Tomatoes are a warm-season crop originating from the tropical highlands of the Andes, which explains their sensitivity to cold. Although perennial in their native climate, they are typically grown as annuals and require consistently high heat to thrive and produce fruit. Their optimal growing range is between 65°F and 80°F. Cold is the main environmental factor that limits tomato cultivation and can quickly lead to plant damage or death.
The Risk of Chilling Injury
Damage to tomato plants can occur well above the freezing point, a condition known as chilling injury. This begins when temperatures fall below the optimal range but remain above 32°F, typically spanning 33°F and 55°F. The threshold where growth slows significantly and potential damage starts is around 50°F (10°C), especially when night temperatures consistently dip below this mark. Low nighttime temperatures disrupt the plant’s circadian rhythm, interfering with chemical reactions and stalling daytime photosynthesis, which reduces overall plant health and yield.
Prolonged exposure to temperatures between 40°F and 50°F slows development and causes visible stress symptoms in foliage. Young plants may display a yellowing or purpling of the leaves. While chilling injury is not immediately lethal, it severely weakens the plant, making it susceptible to disease and leading to poor fruit development. Mature green fruit exposed to temperatures below 55°F for extended periods can suffer internal breakdown and fail to ripen properly, resulting in poor color and a mealy texture.
The Lethal Threat of Freezing
The absolute danger zone for a tomato plant is 32°F (0°C), the temperature at which water freezes. Temperatures dropping to or slightly below this point cause frost, resulting in irreversible damage to the plant’s cellular structure. Because tomato tissues have a high water content, the formation of ice crystals inside the cells causes the cell walls to burst, leading to the rapid collapse of the tissue.
Plant vulnerability depends on its stage of growth and the severity and duration of the freeze event. Tender seedlings and newly transplanted tomatoes are often killed outright by a light frost. More mature plants may survive a brief, light frost if they are hardened off, but they will suffer massive tissue loss. Damaged leaves and stems take on a water-soaked appearance followed by blackening and wilting.
Protecting Plants from Cold Temperatures
To mitigate the threat of cold weather, gardeners should address both chilling injury and lethal freezing. The practice of “hardening off” is important for young transplants. This involves gradually exposing them to cooler temperatures and wind over seven to ten days before planting them outside. This process allows the plant to build stronger cell walls and better tolerate temperature fluctuations.
When an unexpected cold snap or light frost is forecasted, temporary physical protection is the most immediate solution. Covering the plants with lightweight fabric, such as frost cloth or blankets, can provide an extra 4°F to 8°F of protection. This covering must be draped over a support structure, like stakes or tomato cages, to prevent the fabric from touching the foliage. The edges should extend to the ground and be secured to trap heat radiating from the soil.
For extreme or prolonged cold, increasing the thermal mass around the plants offers greater insulation. Systems like water-filled devices absorb solar energy during the day and slowly release stored heat throughout the night, protecting plants from temperatures as low as 26°F. Moving container-grown plants into a protected area, such as a garage or covered porch, is the most effective way to ensure survival during a freeze. Temporary covers should be removed promptly after sunrise once the temperature rises above 32°F to prevent overheating.