Start seeds indoors 6 to 8 weeks before your last spring frost, or direct sow outdoors after soil warms to 60°F

The timing for tomato seeds depends on where you live and whether you start them indoors or plant them straight in the ground. Indoor seed starting — the most common method — means planting seeds in containers under lights inside your home, then moving seedlings outside after the frost danger passes. Direct sowing — planting seeds straight into garden soil — works only in warm climates or late in the season when soil has warmed.

Most gardeners in cooler regions start seeds indoors in late winter or early spring. Count backward from your area's last spring frost date. If your last frost is May 15, start seeds indoors around March 20. If your last frost is June 1, start around April 10. Seeds take 6 to 8 weeks to grow into transplant-ready seedlings, and you need those weeks of indoor growth before outdoor conditions are safe.

Direct sowing outdoors works if your soil temperature reaches at least 60°F and stays there — ideally 65°F or warmer. Soil thermometers cost $10 to $20 and take the guesswork out. Cold soil rots seeds before they sprout. In most of the United States, this happens 2 to 4 weeks after the last frost date, once the ground has warmed.

Key Takeaways

  • Indoor seed starting begins 6 to 8 weeks before your last spring frost date, which you can find through your local cooperative extension office or by searching your zip code online.
  • Direct sowing seeds outdoors requires soil temperature of at least 60°F, measured with a soil thermometer, and typically happens 2 to 4 weeks after your last frost date.
  • Indoor-started seedlings need 12 to 16 hours of light daily from a grow light or bright window, and should be moved to larger pots once they develop their first true leaves.
  • Seedlings started indoors must be hardened off — gradually exposed to outdoor conditions over 7 to 10 days — before transplanting into the garden.

Finding your last spring frost date

Your last spring frost date is the average date when freezing temperatures stop occurring in your area. Tomatoes die in frost, so this date is your important date for moving seedlings outdoors. You can find it through your state's cooperative extension office website — search "[your state] cooperative extension" plus "frost date" — or by entering your zip code into the Almanac's frost date tool.

The date varies by 1 to 2 weeks even within the same county, depending on elevation and proximity to water. If you live in a valley or near a lake, your frost date may be later than the official county average. Asking neighbors or a local garden center what date they use is often more reliable than a general map.

Starting seeds indoors under lights

Fill seed trays or small pots with seed-starting mix — a light, sterile blend designed to drain well and prevent damping off, a fungal disease that kills seedlings. Moisten the mix before planting. Plant one seed per cell or pot, about 1/4 inch deep. Cover the tray loosely with a humidity dome or plastic wrap to keep moisture in while seeds germinate.

Tomato seeds germinate best at 70°F to 80°F. A heating mat under the tray speeds germination to 5 to 10 days. Once seedlings emerge, remove the dome and move the tray under grow lights positioned 2 to 3 inches above the leaves. Seedlings need 12 to 16 hours of light daily. Without enough light, they become tall and spindly — a condition called leggy — and will not transplant well.

Water from below by setting pots in a tray of water and letting them absorb moisture from the bottom, or water gently at soil level. Keep soil moist but not waterlogged. Once seedlings develop their first true leaves — the second set of leaves that appear — thin out weaker seedlings or move each one to a larger pot with potting soil. Pinch off the lowest leaves as the plant grows taller.

Hardening off seedlings before transplanting

Seedlings grown indoors are tender and will wilt or burn if moved directly into outdoor sun and wind. Hardening off — gradually exposing them to outdoor conditions — takes 7 to 10 days and is essential. Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase the time outside and gradually move them into brighter light.

By day 7 or 8, seedlings should spend the full day outside in dappled shade or morning sun. On the final days, expose them to a few hours of direct afternoon sun. Watch the weather — if frost is forecast, bring seedlings back indoors. Once seedlings are hardened off and nighttime temperatures stay above 50°F, they are ready to plant in the garden.

Direct sowing seeds outdoors

Direct sowing works best in warm climates where the growing season is long, or as a second planting in mid-summer for a fall harvest. Plant seeds 1/4 inch deep in warm, moist soil. Space them 2 to 3 inches apart in rows, or plant 3 to 4 seeds in a small hill and thin to the strongest seedling once they have true leaves.

Keep soil consistently moist until seedlings are 2 to 3 inches tall. Once they develop true leaves, thin them to their final spacing — usually 18 to 36 inches apart depending on the variety. Direct-sown seedlings mature slower than transplants because they start from seed in the ground rather than as established plants, so they will produce fruit 2 to 3 weeks later than indoor-started plants.

Timing for succession planting and fall crops

In regions with a long growing season, you can plant tomato seeds in waves. Start the first batch indoors in late winter for spring planting. In early summer, direct sow a second batch outdoors for a fall harvest. Count backward from your first fall frost date — the average date when freezing temperatures return in autumn — to determine when to sow. Tomatoes need 60 to 85 days from transplant to first fruit, depending on variety.

Determinate varieties — compact plants that produce most fruit at once — work better for fall crops because they mature faster. Indeterminate varieties — tall plants that produce fruit all season — need more time and may not finish before frost arrives.

Choosing between indoor starting and direct sowing

Indoor starting gives you a head start and lets you control growing conditions, but requires grow lights, space, and attention for 6 to 8 weeks. Direct sowing is simpler and cheaper, but only works if your soil warms early enough and your growing season is long enough for fruit to mature before frost.

Most gardeners in cooler climates choose indoor starting because it guarantees mature plants by the time warm weather arrives. Gardeners in warm climates or those with short growing seasons may find direct sowing faster and easier. Some gardeners do both — start a main crop indoors and direct sow a smaller second crop in summer.

Frequently Asked Questions

Can I start tomato seeds too early?

Yes. Seedlings started more than 8 weeks before the last frost will become overgrown and root-bound by transplant time, and they do not transplant well. If you start too early, pot them up into larger containers to give roots room to grow, or they will struggle in the garden.

What if my last frost date passes and I haven't started seeds yet?

Direct sow seeds outdoors into warm soil, or buy transplants from a garden center. Direct-sown seeds will produce fruit later than transplants, but you will still have a harvest if your growing season is long enough. Check the days-to-maturity on the seed packet to confirm the variety will finish before your first fall frost.

Do I need a grow light, or can seedlings grow in a window?

A south-facing window works if it gets 12 to 16 hours of direct sun daily, but most home windows do not provide enough light. Seedlings in dim windows become leggy and weak. A basic LED grow light costs $30 to $60 and produces much stronger seedlings. If using a window, rotate the tray daily so seedlings grow straight.

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

Seedlings are ready when they are 6 to 8 inches tall, have several sets of true leaves, and have been hardened off. They should have a sturdy stem and a small root system visible at the bottom of the pot. Leggy, pale seedlings are not ready and will struggle in the garden.

Can I plant tomato seeds in fall for a winter crop?

In warm climates where winter temperatures stay above freezing, yes. In cold climates, tomatoes cannot survive winter outdoors. Check your first fall frost date and count backward 60 to 85 days depending on variety. If that date falls before your first frost, the crop will not mature in time.