Plant tomatoes outside after your last spring frost date, when soil temperature reaches 60°F
The single most important date for outdoor tomato planting is your last spring frost date—the average date when freezing temperatures stop for the season in your area. Tomatoes die when exposed to frost. Soil temperature matters just as much: tomato seeds and seedlings will sit dormant or rot if the ground is colder than 60°F, and they grow fastest when soil reaches 65°F or warmer.
To find your last frost date, search "last frost date" plus your zip code, or check your local cooperative extension office website. The USDA Hardiness Zone Map and almanac sites like almanac.com also provide this information by location. Once you know the date, wait until at least one week after it to transplant seedlings outdoors. This buffer accounts for unexpected cold snaps.
Soil temperature is separate from air temperature. A warm day does not mean the ground has warmed up. Use a soil thermometer (available at garden centers for $10–20) to check the actual temperature 4 inches deep where you will plant. If you do not have a thermometer, wait until nighttime temperatures stay above 50°F for at least a week before planting.
Key Takeaways
- Your last spring frost date is the cutoff—plant tomatoes at least one week after this date to avoid frost damage that kills the plant.
- Soil temperature must be 60°F or higher; check with a soil thermometer rather than relying on air temperature alone.
- In most of the northern United States, this means late May or early June; in the South, it can be as early as late March or April.
- Hardening off seedlings (gradually exposing them to outdoor conditions) for 7 to 10 days before planting reduces transplant shock and improves survival.
- If an unexpected frost is forecast after you plant, cover seedlings with row covers or blankets on the night of the frost.
How frost dates work and why they matter for tomatoes
A frost date is not a single day that happens every year at the same time. It is a statistical average based on 30 years of weather records for your specific location. Your last spring frost date tells you when freezing temperatures are unlikely to return, but it is still possible—just rare. That is why planting one week after the date, rather than on the date itself, gives you a safety margin.
Tomato plants cannot survive a hard frost (32°F or below). Even a light frost (32°F to 28°F) will damage leaves and stop growth. Young seedlings are more vulnerable than mature plants because they have less stored energy. If you plant too early and a frost hits, the plant may die outright or be so damaged that it never produces well.
Different regions have very different frost dates. In Minneapolis, the last frost date is around May 15. In Atlanta, it is around April 15. In San Diego, frost rarely occurs, so tomatoes can go in the ground in late February or early March. Check your specific location rather than assuming a date based on your state.
Soil temperature: why 60°F is the real starting line
Even if the air is warm and frost is no longer a threat, cold soil will stall tomato growth. Seeds will not germinate in soil colder than 50°F, and they germinate very slowly below 60°F. Transplanted seedlings will sit in the ground without growing, and roots may rot if the soil stays wet and cold for too long.
The easiest way to check soil temperature is to push a soil thermometer 4 inches into the ground in the morning, wait 2 minutes, and read it. Do this in the spot where you plan to plant. If it reads below 60°F, wait another week and check again. Soil warms gradually as spring progresses, so checking every few days in late spring will tell you when it is ready.
If you are impatient and plant in cold soil, you can warm it up by laying black plastic mulch or landscape fabric over the bed for one to two weeks before planting. This traps heat and raises soil temperature by several degrees. Remove it before you plant, or cut holes in it for the seedlings.
Hardening off seedlings before moving them outside
Hardening off means gradually exposing indoor-grown seedlings to outdoor sun, wind, and temperature swings so they toughen up before permanent planting. Seedlings started indoors under grow lights are tender and will wilt or burn if moved directly into full sun and outdoor conditions.
Start hardening off about 7 to 10 days before your planned planting date. On day one, set seedlings in a sheltered spot with dappled shade for 2 to 3 hours, then bring them back inside. Each day, increase the time outside and the amount of direct sun they receive. By day 7 or 8, they should be outside all day. If nighttime temperatures will stay above 50°F, leave them out overnight as well.
Watch the weather forecast during hardening off. If a cold night is predicted, bring the seedlings back inside. If a hot, windy day arrives, provide afternoon shade or move them to a windbreak. The goal is to let them adjust gradually, not to shock them with extreme conditions.
