What Temperatures Will Kill Tomato Plants?

Hardiness Zones

• Gardeners in the northern U.S. hardiness zones overcome many challenges to grow tomato plants. Young plants die if set out in the garden before the temperature of the soil rises to 50 degrees. Mature plants are killed by frost and temperatures that go below 40 degrees. For tomato growth in cold climates, keep a greenhouse at 60 to 75 degrees, advises the University of Alaska at Fairbanks Extension Service.

Cold Temperatures

• Tomato plants need a consistently warm temperature that is higher than 50 degrees during the day or night. When the growing season nears the end, it is important to pay attention to cold temperature and frost predictions. Plastic covers and wraps can be used to protect established plants during a drop in temperature. If the night temperature dips below 50 degrees and a frost hits the plants, the tomatoes will no longer produce the flavor enzyme.

High Temperatures

• A tomato plant needs daily watering, particularly in high temperatures. The plant will wilt and die if it is subjected to days of intense heat of 95 degrees or higher, in direct sun, under drought conditions. A temperature over 85 degrees will stop the development of lycopene, the cancer-fighting substance that gives tomatoes their red pigment.

Extended Plant Life

• Trials were conducted on tomato plants in 2000 and 2001 at the University of Arizona, to develop heat tolerance for growth in the desert.