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

The fastest route depends on where you live. If you have a short growing season (frost arrives in May or June), start seeds indoors under lights so plants are already several inches tall when you move them outside. If you live where frost doesn't arrive until September or later, you can plant seeds directly in the garden soil and let them grow there from the start. Either way, tomato seeds need warm soil—at least 70°F—to sprout reliably.

Indoors, seeds sprout in 5 to 10 days under the right conditions. Outdoors, they take 7 to 14 days depending on soil temperature. The main difference is control: indoors you manage light, heat, and moisture precisely. Outdoors, you depend on weather but skip the transplanting step.

Key Takeaways

  • Tomato seeds need soil between 70°F and 80°F to sprout; cooler soil delays germination or causes seeds to rot.
  • Indoor seedlings need 14 to 16 hours of direct light daily from a grow light or sunny window, or they will become tall and weak.
  • Harden off indoor seedlings by moving them outside for a few hours each day for one week before planting them in the garden.
  • Direct-sown seeds in the garden should be planted after the soil has warmed and all frost danger has passed, usually 1 to 2 weeks after your last frost date.
  • Tomato seeds stay viable for 4 to 6 years if stored in a cool, dry place, so old seed packets may still sprout.

Starting seeds indoors under lights

Fill a seed tray or small pots with seed-starting mix—a light, sterile blend of peat moss, coconut coir, or perlite. Regular garden soil is too dense and can introduce fungal diseases. Moisten the mix before you fill the containers so it is damp but not waterlogged. Plant one seed per cell or pot, pressing it about 1/4 inch deep. Tomato seeds are small; you do not need to bury them.

Cover the tray with a humidity dome or plastic wrap to keep moisture in, then place it on a heat mat set to 75°F. Check daily and remove the cover as soon as you see green sprouts—usually within a week. Once seedlings emerge, remove the heat mat and move the tray under a grow light positioned 2 to 3 inches above the tops of the plants. Keep the light on for 14 to 16 hours daily. Without enough light, seedlings stretch upward and become spindly.

Water from below by setting the tray in a shallow pan of water for 10 minutes, then removing it to drain. This keeps the soil moist without waterlogging the stems. Once seedlings have their first true leaves (the second set to appear), you can fertilize weekly with a diluted liquid fertilizer at half strength.

Hardening off and transplanting seedlings outdoors

About a week before your last spring frost date, begin hardening off your seedlings. This means gradually exposing them to outdoor conditions so they adjust to wind, direct sun, and temperature swings. On day one, place them outside in a sheltered, shaded spot for 2 to 3 hours. Each day, increase the time outdoors and move them into more direct light. By day 7, they should be outside all day and can handle full sun.

After hardening off is complete, plant seedlings in the garden when soil temperature reaches 60°F and nighttime lows stay above 50°F. Dig a hole slightly deeper than the root ball and bury the stem up to the first true leaves—tomatoes will root along the buried stem and develop a stronger base. Space plants 24 to 36 inches apart depending on the variety. Water gently after planting and keep the soil consistently moist for the first two weeks.

Direct sowing seeds in the garden

If you prefer to skip the indoor step, wait until soil temperature is at least 70°F and all frost danger has passed. In most regions, this is 1 to 2 weeks after your last frost date. Plant seeds 1/4 inch deep in rows or clusters, spacing them about 2 inches apart. Water gently so the soil stays moist but not soggy.

Seeds will sprout in 7 to 14 days depending on soil warmth. Once seedlings have two true leaves, thin them by removing the weaker plants so survivors are spaced 24 to 36 inches apart. Direct sowing works well in warm climates where the growing season is long, but in cooler regions you may not harvest fruit before frost arrives if you wait to plant seeds outdoors.

Soil, light, and temperature requirements

Tomato seeds germinate best in soil between 70°F and 80°F. Below 70°F, germination slows dramatically. Below 60°F, seeds may rot before they sprout. If your home is cool, a heat mat under seed trays makes a real difference—it cuts germination time in half.

