Tomato plants are natives of warm regions, making them highly susceptible to damage when temperatures drop. Their growth and fruit production are directly tied to consistent heat. Knowing the precise temperature at which a plant suffers stress is important for a successful harvest. Providing protection during cold snaps can extend the time for fruit to mature.
The Critical Temperature Threshold
Tomato plants stop active growth and enter a state of stress when the temperature consistently falls below 50°F (10°C) overnight. Below this point, metabolic processes slow significantly, hindering flower development and fruit ripening.
The true danger threshold, known as chilling injury, occurs when temperatures dip into the 32°F to 50°F range for extended periods. Protection becomes an immediate necessity when the forecast predicts temperatures will drop below 40°F (4.4°C). This colder range can cause significant cellular dysfunction.
The hard freeze point, 32°F (0°C), is a fatal threshold because the water inside the plant’s cells will freeze and expand. Any exposure to a hard freeze will result in the immediate death of the plant’s tender tissues.
How Cold Damages Tomato Plants
Cold damage occurs through two distinct physiological mechanisms: chilling injury and freeze damage.
Chilling injury happens when temperatures are above freezing, typically between 32°F and 50°F. During this time, the integrity of the plant’s cell membranes is compromised, which impairs nutrient absorption and disrupts the process of photosynthesis. This injury manifests as stunted growth, wilting, and sometimes a yellowing or purpling of the leaves.
Freeze damage, conversely, is caused by the formation of ice crystals inside the plant’s tissues at 32°F or below. As water freezes, it expands, causing the cell walls to rupture. This internal damage is irreversible and results in the characteristic appearance of frost damage, where the foliage turns dark, water-soaked, and eventually blackens or wilts.
Essential Timing and Preparation
Proactive monitoring of the weather forecast is the single most important step in cold protection, especially watching for overnight lows and dew points. It is generally advised to take action when the forecast calls for temperatures in the mid-30s Fahrenheit.
An effective pre-cold-snap strategy is to thoroughly water the soil around the plants hours before the temperature is expected to drop. Wet soil holds heat much better than dry soil, and this stored thermal energy will radiate upward to help warm the plant canopy overnight.
Covers must be applied before sunset to trap the residual warmth accumulated in the soil throughout the day. The ground is the source of protection, and covering the plants too late will miss the opportunity to capture this heat. Covers should be taken off shortly after sunrise and once the temperature begins to rise. Leaving covers on too long, particularly plastic materials, can cause the plant to overheat from the greenhouse effect, resulting in sun scald or rapid death.
Practical Methods for Protection
The most effective method for protecting tomatoes involves creating an insulating barrier that does not directly touch the foliage. Commercial row covers, heavy blankets, or burlap work well because they are dense enough to slow radiative heat loss. If using plastic sheeting or tarps, it is crucial to use stakes or tomato cages to create a support structure. Direct contact between cold, wet foliage and a covering material, especially conductive plastic, can draw heat away from the plant and cause localized freeze damage.
For individual plants, the existing tomato cage can be wrapped entirely with a fabric cover and secured at the base to the ground with bricks or soil to prevent cold air from entering. Potted tomato plants offer the easiest solution, as they can simply be moved to a protected location, such as a garage, shed, or under a porch. For a slight boost in microclimate temperature, placing large jugs of water near the base of the plant before covering can help, as the water releases heat as it cools down during the night.