Temperatures near freezing, 32 degrees Fahrenheit, can damage tomato plants. Growing best at temperatures of 65 to 68 F at night and 70 to 75 F during the day, they slow fruit production when temperatures drop below 60 F. Some varieties have been specially bred to set fruit between 40 and 50 F.
Tomato plants are not frost-tolerant. Temperatures 40 F and lower damage tomato plants, causing soft, discolored leaves and stems, and decaying fruit. The plants sometimes recover from frost damage if the lower portion of their stems isn't affected.
Early harvesting and watering helps prevent tomato crops from freezing when winter approaches. Mature, green tomato fruits can ripen after they have been harvested. If frost is forecast, harvest the green tomato fruits that have stopped growing larger. Place the fruits in a single layer in a dark, airy place where temperatures don't fall below 55 F. Over time, the tomato fruits will turn red.
The air temperature above irrigated soil can be up to 5 degrees higher than above dry soil. Water the ground around tomato plants thoroughly in the evening when frost threatens. Spraying water over tomato plants during the coldest part of the night also may prevent the plants from freezing when the temperature is 30 to 32 F; if the temperature drops lower, however, the plants must be continually watered until the temperature rises above 32 F.
Covers and Water Tubes
Floating row covers and tubes filled with water protect tomato plants during frosts. Composed of spun polyolefin fabric, floating row covers offer protection from temperatures between 26 and 32 F and are barriers to insects. Spread floating row covers over hoops of wire, bamboo canes or other structures to prevent the covers from lying directly on the tomato plants. Old bedsheets, blankets, cardboard and other coverings also can be used, but, unlike row covers, they must be removed during the day to allow light to reach the plants.
Water tubes protect young tomato plants from freezing temperatures. The connected tubes filled with water stand vertically around the plants. Closing the tops of water tubes protects plants from temperatures as low as 20 F, but the tubes must be removed when the plants grow large and become crowded.
Plastic Tunnels and Winter Care
Low or high plastic tunnels provide long-term protection from cold temperatures for tomato plants. In areas with long cold seasons, hoops of wire, polyvinyl chloride or other durable materials covered in plastic 1 to 1.5 millimeters thick protect the plants from cold winds and heavy rain, and they provide some protection from freezing temperatures. In high tunnels, which are large enough to walk through, floating row covers give additional protection. On sunny days, these tunnels must be opened to prevent their interior temperatures from rising too high.
Tomato plant growth and fruiting slow during winter. Water the plants when the soil surface is dry, but don't overwater or else the ground will become soggy. Wait until the tomato plants' growth begins in spring before applying fertilizer.