Even though indeterminate tomatoes are technically perennials, they are grown as annuals because of their extreme aversion to cold. Not only do tomatoes stop producing as temperatures sink, they are killed completely by hard frosts and freezes.
Tomatoes' Natural Climate
Tomatoes were originally bred from subtropical plants originating in an area covering parts of Peru and Ecuador and spread northward to Mexico before Columbus set foot in the Western hemisphere. In these areas, tomatoes grew abundantly, bolstered by the steamy, but frost-free temperatures that frequently fell within their optimal temperature range. Tomatoes spread to Europe, where they were at first feared, then later adored for their sweet fruits. Unfortunately, these plants are still adapted to those climates where they originated, making long periods of cool temperatures -- below 55 degrees Fahrenheit -- often as damaging as frosts. Tomato plants grow and produce best between 60 and 85 degrees Fahrenheit.
How Frost Kills
Frost generally occurs on still nights when the ambient temperature drops near or below freezing. Exposed plant tissues are damaged when water vapor in the air condenses on plant surfaces, then freezes. Condensation forms because plant tissues are initially warmer than the surrounding air, though they cool quickly in the night. Often these events happen quickly in the coldest part of the night, creating frost only briefly. Unfortunately for tomatoes, even a brief stint of freezing is enough to end the growing season.
Protecting Tomatoes From Light Frost
If a light frost is predicted, you may be able to protect your tomatoes from the chill injury or freeze damage it can cause. Depending on the size of your plants, you can use a variety of covers to insulate your plants from the colder air outside, from large cardboard boxes for smaller patio tomatoes to large, clear plastic tents for larger garden varieties. Remove the cover in the morning to prevent accidental overheating.
Ripening Tomatoes Indoors
When frost threatens, some gardeners uproot their plants and bring them indoors to ripen the fruit that remains on the vine. This will only work with fully formed fruits that are just barely beginning to turn red, so if you decide to hang your tomatoes upside down in the basement, pick off any underdeveloped fruits. Your tomato plant will die, but the vine creates a handy place to store fruit as it finishes ripening.
- University of California, Davis Vegetable Research and Information Center: Tomatoes
- University of Arkansas Cooperative Extension: Tomatoes
- University of California Statewide Integrated Pest Management Program: Time to Plant Tomatoes
- Texas A&M Agrilife Extension: Frosts and Freezes
- University of Illinois Extension: Protecting the Home Vegetable Garden From Spring Frost
- Oregon State University Extension Service: How to Ripen Green Tomatoes off the Vine
- Colorado State University Extension: How to Ripen Green Tomatoes
- University of Alaska Cooperative Extension Service: A Harvest of Green Tomatoes
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