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

The timing for starting tomato seeds depends on when you can safely move seedlings outside. Count backward from your area's last frost date — the average date when freezing temperatures stop for the season. Tomato seedlings need 6 to 8 weeks indoors under lights before they're ready to transplant into the garden. If your last frost date is May 15, start seeds indoors around March 20 to April 1.

You can find your last frost date by searching online for your zip code, or by calling your local cooperative extension office. The date varies by region — it might be April 1 in the South and June 1 in the North. Starting too early means seedlings will become leggy and weak while waiting for warm soil outdoors. Starting too late means you'll miss peak growing season and harvest fewer tomatoes before fall frost arrives.

Key Takeaways

  • Count backward 6 to 8 weeks from your last spring frost date to find your seed-starting date.
  • Seedlings need 14 to 16 hours of light daily under grow lights or bright windowsills to stay compact and strong.
  • Soil temperature for germination should be 70 to 80 degrees Fahrenheit; seeds sprout in 5 to 10 days at that warmth.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • You can direct-sow seeds outdoors only in warm climates where soil stays above 60 degrees Fahrenheit year-round.

How to calculate your exact seed-starting date

Find your last frost date first — this is the single most important number. Search "last frost date" plus your zip code, or contact your county's cooperative extension office. Write down the date. Then count backward 6 weeks for early varieties (like 'Early Girl' or 'Siberia') or 8 weeks for larger slicing tomatoes (like 'Brandywine' or 'Beefsteak'). Mark that date on your calendar as your seed-starting day.

If you're unsure whether your variety is early or late, check the seed packet — it lists "days to maturity," which tells you how long from transplant to first ripe fruit. Varieties under 65 days are early; 75 days and above are late. Early varieties can start 6 weeks before the frost date. Late varieties need the full 8 weeks, or even 10 weeks if you want large plants with multiple fruit sets by midsummer.

Indoor growing conditions seedlings need to thrive

Light is the biggest factor in growing strong seedlings. Tomato seeds germinate in darkness, but once they sprout, seedlings need 14 to 16 hours of bright light daily. A sunny south-facing windowsill works if it gets at least 6 hours of direct sun, but most home windows don't provide enough. Grow lights (LED or fluorescent) positioned 2 to 3 inches above the seedlings are more reliable. Raise the lights as seedlings grow to keep them at that distance.

Temperature matters at two stages. For germination, keep soil at 70 to 80 degrees Fahrenheit — a heat mat under seed trays speeds sprouting to 5 to 10 days. Once seedlings emerge, they can tolerate cooler air (65 to 70 degrees), but soil should stay warm. A heat mat under the tray for the first 3 to 4 weeks helps roots develop faster. After that, you can remove it.

Moisture should be consistent but not waterlogged. Water from below by setting seed trays in a shallow pan of water and letting soil wick it up, or water gently from above when the surface feels dry. Soggy soil causes damping-off disease, which kills seedlings at the soil line. Use sterile seed-starting mix, not garden soil, to prevent this problem.

When to transplant seedlings into larger containers

Seedlings are ready to move up to larger pots (3 to 4 inches wide) when they develop their first true leaves — the second set of leaves that appear after the initial rounded seed leaves. This usually happens 2 to 3 weeks after sprouting. At this point, roots have grown enough to handle the move, and seedlings need more room to develop strong stems.

Tomato seedlings can be buried deeper than they were in the seed tray. Bury them up to their first true leaves; they'll develop roots along the buried stem, creating a stronger root system. Keep the same light and temperature conditions after transplanting. Continue watering from below and fertilize weekly with a diluted liquid fertilizer (half-strength) once seedlings have their first true leaves.

Hardening off before moving seedlings outdoors

Hardening off is the process of gradually exposing indoor seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start this 7 to 10 days before your transplant date. On day one, place seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase time outdoors and gradually move them into more sun and wind.

By day 7 or 8, seedlings should spend the full day outside in dappled shade or morning sun. By day 10, they can handle full sun for several hours. Watch the weather — if frost is forecast, bring seedlings inside. If temperatures drop below 50 degrees at night, wait another week before transplanting. Seedlings that skip hardening off will wilt, turn purple, or stop growing when planted in the garden.

Direct sowing seeds outdoors in warm climates

If you live in a warm region where soil temperature stays above 60 degrees Fahrenheit year-round, you can sow seeds directly into the garden instead of starting them indoors. This works in southern Florida, southern California, southern Texas, and similar climates. Sow seeds 1/4 inch deep, 2 inches apart, in a row or small mound. Keep soil moist until seedlings emerge in 5 to 10 days.

Thin seedlings to 12 inches apart once they have their first true leaves. Direct sowing saves space indoors and avoids transplant shock, but it means a later harvest because seeds germinate slower than transplants establish. In most of the United States, starting seeds indoors is faster and more reliable because it lets you control temperature and light from day one.

Common mistakes that delay or damage seedlings

Starting too early is the most common error. Seedlings started 10 or 12 weeks before the frost date will be large, woody, and stressed by the time they go outside. They may flower and set fruit indoors, wasting energy on fruit instead of root development. Stick to 6 to 8 weeks; it's the sweet spot for transplant-ready size.

Insufficient light creates tall, thin, pale seedlings that flop over and struggle after transplanting. If you don't have grow lights, a bright south-facing window is your second choice, but rotate trays daily so seedlings don't lean toward the light. Overwatering causes damping-off and root rot. Water only when the soil surface feels dry, and always use sterile seed-starting mix.

Skipping hardening off shocks seedlings into dormancy or death. Even if it takes extra time, the 7 to 10 day hardening process is non-negotiable. Planting too deep in the garden buries the stem too far and can cause rot. Plant at the same depth they were in their container, or slightly deeper for leggy seedlings — but not more than one-third of the stem.

Frequently Asked Questions

Can I start tomato seeds in a sunny window without grow lights?

A south-facing window with at least 6 hours of direct sun can work, but seedlings often become leggy and weak because window light is less intense than grow lights. If you use a window, rotate trays daily and keep seedlings as close to the glass as possible. Grow lights are inexpensive and produce much stronger plants, so they're worth the small investment if you plan to start seeds every year.

What if I missed my seed-starting date?

You can still start seeds a few weeks late, but expect a shorter harvest season. Start them as soon as possible and skip the hardening-off period if frost danger has passed. You can also buy transplants from a garden center — they're already at the size you would have grown indoors, so you just harden them off and plant them out.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination and strengthens roots, but it's not required. Seeds will sprout in a warm room (70 to 75 degrees) without one, just more slowly — 10 to 14 days instead of 5 to 10. If your home is cool or you're starting seeds in early spring, a heat mat is worth using for the first 3 to 4 weeks.

Can I start seeds indoors year-round and transplant whenever I want?

No. Tomatoes need warm soil (at least 60 degrees, ideally 70 degrees or warmer) to grow after transplanting. Planting before soil warms will stall growth and invite disease. Always wait until after your last frost date and until soil has warmed — usually 1 to 2 weeks after the frost date, depending on your region.

How do I know if my seedlings are ready to transplant outside?

Seedlings are ready when they're 6 to 8 inches tall, have 4 to 6 true leaves, and have been hardened off for 7 to 10 days. They should look sturdy and green, not pale or leggy. If they're flowering indoors, they're too old and stressed — plant them anyway, but they may produce fewer tomatoes overall.