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

Growing tomatoes from seed gives you far more variety than buying transplants at a garden center, and costs much less per plant. You have two main routes: start seeds indoors under lights in late winter, then move seedlings outside after frost danger passes, or sow seeds directly into garden soil once the ground is warm enough. Indoor starting takes longer but works in any climate. Direct sowing is faster and simpler if you live where summers are long and warm.

The timing depends on where you live. Count backward from your area's last spring frost date—you can find this through your local cooperative extension office or by searching "[your city] last frost date." If your last frost is May 15, start seeds indoors around March 15. If you plan to direct sow instead, wait until soil temperature reaches at least 60°F, ideally 70°F or warmer.

Key Takeaways

  • Indoor seeds need sterile seed-starting mix, consistent moisture, and bright light from a grow light or sunny window to prevent leggy seedlings.
  • Tomato seeds germinate in 5 to 10 days at 70°F to 80°F and do not need light until they sprout.
  • Seedlings are ready to move outside 6 to 8 weeks after planting, once they have true leaves and nighttime temperatures stay above 50°F.
  • Direct sowing outdoors skips the indoor step entirely but only works in regions with at least 70 to 85 frost-free days, depending on the variety.
  • Hardening off—gradually exposing seedlings to outdoor sun and wind over 7 to 10 days—prevents transplant shock and sunburn.

Starting seeds indoors: containers, soil, and light setup

Use any container with drainage holes: seed trays, yogurt cups, newspaper pots, or recycled takeout containers. Fill them with seed-starting mix, not garden soil or potting soil. Seed-starting mix is lighter and sterile, which prevents damping-off disease—a fungal infection that kills seedlings at soil level. Moisten the mix before filling containers so it absorbs water evenly.

Plant one seed per cell or small pot, pressing it about 1/4 inch deep into the soil. Water gently so the seed stays in place. Cover the container with a humidity dome, plastic wrap, or a clear lid to keep moisture in while seeds germinate. You do not need light yet—seeds germinate in darkness. Keep the soil warm: 70°F to 80°F is ideal. A heat mat under the containers speeds germination to 5 to 10 days; without one, it may take 2 to 3 weeks.

As soon as the first green shoots appear, remove the cover and move containers under bright light. A south-facing window works if it gets at least 6 hours of direct sun, but a grow light positioned 2 to 3 inches above seedlings is more reliable. Run the light for 14 to 16 hours per day. Without enough light, seedlings become tall and spindly—a condition called "leggy"—and will not transplant well.

Watering, feeding, and preventing common seedling problems

Keep seed-starting mix consistently moist but not waterlogged. Water from below by setting containers in a tray of water and letting soil wick it up, or water gently from above. Soggy soil causes damping-off and root rot. Dry soil kills seedlings just as quickly. Check daily, especially under grow lights where soil dries faster.

Seed-starting mix contains no nutrients, so seedlings need feeding once they develop their first true leaves—the second set of leaves that appear after the initial rounded seed leaves. Mix a diluted liquid fertilizer (half strength) and feed every 7 to 10 days. Tomato seedlings are also prone to blossom end rot later in life if calcium is low, so use a balanced fertilizer with calcium, or add crushed eggshells to the soil before planting.

If seedlings become leggy despite good light, the room may be too warm. Tomato seedlings grow stockier in cooler conditions—65°F to 70°F is better than 75°F or higher. If seedlings develop purple or reddish leaves, they are cold or hungry; move them to warmer light and feed them. If leaves yellow and drop, overwatering is the likely cause—let soil dry slightly between waterings.

Transplanting seedlings into larger pots

When seedlings develop their first true leaves and are about 2 inches tall, move them into larger containers—4-inch pots or cell packs. This gives roots more room and prevents them from becoming root-bound. Gently loosen soil around each seedling, support it by a leaf (not the stem), and plant it at the same depth it was growing. Tomato seedlings can be planted deeper than they were in the seed tray; they will develop roots along the buried stem.

Water after transplanting and return seedlings to the grow light. Continue feeding every 7 to 10 days. By week 6 or 7, seedlings should be 6 to 8 inches tall with several sets of true leaves and a sturdy stem. This is when they are ready to harden off and move outside.

