Tomato plants are warm-weather crops highly sensitive to cold temperatures. Gardeners often worry when a cold snap is forecast, particularly when temperatures hover near 40°F. Tomato plants thrive in consistent warmth, typically between 70°F and 85°F during the day. They begin to experience significant physiological disruption when temperatures drop below 50°F. Understanding the difference between cold stress and freezing is important for plant survival.
Survival Thresholds: Chilling vs. Freezing
The 40°F temperature threshold falls into the range of non-freezing cold stress known as chilling injury. Chilling injury generally occurs when temperatures are between 32°F and 50°F, causing subtle yet disruptive damage to tomato plants. While 40°F is not the immediate death point, it is a significant temperature that causes internal cellular dysfunction.
At 40°F, the plant’s metabolic processes slow down dramatically, and the fluidity of the cell membranes is compromised. Cell membranes are made of lipids, and the cold temperature causes these fatty acids to become rigid and less flexible, disrupting the movement of essential compounds. This loss of membrane integrity is the primary event of chilling damage and impairs the plant’s ability to regulate water and nutrient uptake.
Temperatures at or below 32°F are the true death threshold because they cause freezing injury, where ice crystals form inside the plant tissues, leading to irreversible cellular damage. The formation of ice outside the cell walls draws water out of the cells, causing them to collapse from desiccation. While exposure to 40°F is generally survivable, 32°F or below often results in the swift death of the plant’s tissue.
Recognizing Cold Stress and Chilling Injury
When tomato plants are exposed to temperatures below 50°F, they begin to show specific symptoms of cold stress, although these signs may not be immediate. The plant’s ability to absorb and transport water is quickly reduced, which can lead to the wilting of leaves, even when the soil is moist. This dysfunction can also present as symptoms similar to nutritional deficiency because the roots struggle to absorb and move ions to the above-ground parts of the plant.
Visible signs of damage often include a purplish or bronze discoloration on the leaves, especially on the undersides or near the veins. If the exposure is severe, areas of the foliage may develop water-soaked spots or necrotic lesions, which eventually turn brown or black. For plants that are flowering or setting fruit, cold temperatures can interfere with pollination, leading to blossom drop or the development of misshapen fruit, known as “catfacing”.
Chilling injury also disrupts the plant’s circadian rhythm, which is the internal timekeeping mechanism that regulates specific chemical reactions. Low night temperatures can delay the processes that would normally take place after dawn, resulting in less efficient daytime photosynthesis. The symptoms of stunted growth, leaf damage, and increased susceptibility to disease may become more apparent days after the cold event has passed.
Immediate Protection Strategies
When temperatures near 40°F or lower are forecast, immediate action is necessary to minimize the impact of chilling injury. Container-grown tomato plants should be moved to a protected location, such as a garage, porch, or indoors, shielding them from cold air and wind. For plants growing directly in the ground, physical barriers are the most effective method of protection.
Insulating in-ground plants can be achieved by covering them with a lightweight fabric, such as a sheet, blanket, or commercial row cover. The covering should be draped over a support structure, like stakes or hoops, to prevent the material from directly touching the foliage, as contact can transfer cold and cause damage. Ensure the edges of the covering extend to the ground, trapping the residual warmth released by the soil.
To maximize heat retention, water the soil thoroughly before sunset on the day the cold is expected. Moist soil retains heat much better and releases it slowly overnight, providing a beneficial buffer against dropping air temperatures. Utilizing temporary structures, such as cloches or Wall O Water devices, can also provide significant insulation, sometimes protecting the plant down to temperatures well below 32°F. These covers must be removed once the temperature warms up in the morning to allow for sunlight and airflow.