Move tomato plants outside after the last frost date in your area, once soil and air temperatures stay above 50°F during the day and night

Tomato plants are frost-sensitive and will die if exposed to freezing temperatures. The safest time to move them outside is one to two weeks after your region's last spring frost date—the average date when the final hard freeze occurs. You can find your last frost date by searching "[your city] last frost date" or checking the USDA Plant Hardiness Zone Map.

Even if frost is not a risk, tomato plants struggle in cold soil and air. Soil below 50°F slows root growth and nutrient uptake, stunting the plant. Air temperatures below 55°F at night cause the plant to stop growing and can trigger disease. Wait until nighttime temperatures consistently stay above 50°F, and soil temperature reaches at least 60°F before moving seedlings outside permanently.

Key Takeaways

  • Check your local last frost date—tomato plants will not survive a hard freeze, so moving them out too early can kill them.
  • Soil temperature matters as much as air temperature; soil below 50°F prevents roots from absorbing water and nutrients even if the air feels warm.
  • Harden off seedlings by exposing them to outdoor conditions for a few hours each day over one to two weeks before planting them in the ground.
  • If you move plants outside and a late frost is forecast, cover them with a blanket or move them back indoors for the night.

How to Find Your Last Frost Date

Your last frost date depends on where you live. Search online for "[your city name] last frost date" or visit the USDA Plant Hardiness Zone Map at planthardiness.ars.usda.gov. Enter your zip code to see your zone and the average last frost date for spring. This date is based on 30 years of weather records and is the safest benchmark for moving frost-sensitive plants outside.

Keep in mind that the last frost date is an average, not a may provide. Some years frost arrives later; some years it does not arrive at all. If you move plants out right on the frost date and an unexpected freeze occurs, you may lose them. Many gardeners wait an extra week or two after the frost date to be safer, especially if they live in an area with unpredictable spring weather.

Hardening Off Seedlings Before Moving Them Outside

Hardening off means gradually exposing indoor seedlings to outdoor sun, wind, and temperature swings so they adjust without shock. Seedlings grown indoors under lights are tender and can wilt, bleach, or die if moved directly into full sun and wind. Hardening off takes one to two weeks and prevents this damage.

Start by placing seedlings in a sheltered, shaded spot outdoors for two to three hours on the first day. Bring them back inside. Each day, increase the time outside by one to two hours and move them into brighter light. By day seven to ten, they should spend the full day outside in dappled shade or morning sun. By day ten to fourteen, they can handle full sun and stay outside overnight if temperatures stay above 50°F. After hardening off, they are ready to plant in the ground or move to a permanent outdoor location.

Soil Temperature and Why It Matters

Tomato roots do not grow well in cold soil, even if the air is warm. Soil below 50°F slows root development and nutrient uptake. Soil below 60°F causes roots to absorb water slowly, which can lead to wilting and disease. Soil at 65°F or warmer is ideal for tomato growth.

Check soil temperature with a soil thermometer, available at garden centers or online. Push the probe two to three inches into the ground in the morning. If the soil is still cold, wait another week or two before planting. In cooler climates, you can warm the soil faster by laying black plastic mulch over the bed for one to two weeks before planting. This traps heat and raises soil temperature by several degrees.

What to Do If Frost Is Forecast After You Plant

If you have already moved tomato plants outside and a frost warning is issued, bring them back indoors or cover them. For plants in pots, move them into a garage, shed, or house for the night. For plants in the ground, cover them with a blanket, sheet, or frost cloth in the late afternoon before the freeze. Anchor the cover to the ground so wind does not blow it away. Remove the cover the next morning when temperatures rise above freezing.

If you cannot cover the plants and frost occurs, check them the next morning. Frost-damaged leaves will be blackened and wilted. The plant may recover if the stem and roots survived. Remove the damaged leaves and wait a few days to see if new growth appears. If the entire plant is blackened and mushy, it is dead and should be removed.

Timing for Different Tomato Varieties

Determinate tomatoes (bush types that grow to a fixed size) and indeterminate tomatoes (vining types that grow all season) can both be moved outside on the same schedule—after the last frost date and when soil and air temperatures are warm enough. The variety does not change the frost sensitivity or temperature needs.

Heirloom and specialty tomatoes are no more cold-sensitive than standard varieties, but they may be slower to mature. If you live in a short growing season, start heirloom seeds indoors earlier so they have time to fruit before fall frost. Cherry tomatoes mature faster than large slicing tomatoes, so they can be planted on the same schedule but will produce fruit sooner.

Indoor Seedlings vs. Nursery Transplants

Seedlings you grow indoors from seed need hardening off before moving outside. Nursery transplants sold at garden centers have usually been hardened off already, but check by looking at the leaves and stems. If they are pale green and thin, they may not have been hardened off yet. If they are dark green and sturdy, they are ready to plant outside after the frost date.

Even hardened nursery transplants should not go outside until soil and air temperatures are warm enough. A transplant that looks healthy can still be killed by cold soil or frost. Follow the same temperature guidelines—wait until nighttime air stays above 50°F and soil reaches 60°F or warmer.

Frequently Asked Questions

Can I plant tomatoes outside before the last frost date if I use a cold frame or row cover?

A cold frame or row cover can protect plants from light frost, but it does not solve the cold soil problem. Soil below 60°F still slows root growth and nutrient uptake. If you use a cover, wait until soil temperature is at least 60°F, then cover the plants to protect them from frost. Remove the cover during the day so plants get sun and air circulation.

What if my area does not have a clear last frost date?

Search for your USDA Plant Hardiness Zone on the USDA website, which gives an average last frost date for your region. If you live in a very mild climate where frost rarely or never occurs, you can plant tomatoes outside once soil reaches 60°F and nighttime air stays above 50°F, regardless of the frost date.

How do I know if my seedlings are ready to harden off?

Seedlings are ready to harden off once they have two to four true leaves (the leaves that look like tomato leaves, not the first round seed leaves). They should be sturdy and dark green. Weak, pale seedlings need more time under lights before hardening off begins.

Can I plant tomatoes in a greenhouse or cold frame earlier than outdoors?

Yes. A greenhouse or cold frame traps heat and can be 10 to 20 degrees warmer than outdoor air. You can plant tomatoes in a greenhouse or cold frame one to two weeks earlier than in open ground, as long as you ventilate on warm days to prevent overheating and monitor for frost on cold nights.

What happens if I plant tomatoes too early and they get cold?

Cold-stressed tomato plants may stop growing, develop purple or yellow leaves, or wilt. If temperatures stay cold for more than a few days, the plant may develop disease or die. If a single cold night occurs, the plant may recover once temperatures warm. If you plant too early and cold weather persists, dig up the plants and replant them indoors or in a warmer location.