Planting depth and spacing for transplants
When you move seedlings into the ground, bury them deeper than they were in their pots. Tomatoes can develop roots all along the buried stem, which creates a stronger root system. Bury the seedling up to the first true leaves (the leaves that look like tomato leaves, not the round seed leaves). If the seedling is leggy or tall, you can bury it even deeper, laying it at an angle in the soil.
Space seedlings 24 to 36 inches apart, depending on the variety. Determinate (bush) tomatoes can be closer; indeterminate (vining) tomatoes need more room for air circulation. Crowded plants are more prone to disease. If you are planting in rows, space rows 3 to 4 feet apart so you can walk between them and reach the fruit.
Water the seedling well after planting, and keep the soil consistently moist (not waterlogged) for the first two weeks while roots establish. Mulch around the base with 2 to 3 inches of straw or wood chips, keeping it a few inches away from the stem to prevent rot.
What to do if frost threatens after you plant
Even after you plant, an unexpected frost can occur in late spring. If the forecast calls for frost within a few days of planting, cover the seedlings with row covers (lightweight fabric that lets light through), old blankets, or even newspaper held down with soil or stones. Leave the cover on until the temperature rises above freezing the next morning.
Do not use plastic sheeting unless you remove it during the day, because plastic touching the plant can conduct cold and damage it further. Row covers and blankets are safer because they insulate without direct contact. If a seedling is damaged by frost (leaves turn black or mushy), do not pull it out when ready. Wait a few days to see if new growth emerges from the stem. Many tomato plants recover from light frost damage if the stem is still alive.
Regional planting timelines
Planting dates vary widely across North America. The table below shows approximate last frost dates and typical outdoor planting windows for major regions, but your specific location may differ. Always check your local frost date rather than relying on your state or region alone.
| Region | Last Frost Date (Approximate) | Typical Planting Window |
|---|---|---|
| Upper Midwest (Minnesota, Wisconsin) | May 10–20 | Late May to early June |
| Mid-Atlantic (Pennsylvania, New York) | May 1–15 | Mid-May to early June |
| Upper South (Kentucky, Tennessee) | April 15–May 1 | Late April to mid-May |
| Deep South (Georgia, Alabama) | April 1–15 | Mid-April to early May |
| Southwest (Arizona, New Mexico) | March 15–April 15 | Late March to mid-April |
| Pacific Northwest (Oregon, Washington) | May 1–15 | Mid-May to early June |
| California Coast | April 15–May 15 | Late April to early June |
Use this table as a starting point, but your exact planting window depends on your microclimate—whether you are in a valley, on a hilltop, near water, or in an urban area. These factors can shift your frost date by one to two weeks. Your local cooperative extension office has the most accurate frost date for your specific address.
Frequently Asked Questions
Can I plant tomatoes if the air temperature is warm but the soil is still cold?
No. Warm air does not warm the soil fast enough. Cold soil will slow or stop root growth, and seedlings may rot. Wait until soil reaches 60°F, even if the air temperature is 70°F or higher. A soil thermometer is the only reliable way to know.
What happens if I plant too early and a frost kills my seedlings?
You will need to replant. Keep extra seedlings indoors or buy new ones from a garden center. This is why waiting until one week after the last frost date is worth the patience—replanting costs time and money.
Do I need to harden off seedlings if I buy them from a garden center?
Yes. Garden center seedlings are also grown indoors under controlled conditions and need hardening off. The process is the same: gradually expose them to outdoor sun and temperature over 7 to 10 days before planting.
Can I plant tomatoes in containers instead of in the ground to avoid frost risk?
Yes, and containers warm up faster than ground soil in spring. You can move containers indoors if frost threatens. Use containers at least 5 gallons for determinate varieties and 10 gallons for indeterminate types, and fill them with potting soil (not garden soil).
What if my area has unpredictable spring weather with warm days and cold nights?
Wait until nighttime temperatures stay consistently above 50°F for at least a week. This is more reliable than a single frost date, especially in regions with erratic spring weather. You can also plant in stages: put out a few seedlings first, and if they survive, plant the rest a week later.