Start seeds indoors 6 to 8 weeks before your last spring frost

The single most important date for tomato seeds is your local last spring frost date. Tomatoes cannot survive frost, so you work backward from that date. Count back 6 to 8 weeks and start your seeds indoors on that day. If your last frost date is May 15, you start seeds around March 20 to April 1.

You can find your frost date by searching "[your city] last spring frost date" or by entering your zip code into the USDA Plant Hardiness Zone Map at planthardiness.ars.usda.gov. The date varies by hundreds of miles — a city 50 miles north may frost two weeks later than a city to the south. Using your actual local date matters more than using a regional average.

Starting too early means seedlings grow tall and weak while waiting for warm soil outdoors. Starting too late means you transplant small, fragile plants into the garden when they need more time to establish roots. The 6 to 8 week window gives you sturdy plants with true leaves, ready to harden off and move outside.

Key Takeaways

  • Find your last spring frost date by zip code on the USDA Plant Hardiness Zone Map, then count back 6 to 8 weeks to set your seed-starting date.
  • Start seeds indoors under grow lights or in a sunny window, keeping soil warm (70 to 80 degrees Fahrenheit) and moist until seedlings emerge.
  • Harden off seedlings by moving them outside to a sheltered spot for a few hours each day, starting one to two weeks before transplanting.
  • Transplant seedlings into the garden only after the last frost date has passed and soil temperature reaches at least 60 degrees Fahrenheit.
  • If you miss the indoor window, you can direct-sow seeds outdoors after the last frost date, though they will fruit later than transplanted seedlings.

What happens if you start seeds too early or too late

Seeds started more than 8 weeks before the frost date will be ready to plant long before the soil is warm enough. Seedlings sitting in pots for weeks become leggy — tall, thin, and weak — because they are reaching for light while waiting. Leggy plants transplant poorly and take longer to recover in the garden.

Seeds started fewer than 6 weeks before the frost date produce small, underdeveloped seedlings that go into the ground before they have enough roots or true leaves. These plants are slower to grow, slower to flower, and slower to set fruit. You lose weeks of growing time in summer when the plant should be producing tomatoes.

The 6 to 8 week window is a target, not a hard rule. If your frost date is uncertain or you live in a place where frost can arrive early or late, starting at 7 weeks gives you a middle ground. You can also start a second batch two weeks later if you want insurance against a late frost killing your first transplants.

Indoor seed-starting conditions: light, warmth, and moisture

Tomato seeds germinate best in warm soil — between 70 and 80 degrees Fahrenheit. A seedling heat mat under your seed tray speeds germination from 7 to 10 days down to 5 to 7 days. Without heat, germination takes longer and some seeds may not sprout at all, especially if your house is cool.

Keep the soil moist but not waterlogged while seeds are germinating. A spray bottle works better than pouring water, which can wash seeds or create standing water that causes rot. Once seedlings emerge, they need strong light — at least 14 to 16 hours per day. A sunny south-facing window works if you have one; otherwise, use a grow light positioned 2 to 3 inches above the seedlings.

As seedlings grow their first true leaves (the second set of leaves that look like actual tomato leaves), they need less warmth and more light. You can turn off the heat mat once they sprout. Water when the top of the soil feels dry to the touch. Seedlings that are too wet develop damping-off disease, a fungal rot that kills them at the soil line.

Hardening off: moving seedlings from indoors to outdoors

Seedlings grown indoors are tender and will wilt or die if moved directly into the garden. Hardening off means gradually exposing them to outdoor conditions — wind, direct sun, and temperature swings — over 7 to 10 days. Start one to two weeks before your last frost date.

On day one, place seedlings in a sheltered spot outdoors (against a wall, under an eave, or in a cold frame) for one to two hours in the morning. Bring them back inside. Each day, add an hour and move them to a slightly more exposed location. By day 7 or 8, they can stay outside all day. By day 10, they can stay out overnight if temperatures stay above 50 degrees Fahrenheit.

Watch the weather forecast during hardening off. If a frost or hard freeze is predicted, bring seedlings inside. If a heat wave arrives, move them to shade and water more often. The goal is to toughen the plants without shocking them into dormancy or death.

Transplanting into the garden after the last frost date

Plant seedlings into the garden only after two conditions are met: the last frost date has passed, and soil temperature is at least 60 degrees Fahrenheit (ideally 65 or warmer). You can check soil temperature with a soil thermometer or by feeling the soil 4 inches deep — it should feel warm, not cool.

Dig a hole slightly deeper than the seedling's root ball. Tomatoes can be planted deeper than they were growing in the pot — bury the stem up to the first true leaves, and roots will form along the buried stem, creating a stronger plant. Space plants 24 to 36 inches apart depending on the variety (check the seed packet). Water well after planting.

If you harden off correctly and plant at the right time, seedlings usually recover from transplant shock within a few days and resume growing. If you rush and plant too early into cold soil, growth stalls for weeks.

Direct-sowing seeds outdoors if you missed the indoor window

If you did not start seeds indoors, you can plant seeds directly into the garden soil after the last frost date and soil reaches 60 degrees Fahrenheit. Plant seeds half an inch deep, 6 inches apart, and thin seedlings to 24 to 36 inches apart once they have two true leaves.

Direct-sown tomatoes will fruit 2 to 4 weeks later than transplanted seedlings because they spend time as small plants in the ground instead of growing indoors. In short-season areas (frost dates in late May or June), this delay can mean fewer tomatoes before fall frost arrives. In warm climates with long growing seasons, the difference is minor.

Direct sowing works best for determinate varieties (bush types that fruit all at once) and for gardeners in warm zones. Indeterminate varieties (vining types that produce all summer) benefit more from the head start that indoor seeding provides.

Frequently Asked Questions

Can I start tomato seeds in February if I have grow lights?

You can, but the seedlings will be large and leggy by planting time. Unless your last frost date is in late March or early April, starting in February means waiting 10 to 12 weeks before transplanting — too long indoors. Start closer to your actual frost date so seedlings are ready when the soil is warm.

What if my area has unpredictable frost dates?

Use the average last frost date from the USDA map, not the earliest possible frost date. If frost arrives early, you lose some plants; if it does not, you have extra time to plant more. Starting at 7 weeks (the middle of the 6 to 8 week range) gives you a buffer. You can also start a second batch of seeds two weeks later as backup.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination and is helpful if your house is cool, but it is not required. Seeds will germinate without one, just more slowly — sometimes 10 to 14 days instead of 5 to 7. If you have a warm windowsill or a spot near a heating vent, you may not need a mat.

Can I start seeds outdoors in a cold frame?

Yes, if your last frost date is at least 8 weeks away. A cold frame acts like a mini-greenhouse, warming soil and protecting seedlings from wind. You still need to harden them off before moving them to open garden beds, but they skip the indoor stage. This works best in mild climates.

What if seedlings get too tall before planting time?

Bury the extra stem when you transplant — tomatoes will root along the buried portion and create a stronger plant. You can also pinch off the top growing tip to encourage branching, though this delays flowering slightly. Improve light conditions (move lights closer or add more hours) to prevent legginess in future years.