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

The timing depends on two things: your location's last frost date and whether you start seeds indoors or plant them straight in the ground. Indoor seed starting gives you a head start — seedlings are ready to transplant when outdoor soil warms up. Direct sowing is simpler but means waiting longer for fruit, since seeds need warmer soil to germinate.

Most gardeners in cold climates start seeds indoors in late winter or early spring, then move seedlings outside after the last frost passes and soil temperature stays above 60°F. In warm climates where frost rarely or never occurs, you can sow seeds directly into garden soil or containers anytime soil is warm enough. The difference in harvest time can be 3 to 4 weeks — indoor-started plants produce fruit earlier.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your area's last spring frost date, which you can find through your local cooperative extension office or by searching your zip code online.
  • Direct sow seeds outdoors only after soil temperature reaches 60°F, since seeds will rot in cold, wet soil below that threshold.
  • Indoor seedlings need 12 to 16 hours of light daily and should be moved to a sunny windowsill or under grow lights as soon as they sprout.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting them into the garden.

Finding your last frost date

Your last frost date is the average date of the final frost in spring for your area. Tomato seeds and seedlings die when exposed to frost, so this date marks the earliest safe day to plant outdoors. You can find it by entering your zip code into the USDA Hardiness Zone map on planthardiness.ars.usda.gov, or by calling your local cooperative extension office — they have the exact date for your county.

Frost dates vary widely by location. A gardener in zone 5 might have a last frost date in mid-May, while someone in zone 9 might never experience frost. Once you know your date, count backward 6 to 8 weeks to find when to start seeds indoors. If your last frost is May 15, start seeds indoors around March 20 to April 1.

Starting seeds indoors

Indoor seed starting requires a seed-starting mix (a light, sterile soil blend), containers with drainage holes, and a light source. Fill containers with moistened seed-starting mix, plant seeds about a quarter-inch deep, and keep soil consistently moist but not waterlogged. Seeds germinate in 5 to 10 days at room temperature — they do not need heat mats, though soil warmth between 70°F and 80°F speeds germination.

Once seedlings sprout, light becomes critical. Place them under grow lights or in a bright south-facing window, keeping lights 2 to 3 inches above the leaves for 12 to 16 hours daily. Without enough light, seedlings become leggy and weak. When the first true leaves appear (the second set of leaves), thin seedlings so each has its own space, or transplant them into individual small pots filled with potting soil.

Water seedlings from below by setting pots in a tray of water for 10 to 15 minutes, then removing them to drain. This prevents damping off, a fungal disease that kills seedlings at soil level. Keep the growing area at 65°F to 75°F during the day. About 2 weeks before your last frost date, begin hardening off — move seedlings outdoors to a sheltered spot for a few hours, gradually increasing exposure over 7 to 10 days.

Direct sowing seeds outdoors

Direct sowing means planting seeds straight into garden soil or containers outside. Wait until soil temperature reaches 60°F — use a soil thermometer to check, or wait until nighttime temperatures stay above 50°F consistently. Seeds planted in cold soil will sit dormant and may rot before germinating.

Prepare soil by mixing in compost or aged manure to improve drainage and fertility. Plant seeds half an inch deep, 2 to 3 inches apart, in rows or clusters. Keep soil moist until seedlings emerge in 5 to 10 days. Once they have two true leaves, thin them to 12 to 24 inches apart depending on the variety — check the seed packet for spacing. Direct-sown plants usually flower and fruit 2 to 3 weeks later than transplanted seedlings.

Soil temperature and outdoor conditions

Tomato seeds and seedlings are sensitive to cold. Soil below 60°F slows germination dramatically and increases rot risk. Seedlings exposed to frost (32°F or below) will die or suffer severe damage. Even temperatures between 40°F and 50°F stunt growth and can cause leaves to turn purple.

Before transplanting seedlings outdoors, check the 10-day forecast. If frost is predicted, delay planting or cover plants with row covers or cloches overnight. Once plants are established and growing, they tolerate cool nights better, but young seedlings are vulnerable. Soil warms faster in raised beds and containers than in ground-level garden beds, so these are good options if your area has a short growing season.

Timing by climate zone

In cold climates (zones 3 to 5), start seeds indoors in March or early April and transplant after the last frost in May or early June. In moderate climates (zones 6 to 7), start seeds in February or March and transplant in April or May. In warm climates (zones 8 to 10), you can start seeds indoors in January or February, or direct sow in March or April when soil is warm.

In very warm climates where summer temperatures exceed 95°F regularly, consider planting in late winter or early spring so plants mature before extreme heat arrives. Tomatoes stop setting fruit when nighttime temperatures stay above 75°F, so timing matters for total yield. If you live in a hot climate, you may get better results planting in late summer for a fall harvest instead.

Container growing and season extension

Growing tomatoes in containers lets you move plants indoors if frost threatens and gives you more control over soil temperature and moisture. Start seeds indoors the same way, but transplant seedlings into 5-gallon containers filled with potting soil once they have several true leaves. Containers warm faster than ground soil, so you can move them outside a week or two earlier than garden plants.

You can also extend the season by covering plants with row covers, cloches, or cold frames on cool nights. These straightforward structures trap heat and protect seedlings from frost. In fall, moving containers to a sunny sheltered spot or covering them can add 2 to 4 weeks to your harvest window.

Frequently Asked Questions

Can I plant tomato seeds in winter?

Only in warm climates where soil stays above 60°F year-round. In cold climates, winter soil is frozen or too cold for seeds to germinate. If you want to start seeds in winter, do it indoors under lights, then transplant seedlings outdoors after the last frost.

What happens if I plant seeds too early?

Seeds planted in cold soil below 60°F germinate slowly or rot before sprouting. Seedlings transplanted before the last frost will be killed or severely damaged by frost. Starting seeds indoors too early means seedlings become overgrown and leggy before it is warm enough to move them outside.

Do I need grow lights to start seeds indoors?

A bright south-facing window works if you have one, but grow lights give more consistent results. Without enough light, seedlings stretch toward the window and become weak. Inexpensive LED shop lights on a timer are a reliable option and cost less than $30.

How deep should I plant tomato seeds?

Plant seeds about a quarter-inch deep indoors and half an inch deep outdoors. Seeds planted too deep may not have enough energy to push through soil. Seeds planted too shallow may dry out before germinating.

Can I transplant seedlings before the last frost date?

No — frost will kill them. Wait until nighttime temperatures stay above 50°F consistently and your area's last frost date has passed. If you transplant early and frost is forecast, cover plants with row covers or move containers indoors overnight.