Hardening off: preparing seedlings for outdoor conditions

Seedlings grown indoors are tender 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 1 to 2 hours. Each day, increase the time outside and move them into brighter light. By day 7 or 8, they should tolerate full sun and outdoor wind.

Begin hardening off only when nighttime temperatures stay above 50°F. If frost is forecast during the hardening-off period, bring seedlings back indoors. Harden off during a calm week if possible—strong wind can damage tender stems. Water seedlings well before moving them outside, as they dry out faster in sun and wind than indoors.

Planting seedlings in the garden

Plant seedlings outdoors once nighttime temperatures consistently stay above 50°F and all frost danger has passed. Space plants 24 to 36 inches apart, depending on the variety—check the seed packet. Dig a hole slightly deeper than the root ball and plant seedlings at the same depth they were growing in pots, or even deeper. Tomatoes develop roots along buried stems, so planting deep creates a stronger root system.

Water thoroughly after planting. Mulch around each plant with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature. Stake or cage plants early—waiting until they are large makes it harder to avoid damaging roots. Water deeply and consistently; tomatoes need about 1 to 2 inches per week depending on heat and rainfall.

Direct sowing seeds outdoors

If you live in a warm climate with a long growing season, you can skip indoor starting and sow seeds directly into the garden. Wait until soil temperature reaches 70°F or warmer and all frost danger has passed. Plant seeds 1/4 inch deep in groups of 3 to 4, spaced 24 to 36 inches apart. Water gently and keep soil consistently moist until seedlings emerge in 5 to 10 days.

Once seedlings have their first true leaves, thin them to one plant per spot by snipping off extras at soil level—do not pull them out, as this disturbs roots of the remaining plant. Direct-sown seedlings catch up to transplants by mid-summer and often produce earlier because they avoid transplant shock. This method works best in regions with at least 70 to 85 frost-free days, depending on whether you are growing early, mid-season, or late varieties.

Choosing tomato varieties for seed starting

Determinate varieties (bush types) grow to a fixed size and produce most fruit at once—good for canning or sauce-making. Indeterminate varieties (vining types) grow continuously and produce fruit all season until frost—better for fresh eating. Both start from seed the same way, but indeterminate plants need more space, staking, and pruning.

Check the seed packet for days to maturity. Early varieties mature in 50 to 70 days; mid-season in 70 to 85 days; late varieties in 85 days or more. If your growing season is short, choose early or mid-season varieties. Heirloom varieties often take longer to mature but offer unique flavors. Hybrid varieties are often faster and more disease-resistant. All grow from seed equally well.

Frequently Asked Questions

Can I use regular potting soil instead of seed-starting mix?

Regular potting soil is heavier and may contain fungi that cause damping-off disease in seedlings. Seed-starting mix is lighter, sterile, and drains better. If you must use potting soil, mix it half-and-half with perlite or coconut coir to lighten it. Avoid garden soil entirely—it compacts and harbors disease.

What should I do if my seedlings are tall and spindly?

Leggy seedlings mean insufficient light. Move them closer to a grow light—2 to 3 inches above the tops—or place them in a sunnier window. Lower room temperature to 65°F to 70°F if possible; warmth encourages stretching. If seedlings are already planted outside, bury them deeper; they will develop roots along the stem and become sturdier.

How often should I water seedlings under a grow light?

Check soil daily; it dries faster under lights than in a window. Water when the top 1/4 inch of soil feels dry to the touch. Watering from below by setting pots in a tray of water prevents overwatering and reduces disease. Never let soil dry completely or stay soggy for more than a day.

Can I save seeds from tomatoes I grow?

Yes, but only from heirloom and open-pollinated varieties. Hybrid tomatoes produce seeds that do not grow true to the parent plant. To save seeds, let a ripe tomato fully mature on the plant, scoop out seeds and pulp into a jar, ferment for 2 to 3 days to remove the gel coating, rinse, and dry on a paper towel for 2 to 3 weeks before storing in a cool, dry place.

What is the difference between determinate and indeterminate tomatoes?

Determinate (bush) tomatoes grow to a fixed height, usually 3 to 4 feet, and produce most fruit within a short window—ideal for canning. Indeterminate (vining) tomatoes grow continuously, reaching 6 feet or taller, and produce fruit all season until frost—better for fresh eating. Both start from seed the same way, but indeterminate plants need more support and pruning.