Will Frost Hurt a Tomato Plant's Fruit?
Tomatoes (Solanum lycopersicum) are heat-loving plants that can be easily damaged or killed by frost and freezing temperatures. Tomato plant frost damage occurs when temperatures drop below freezing, and colder temperatures, even if above freezing, will stunt the plant’s growth. You can employ a few strategies to protect your plants and extend the growing season.
Tomato fruits that have been frozen should be discarded. You can extend the growing season by covering tomato plants in the evenings before a frost is expected.
Ideal Temperatures for Tomatoes
Tomatoes need sun and plenty of it but not just sun – they need warmth. Plant them outside after your area’s last frost date, usually May to early June in most United States locales. (Use the Old Farmer's Almanac to find your last frost date.) Flowers set fruit only when night temperatures are between 55 and 75 degrees Fahrenheit.
Temperatures below 50 degrees will stunt a tomato and impair its growth, while fruit may not ripen if the temperatures are below 60 degrees. Tomatoes are also sensitive to swings in temperature as well as very high temperatures. Missouri Botanical Garden reports that night temperatures below 55 degrees or above 70 degrees impact fruit ripening and can cause blossom drop in either spring or summer.
At the same time, day temperatures over 90 degrees can also contribute to a poor fruit set, especially if combined with very low humidity or drought. In summer when it's very hot, be sure to water your plants daily.
Tomato Plant Frost Damage
Tomatoes cannot withstand freezing temperatures, so use protection when a frost is expected. This is true of both the plants themselves as well as the fruit.
What temperature will kill a tomato plant? It depends on whether the plant tissue actually freezes. Michigan State University Extension reports that a frost occurs when temperatures hit 32 degrees Fahrenheit or lower. Depending on other conditions and if the frost is brief, the plant may not actually freeze. When freezing occurs, however, the plant and its fruit will not recover.
New buds and actively growing vegetative growth are particularly sensitive. While the damage may not be immediately apparent, plants will wilt and become blackened or otherwise distorted, according to the Missouri Botanical Garden. When the water inside plant cells freezes, the cells expand until the cell walls rupture and the cells collapse.
If the plant is established and a brief freeze occurs, it’s possible that the plant itself can withstand the cold, although new shoots and leaves will likely die off. This is also true of the fruit; once it actually freezes, it will not survive.
How to Extend Your Tomato Growing Season
You can extend the growing season for your tomatoes by protecting them with coverings. You can also harvest green tomatoes if you know that a frost is expected. If the fruit has frozen, discard it and consider your tomatoes finished for the season. If your plants and fruit are not yet damaged, try these techniques.
To protect the plants from frost, cover them using lightweight blankets, old sheets, tarps, clear plastic or any other covering that you can wrap around them without damaging their structure. This works when first planting them in the spring and also in the fall.
When a hard freeze is coming and you know the season is over, harvest any green, mature fruit – don’t bother with the immature fruits, as they will never ripen. Missouri Botanical Garden recommends that you wrap each tomato in a sheet of newspaper and pack them in a paper bag or cardboard box in a cool location, such as a garage or basement. When you’re ready to ripen them, take them out and place them in a dry, 70-degree area.
If you are looking for frost-resistant tomatoes, note that there are no cultivars that can withstand a real freeze. If you live in a northern climate where the growing season is short, University of Illinois Extension recommends choosing a cultivar that ripens quickly. Tomatoes' ripening time (from seed germination to harvest) ranges from about 45 to 90 days.
- The Old Farmer’s Almanac: First and Last Frost Dates
- Missouri Botanical Garden: Frost Injury and Ice Damage
- Missouri Botanical Garden: Tomato Fruit Problems
- Missouri Botanical Garden: Gardening Help FAQs
- Michigan State University Extension: Identifying and Preventing Freeze Damage in Fall Vegetables
- University of Illinois Extension: Tomato
Shelley has been writing and editing garden stories for 10 years, and has a Master Gardeners certificate in Oregon.