Once seedlings emerge, light becomes critical. A sunny south-facing window works if you have one, but most indoor growers need a grow light—a fluorescent or LED fixture positioned close to the plants. Without adequate light, seedlings stretch toward the window and become weak. Rotate trays every few days so growth stays even.

Indoors, seedlings tolerate temperatures between 65°F and 75°F during the day. Nighttime temperatures 10 degrees cooler are fine and actually encourage stockier growth. Outdoors, seedlings can handle brief dips to 50°F once hardened off, but growth slows below that.

Watering and fertilizing young plants

The most common mistake is overwatering. Seedlings need moist soil, not wet soil. Check the surface daily; water only when the top feels dry to your finger. Watering from below—by setting the tray in a pan of water—prevents fungal diseases that thrive on wet leaves.

Seed-starting mix contains no nutrients, so seedlings rely on stored energy in the seed for the first 2 to 3 weeks. Once the first true leaves appear, begin fertilizing weekly with a balanced liquid fertilizer diluted to half strength. Continue weekly feeding until you transplant seedlings outdoors, then switch to a slower-release fertilizer in the garden soil.

Choosing tomato varieties and seed sources

Seed packets list days to maturity—the number of days from transplanting to first ripe fruit. Early varieties mature in 50 to 70 days; mid-season varieties in 70 to 85 days; late varieties in 85+ days. If your growing season is short, choose early or mid-season varieties. Heirloom varieties often take longer but offer distinctive flavors. Hybrid varieties tend to mature faster and produce more uniformly.

Buy seeds from reputable sources: seed companies, garden centers, or online retailers that store seeds in cool conditions. Old seed packets from previous years may still sprout if stored properly—tomato seeds remain viable for 4 to 6 years in a cool, dry place. Test old seeds by placing a few on a damp paper towel, rolling it up, and checking for sprouting after a week.

Common problems and how to prevent them

Damping off is a fungal disease that kills seedlings at soil level, causing them to topple over. Prevent it by using sterile seed-starting mix, watering from below, and ensuring good air circulation with a small fan running a few hours daily. If seedlings show signs of damping off, remove affected plants when ready and reduce watering.

Leggy seedlings—tall, thin plants with large gaps between leaves—result from insufficient light. Move the grow light closer or add more hours of light daily. Seedlings that are pale or slow-growing need fertilizer; begin weekly feeding once true leaves appear.

If seedlings wilt suddenly after transplanting outdoors, they may have experienced transplant shock or been exposed to wind or sun too quickly. Harden off gradually over a full week, and water deeply after planting. If wilting persists, check soil moisture; the root ball may have dried out.

Frequently Asked Questions

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

Regular potting soil is heavier and retains more moisture, which can cause seeds to rot before they sprout. Seed-starting mix is lighter and drains better. If you must use potting soil, mix it 50/50 with perlite or coconut coir to improve drainage.

What if I don't have a grow light?

A sunny south-facing window can work if you have at least 6 hours of direct sun daily and rotate trays every few days. Seedlings will likely stretch somewhat, but you can bury the stem deeper when transplanting. Grow lights are inexpensive and give more reliable results.

How do I know when seedlings are ready to transplant outside?

Seedlings are ready when they are 6 to 8 inches tall, have several true leaves, and have been hardened off for a week. The soil should be warm (at least 60°F) and frost danger should have passed. If seedlings are still in small cells, they may be root-bound; transplant them to larger pots indoors first if needed.

Can I save seeds from my tomato plants?

Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes produce seeds that won't grow true to type. Scoop seeds and gel from a ripe tomato into a jar, add a little water, and let it ferment for 2 to 3 days. Rinse, dry on a paper towel for 2 to 3 weeks, then store in a cool, dry place.

Why are my seedlings turning purple?

Purple or reddish leaves usually mean the soil is too cold, slowing nutrient uptake. Move seedlings to a warmer location or use a heat mat. Once soil warms, color should return to normal within a few days.