Tomatoes are tropical perennials native to the Andes Mountains, thriving only in warmth. Outside of frost-free regions, gardeners treat them as annual plants, meaning the onset of cold weather signals the end of the growing season. Understanding the plant’s sensitivity to temperature allows for strategies to extend the harvest, save the plant, or utilize the remaining fruit.
Understanding Tomato Cold Sensitivity
Tomato plants are highly susceptible to cold stress due to their tropical origins. Metabolic functions slow significantly below 50°F (10°C), resulting in stunted growth and halting flowering and fruit set. This chilling injury is not immediately lethal but reduces the plant’s vigor and increases its vulnerability to disease.
The true threat is freezing temperatures, where water within the plant’s cells turns to ice. This ice formation ruptures the cell walls, leading to irreversible damage and the collapse of the plant’s structure. Any temperature at or below 32°F (0°C) is a killing frost for a tomato plant, making survival outdoors during winter impossible.
Short-Term Outdoor Protection Strategies
When an unexpected cold snap or mild, early frost threatens, temporary measures can protect plants still rooted outdoors. These strategies use the insulating properties of water and physical barriers to raise the temperature surrounding the foliage. Deeply watering the soil late in the afternoon before a cold night helps, as wet soil retains and radiates more heat than dry soil.
Physical coverings are the most common form of short-term protection. Old blankets, burlap sacks, or specialized row covers can be draped over stakes or cages to create a temporary thermal layer. The cover must extend all the way to the ground to trap rising heat and should not rest directly on the foliage, as cold transfers through contact points. Inverted buckets or large cloches can also protect individual plants briefly.
Remove these covers promptly the following morning, especially if the sun is out. Leaving a cover on when temperatures rise can cause the plant to overheat or trap excessive moisture, encouraging fungal diseases. These methods are designed only to buy a few extra weeks of harvesting time, not to sustain the plant through the entire winter.
Overwintering Tomato Plants Indoors
To keep a favorite or heirloom variety alive long-term, the entire plant must be moved indoors before the first hard frost. This process, known as overwintering, requires selecting a healthy plant and preparing it for a controlled environment. The plant should be pruned back drastically, removing most foliage, flowers, and fruit, allowing the plant to focus energy on survival.
The prepared plant must be repotted using fresh, sterile potting mix to minimize bringing outdoor pests or diseases inside. The indoor environment should maintain a temperature range between 60°F and 70°F. This range is warm enough to sustain the plant without encouraging rapid, weak growth. Consistent temperature is important, as large fluctuations stress the plant.
Light is the most challenging requirement for successful indoor overwintering. A sunny window alone is often insufficient, especially during winter’s shorter days. Supplemental lighting, such as LED grow lights set on a timer for 12 to 16 hours daily, is necessary to keep the plant healthy. Under these conditions, the plant enters a state of semi-dormancy, requiring minimal water and no fertilizer until spring.
Salvaging the Late-Season Harvest
When the plant cannot be saved, attention must turn to the remaining green fruit before a killing frost arrives. All fruit, regardless of maturity level, should be harvested before the temperature drops to 32°F. Immature tomatoes that have reached full size but not yet changed color are the best candidates for indoor ripening.
Tomatoes are climacteric fruit, meaning they ripen after picking by producing ethylene gas, a natural plant hormone. This process requires only a controlled temperature and gas accumulation, not sunlight. Placing green tomatoes in a loosely covered paper bag or cardboard box traps the ethylene, accelerating ripening.
Adding a ripe apple or banana introduces additional ethylene, further speeding up the process. The ideal ripening temperature is between 60°F and 70°F; temperatures below 50°F permanently halt the process, resulting in flavorless fruit. Small or damaged tomatoes can be reserved for alternative culinary uses, such as frying or pickling.