Tomato plants stop growing below 50°F and die if exposed to frost
Tomato plants are warm-season crops. They cannot survive freezing temperatures, and they slow down or stop producing fruit when nights drop below 55°F. The plant itself may not die until the temperature reaches 32°F (the freezing point), but by then the damage is already done — the leaves blacken, the stems collapse, and the fruit stops ripening.
The practical lower limit for growing tomatoes outdoors is 50°F during the day and 55°F at night. Below that range, the plant becomes stressed. Photosynthesis slows, nutrient uptake stalls, and flowering stops. If you live in a region where nighttime temperatures regularly dip below 50°F, you will need to start seeds indoors, transplant seedlings after the last frost date, and cover plants with frost cloth or move them inside before the first frost arrives in fall.
Key Takeaways
- Tomato plants grow best between 70°F and 85°F during the day and 60°F to 70°F at night.
- Growth slows noticeably below 55°F at night, and the plant stops flowering and setting fruit below 50°F.
- Temperatures above 90°F during the day can cause flowers to drop and fruit to stop developing, especially if nighttime temperatures stay above 75°F.
- Frost (32°F or below) kills tomato plants outright, so you must protect them or bring them indoors before the first freeze.
- The time between your region's last spring frost and first fall frost determines how long you can grow tomatoes outdoors.
The ideal temperature range for fruit production
Tomatoes produce the most fruit and ripen it fastest when daytime temperatures stay between 70°F and 85°F and nighttime temperatures remain between 60°F and 70°F. This is the sweet spot where the plant has enough warmth to flower reliably, set fruit, and develop sugars in the ripening fruit.
When nighttime temperatures climb above 70°F, the plant uses more energy on growth and less on fruit production. Pollen becomes less viable, flowers drop without setting fruit, and the plant may stop flowering altogether. This is why tomatoes grown in very hot climates sometimes produce less fruit during the hottest weeks of summer, then resume production when temperatures cool slightly in late summer or early fall.
What happens when temperatures exceed 90°F
Heat stress begins around 90°F during the day, especially if nighttime temperatures stay above 75°F. At these temperatures, pollen becomes sterile and flowers drop before they can set fruit. The plant may look healthy and green, but it will not produce tomatoes until conditions cool.
Fruit that is already developing may stop ripening and remain green or pale. The skin may also develop sunscald — pale, papery patches where the fruit has been damaged by intense sun combined with heat stress. If you live in a region where summer temperatures regularly exceed 95°F, choose heat-tolerant varieties (often labeled as such on seed packets) and provide afternoon shade with shade cloth or by planting taller crops nearby.
Protecting plants from spring and fall frost
Frost occurs when the air temperature drops to 32°F or below. A light frost (32°F to 28°F) may kill the leaves and stems of a tomato plant but leave the roots alive; a hard frost (below 28°F) kills the entire plant. You cannot save a tomato plant once it has frozen solid.
The standard approach is to know your region's last spring frost date and first fall frost date. These dates mark the average day when frost last occurs in spring and first occurs in fall. You can find them by searching "[your city] frost dates" or by contacting your local cooperative extension office. Start seeds indoors 6 to 8 weeks before the last spring frost date, then transplant seedlings outdoors only after that date has passed and nighttime temperatures stay above 50°F.
In fall, watch the forecast. When frost is predicted, cover plants with frost cloth, old blankets, or plastic sheeting in the late afternoon, then remove the covering the next morning once temperatures rise. Alternatively, pick all remaining fruit (even green tomatoes) before the frost arrives and ripen them indoors on a windowsill or in a paper bag.
How soil temperature affects transplant success
Soil temperature matters as much as air temperature. Tomato roots grow slowly in soil below 60°F and stop growing altogether below 50°F. Transplanting seedlings into cold soil stresses them and can cause the roots to rot before they establish.
Wait until soil has warmed to at least 60°F before transplanting seedlings outdoors. In most regions, this happens one to two weeks after the last spring frost date. You can check soil temperature with an inexpensive soil thermometer (available at garden centers) or by feeling the soil — if it feels cold to the touch, wait another week. Planting too early into cold soil is one of the most common reasons transplants fail to thrive.
Hardening off seedlings before moving them outside
Seedlings started indoors are tender. Moving them directly from a warm, sheltered indoor environment to outdoor sun and wind shocks the plant. Hardening off — gradually exposing seedlings to outdoor conditions over 7 to 10 days — toughens the stems and leaves so the plant can handle the temperature swings and wind it will face outside.
Start by placing seedlings in a sheltered spot outdoors (against a wall, in a cold frame, or under a porch) for 2 to 3 hours on the first day. Each day, increase the time outdoors and expose them to more direct sun and wind. By day 7 to 10, they should be able to stay outside all day and night. This process is especially important if you are transplanting in spring when outdoor temperatures are still cool and variable.
Using row covers and cold frames to extend the season
A frost cloth (also called row cover or frost blanket) is a lightweight fabric that traps heat and protects plants from light frost. It allows light and water through but creates a small air pocket that can raise the temperature under the cloth by 2°F to 8°F, depending on the fabric weight and how well it is sealed at the edges.
A cold frame is a bottomless box with a transparent lid (glass or plastic) that sits over plants. It works like a miniature greenhouse, trapping solar heat during the day and releasing it slowly at night. Cold frames are more effective than frost cloth for protecting plants from hard frost, but they require more setup and must be opened on warm days to prevent overheating.
Both methods extend the growing season by a few weeks in spring and fall. In spring, they allow you to transplant earlier than you otherwise could. In fall, they allow you to keep plants producing fruit into late autumn or even early winter in mild climates.
Frequently Asked Questions
Can I grow tomatoes indoors under lights if outdoor temperatures are too cold?
Yes. Tomatoes grown indoors under grow lights need 14 to 16 hours of light per day, warm soil (65°F to 75°F), and good air circulation. They will produce fruit, but yields are usually lower than outdoor plants because indoor light is less intense than sunlight. This method works best for determinate (bush) varieties and cherry tomatoes rather than large slicing types.
What is the difference between the last frost date and when I can plant tomatoes?
The last frost date is when frost stops occurring, but soil may still be cold. Tomatoes need soil at least 60°F and nighttime air temperatures above 50°F. This usually happens one to three weeks after the last frost date, depending on your region. Check soil temperature before planting rather than relying on the date alone.
If I pick green tomatoes before frost, will they ripen indoors?
Yes, but slowly. Green tomatoes picked before frost will ripen indoors at room temperature (65°F to 75°F) over two to four weeks. They will not develop the same flavor as tomatoes that ripened on the vine, but they will turn red and become edible. Place them in a paper bag with a ripe banana or apple to speed ripening slightly.
Why do my tomato flowers drop in summer even though it is warm?
Flowers drop when nighttime temperatures stay above 75°F or daytime temperatures exceed 95°F. This is heat stress, not a disease. Provide afternoon shade, water consistently, and wait for cooler weather. The plant will resume flowering once temperatures moderate.
Can I move potted tomatoes indoors for winter?
Yes, but the plant will produce less fruit indoors because light is weaker than outdoors. Move the pot inside before the first frost, place it in the brightest window available, and expect slower growth and fewer flowers. Many gardeners straightforward let the plant die and start fresh the following spring.