Plant seedlings outdoors after the last frost date in your area, when soil has warmed to at least 60°F

The single most important date for tomato seedlings is your local last frost date—the average day when freezing temperatures stop for the season. Tomatoes cannot survive frost. Plant seedlings in the ground only after that date has passed and nighttime temperatures stay above 50°F consistently. You can find your last frost date by entering your zip code at almanac.com/gardening/frostdates or asking your local cooperative extension office.

Soil temperature matters as much as air temperature. Tomato roots grow poorly in cold soil and may rot before they establish. Wait until soil reaches 60°F at a depth of 4 inches—ideally 65°F or warmer. Use a soil thermometer to check, or wait two weeks after your last frost date if you do not have one. Planting too early is the most common mistake; a seedling sitting in 50°F soil for weeks will stall, yellow, and attract disease.

Seedlings are usually ready to transplant 6 to 8 weeks after you started them from seed indoors. If you started seeds in mid-February, you would transplant around late April in most regions. Check your seedlings: they should be 6 to 8 inches tall, have their first true leaves (the second set of leaves that appear), and show a sturdy stem. Leggy, pale seedlings started too late in the season are harder to establish than stocky ones.

Key Takeaways

  • Never plant seedlings outdoors before your area's last frost date, which you can find by zip code at almanac.com/gardening/frostdates.
  • Soil temperature must reach at least 60°F (ideally 65°F) at 4 inches deep, which usually happens 1 to 2 weeks after the last frost date.
  • Seedlings are ready to transplant when they are 6 to 8 inches tall with sturdy stems and their first true leaves showing.
  • Planting seedlings in cold soil causes stunted growth, yellowing, and root rot—waiting an extra week is safer than planting too early.

How to harden off seedlings before planting

Seedlings started indoors are soft and will wilt or burn if moved directly into full sun and wind. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours. Each day, increase the time outside and move them into more direct light. By day 7 or 8, they should spend the full day outside; by day 10, they can stay out overnight if temperatures stay above 50°F.

Watch the weather forecast during hardening off. If frost or cold snaps are predicted, bring seedlings back indoors. A seedling that has been hardened off will have thicker, darker leaves and a stockier stem than one kept indoors. This toughened plant will establish faster in the garden and produce fruit earlier. The hardening-off period is not optional—skipping it leads to wilted, sunburned plants that take weeks to recover.

Planting depth and spacing in the garden

Bury tomato seedlings deeper than they grew in their pot—up to the lowest true leaves. Tomatoes will grow roots all along the buried stem, creating a stronger root system. Dig a hole slightly deeper than the seedling's root ball, set the plant in, and backfill with soil. Space seedlings 24 to 36 inches apart depending on the variety; determinate (bush) types need less space, while indeterminate (vining) types need more.

Water seedlings when ready after planting and keep soil consistently moist (not waterlogged) for the first 2 to 3 weeks while roots establish. A layer of mulch 2 to 3 inches thick around the base helps retain moisture and keeps soil temperature steady. Do not mulch directly against the stem—leave a 2-inch gap to prevent rot. This spacing and mulching routine prevents the most common early-season problems: drought stress and fungal diseases at the soil line.

Regional timing examples

Timing varies widely by location. In the upper Midwest and Northeast, last frost dates fall between May 10 and May 31, so seedlings go in the ground in late May. In the mid-Atlantic and upper South, last frost dates are typically April 15 to May 1, allowing planting in mid-to-late April. In the Deep South and Gulf Coast, last frost dates arrive in March or early April, so seedlings can go in by late March or April.

If you live in a region with a short growing season, start seeds indoors 8 weeks before your last frost date to give plants time to mature before fall frost arrives. In longer-season areas, you can start seeds later and still harvest a full crop. Check with your local cooperative extension for specific recommendations for your county—they track local frost dates and soil temperatures year to year and can tell you the best window for your exact location.

What to do if frost threatens after planting

If an unexpected frost or freeze is forecast after you have planted seedlings, cover them with row covers, blankets, or even upside-down buckets for the night. Remove covers the next morning when temperatures rise. A seedling that has hardened off can usually survive a light frost (32°F for a few hours) if the soil is moist, but frost below 28°F will kill tender growth.

If seedlings are damaged by frost, cut off the blackened leaves and stems—the plant may recover and regrow from undamaged nodes lower on the stem. Keep the soil moist and give the plant full sun to encourage new growth. Recovery takes 2 to 3 weeks, which is why waiting until after the last frost date is the safer choice than gambling on an early planting.

Starting seeds indoors on the right schedule

To time your seedlings correctly, count backward from your last frost date. Tomato seeds take 6 to 8 weeks to grow into transplant-ready seedlings. If your last frost date is May 15, start seeds indoors around March 15 to March 20. If your last frost date is April 15, start seeds around February 15 to February 20. Mark these dates on a calendar when you know your frost date, so you start seeds at the right time each year.

Seeds germinate in 5 to 10 days in warm soil (70 to 80°F) under grow lights or on a sunny windowsill. Once seedlings emerge, keep them under lights 14 to 16 hours a day and water from below to prevent damping off (a fungal disease that kills seedlings at soil level). Thin seedlings to one per cell or pot once they have their first true leaves. This indoor phase sets up everything that comes later—seedlings that are the right size and strength at planting time will outproduce rushed or delayed ones.

Frequently Asked Questions

Can I plant seedlings before the last frost date if I use row covers?

Row covers provide some frost protection, but they are not reliable for tomatoes. Tomatoes are sensitive to cold soil and air, and covers only delay the problem. Plant after the last frost date and use covers only as backup if an unexpected frost arrives after planting.

What if my seedlings are too tall and leggy?

Leggy seedlings happened because they did not get enough light indoors. Plant them deeper than usual—bury the stem up to the lowest leaves, and they will root along the buried portion. Water well and give them full sun once planted. They will catch up, though they may flower a week or two later than stocky seedlings.

How do I know if soil is warm enough without a thermometer?

A straightforward test: if you can comfortably hold your bare hand in the soil at 4 inches deep for 10 seconds, it is warm enough. Another rule of thumb: soil reaches the right temperature about 2 weeks after your last frost date in most regions. When in doubt, wait—cold soil is the bigger risk.

Can I plant seedlings in containers instead of the ground?

Yes. Use containers at least 5 gallons for determinate varieties and 10 gallons for indeterminate types. Follow the same hardening-off process and plant at the same time relative to your last frost date. Container soil warms faster than garden soil, so you may be able to plant a few days earlier, but do not skip hardening off.

What happens if I plant seedlings too early and they get frosted?

Frost will blacken leaves and kill tender growth. If the stem survives, the plant may regrow from lower nodes, but it will be set back by 2 to 3 weeks. This is why waiting until after the last frost date is worth the patience—a delayed start is better than replanting.