Plant tomatoes outside after your last spring frost date, when soil temperature reaches 60°F and daytime air stays above 50°F

The single most important date for outdoor tomato planting is your last spring frost date — the average date of the final frost in your area. Tomatoes die when exposed to frost. Planting before that date, no matter how warm a few days feel, will kill seedlings or young plants overnight. You can find your frost date by entering your zip code into the USDA Hardiness Zone Map or by calling your local cooperative extension office.

Beyond the frost date, soil and air temperature matter just as much. Tomato seeds and transplants need soil at least 60°F to grow; below that, they sit dormant and rot easily. Daytime air temperature should stay consistently above 50°F. If nighttime temperatures still dip into the 40s regularly, wait another week or two. Tomatoes planted into cold soil and cold air grow slowly, stay weak, and attract disease.

Key Takeaways

  • Your last spring frost date is the hard important date — plant after that date, never before, because a single frost will kill tomato plants.
  • Soil temperature must reach 60°F and stay there; use a soil thermometer to check, not just air temperature.
  • Daytime air temperature should be above 50°F consistently, and nighttime lows should stay above 45°F for at least a week before planting.
  • In cold climates, waiting until soil is 65°F or warmer produces stronger plants and faster growth than planting into marginal conditions.
  • If you plant too early and a frost warning appears, cover plants with blankets, cardboard, or row cloth the night before.

How to Find Your Last Frost Date

Your last frost date depends entirely on where you live. The USDA Hardiness Zone Map divides North America into zones based on average minimum winter temperatures, and each zone has a corresponding frost date range. Enter your zip code at planthardiness.ars.usda.gov to see your zone and the frost date for your area.

If you prefer to call, your county's cooperative extension office has the exact frost date for your location and can answer questions about your specific microclimate — a valley, hilltop, or area near water may frost later or earlier than the county average. Search "[your county] cooperative extension" online to find the phone number.

Write down your frost date and mark it on a calendar. This is the earliest safe day to plant tomatoes outside. If you plant seedlings indoors under lights, count backward six to eight weeks from your frost date to know when to start seeds.

Checking Soil Temperature Before Planting

Air temperature and soil temperature are not the same. A warm day in spring can feel like planting weather while the soil is still cold and wet. Cold soil slows root growth, invites fungal disease, and causes transplants to sit in place for weeks without growing. A soil thermometer costs five to ten dollars and removes the guesswork.

Push the probe three to four inches into the soil in the spot where you plan to plant. Wait five minutes for a reading. Tomatoes need soil at least 60°F; 65°F or warmer is better. Check the soil temperature in the morning, when it is coolest, to get a realistic picture. If soil is still below 60°F, wait a week and check again. Soil warms fastest in full sun and in dark-colored soil that absorbs heat.

If you do not have a soil thermometer, feel the soil with your hand. Soil that feels cold and clammy is too cold. Soil that feels cool but not cold, and crumbles easily, is usually ready. This is not as reliable as a thermometer, but it is better than guessing by air temperature alone.

Regional Timing Examples

Frost dates vary widely across North America. In the warmest zones (zone 10 and above, found in southern Florida, southern California, and southern Texas), the last frost date falls in January or February, and tomatoes can go outside in late winter. In zone 9 (parts of the Gulf Coast and southern Arizona), the last frost is typically in late March or early April. In zone 8 (much of the upper South), it falls in mid-April. In zone 7 (the mid-Atlantic and lower Midwest), it is late April or early May. In zone 6 (the upper Midwest and Northeast), it is mid-May to late May. In zone 5 and colder (northern New England, upstate New York, Minnesota, and parts of the Mountain West), the last frost does not arrive until late May or early June.

These are averages. A frost date of May 15 means that frost occurs on or after that date in half the years and before it in the other half. If you are risk-averse, wait until a week or two after your frost date to plant. If you are willing to cover plants if frost threatens, you can plant a few days before.

What to Do If Frost Threatens After Planting

Even careful planners sometimes face an unexpected frost warning after planting. Tomato plants can survive a light frost (32°F to 28°F) if you protect them the night before. Cover plants with blankets, old sheets, cardboard boxes, or commercial row cloth. Drape the cover loosely so it does not touch the leaves, and anchor it to the ground with soil, rocks, or stakes so wind does not blow it away. Remove the cover the next morning once temperatures rise above 40°F.

Do not use plastic sheeting alone — it conducts cold and can damage plants. Cloth or paper is better. Water the soil well before covering; moist soil holds heat better than dry soil. If frost dips below 28°F, even covered plants may not survive, but protection gives them a fighting chance.

Hardening Off Seedlings Before Planting

If you started tomato seeds indoors under lights, seedlings grown in a warm, still, protected environment need time to adjust to outdoor conditions before planting. This process is called hardening off. Outdoor air is drier, windier, and more intense than indoor light, and sudden exposure stresses plants.

Begin hardening off about a week before your planned planting date. Place seedlings outdoors in a sheltered, shaded spot for one to two hours. Each day, gradually increase the time outside and move them into more sunlight. By the end of the week, seedlings should spend the full day and night outdoors in dappled sun or morning sun. Then they are ready for full sun and permanent planting.

If you buy transplants from a nursery, ask whether they have already been hardened off. Nursery plants often have, but seedlings you grew yourself have not. Skipping hardening off causes transplant shock — wilting, leaf burn, and stunted growth — even if soil and air temperature are ideal.

Spacing and Depth When Planting Outside

Once soil and air temperature are right and seedlings are hardened off, dig a hole slightly deeper than the root ball. Tomatoes can be planted deeper than they grew in their pot — bury the stem up to the first true leaves, and roots will form along the buried stem, creating a stronger root system. Space plants 24 to 36 inches apart, depending on variety. Determinate (bush) types need less space; indeterminate (vining) types need more.

Water well after planting and keep soil consistently moist for the first two weeks while roots establish. Mulch around the base with two to three inches of straw or wood chips, leaving a gap around the stem to prevent rot. Do not fertilize for the first two to three weeks; transplants have enough nutrients from potting soil. After that, feed every two to three weeks with a balanced fertilizer or one slightly higher in phosphorus to encourage flowering.

Frequently Asked Questions

Can I plant tomatoes if the forecast shows frost in a few days?

No. Wait until your frost date has passed and nighttime temperatures stay above 45°F for at least a week. A single frost will kill young plants. If you have already planted and frost threatens, cover plants overnight with blankets or row cloth.

What if my soil is still cold but the air feels warm?

Wait. Soil temperature is more important than air temperature for tomato root growth. Cold soil causes rot, disease, and slow growth even if daytime air is warm. Use a soil thermometer to check; if soil is below 60°F, wait another week and check again.

Is it better to plant seeds or transplants outside?

Transplants are faster and more reliable. Seeds need warm soil (70°F or higher) and take six to eight weeks to reach planting size. Most home gardeners start seeds indoors under lights and transplant outside after hardening off. Direct seeding outdoors works in warm climates where soil stays above 70°F.

Can I plant tomatoes in containers instead of the ground?

Yes. Container soil warms faster than ground soil in spring, so you can plant a week or two earlier. Use a pot at least five gallons for determinate varieties and ten gallons for indeterminate types. Fill with potting mix, not garden soil, and water more often because containers dry out faster.

What happens if I plant tomatoes too early?

Early-planted tomatoes in cold soil sit dormant, attract fungal disease, and may rot before they grow. Even if they survive, they grow slowly and stay weak. Waiting until soil and air are warm produces stronger plants that grow faster and produce fruit sooner overall.