membranes. This causes fluids to leak out, leading to immediate tissue browning, blackening, and death. Additionally, temperatures below 55°F (13°C) during the flowering stage can cause abnormal pollen development and blossom drop, reducing the plant’s ability to set fruit.
Practical Strategies for Covering and Uncovering
Covering Techniques
Choosing the right material and technique determines the effectiveness of the protection against cold air and frost. Effective covers work by trapping the heat radiated from the soil throughout the night, creating a warmer microclimate around the plant. Suitable materials include specialized row covers, cotton sheets, blankets, or large cardboard boxes and buckets for smaller plants. Plastic sheeting must be supported so it does not directly contact the foliage, as cold plastic can transfer chilling damage.
The cover should extend all the way to the ground and be securely anchored with soil, rocks, or bricks to prevent warm air from escaping. For added warmth during severe cold, a secondary layer, such as a quilt over a plastic frame, can increase insulation. Some gardeners use miniature incandescent Christmas lights strung through the plant canopy to generate heat beneath the cover.
When to Uncover
Removing the covers in the morning is important. Covers should be taken off as soon as the temperature rises above the danger zone and the sun becomes strong. Leaving the covers on too long creates a greenhouse effect, rapidly increasing the temperature and humidity. This can cause the foliage to overheat, leading to sunscald or scorching. If there is any risk of frost, the covers should remain in place until the frost has visibly melted and temperatures are safely above 40°F.