Plant tomato seeds indoors 6 to 8 weeks before your last frost date, or directly in the garden after soil warms to 60°F

Tomato seeds need warmth and moisture to sprout, but not much else. You can start them indoors in seed trays under a grow light, or wait until late spring and plant them straight into garden soil. Indoor starting gives you a head start on the growing season and lets you control conditions; direct sowing is simpler if you live where frost comes late. Either way, seeds germinate in 5 to 10 days if kept warm and moist.

The timing matters more than the method. Tomatoes need 6 to 8 weeks of growth before they go in the ground, so count backward from your area's last spring frost date. If your last frost is May 15, start seeds indoors around March 15. If you miss that window, direct sow once soil reaches 60°F and frost danger has passed.

Key Takeaways

  • Indoor seeds need a warm spot (70–75°F), moist seed-starting mix, and light once they sprout; outdoor seeds need soil warmer than 60°F and consistent moisture until they emerge.
  • Plant seeds ¼ inch deep in either setting—too deep and they won't reach the surface, too shallow and they dry out.
  • Seedlings started indoors need 14 to 16 hours of light daily from a grow light or bright window to stay stocky and strong.
  • Harden off indoor seedlings by moving them outside for a few hours each day for a week before transplanting them into the garden.

Starting seeds indoors in trays or pots

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 rot seeds. Moisten the mix before planting so it holds together but isn't soggy. Plant one seed per cell or pot, pressing it ¼ inch into the surface and covering it lightly.

Place the tray in a warm spot—a heat mat under the tray works best and keeps soil at 70–75°F, the ideal range for germination. A sunny windowsill is too cold; a dark closet with a heat mat is better than a cold, bright shelf. Keep the soil moist but not waterlogged by misting with a spray bottle. Seeds should sprout in 5 to 10 days.

Once sprouts appear, move the tray into bright light when ready—a south-facing window or a grow light positioned 2 to 3 inches above the seedlings. Without enough light, seedlings stretch tall and weak. Run the light for 14 to 16 hours daily. Keep soil moist and allow air to circulate by running a small fan on low speed nearby, which strengthens stems.

Planting seeds directly in the garden

Wait until soil temperature reaches 60°F and all frost danger has passed. Check your soil with a soil thermometer pushed 2 inches deep in the morning. Prepare a spot with full sun (at least 6 hours daily) and soil mixed with compost or aged manure to improve drainage and add nutrients.

Make a shallow hole ¼ inch deep, drop in one seed, and cover it gently. Space seeds 12 inches apart if you plan to thin seedlings later, or plant them closer and remove weaker sprouts once they have two true leaves. Water gently so the soil stays moist but not waterlogged. Seedlings should emerge in 5 to 10 days.

Once seedlings have two true leaves (the second set to appear, which look like actual tomato leaves), thin them to one plant every 18 to 24 inches. Snip weaker seedlings at soil level rather than pulling them, which can disturb roots of the keeper plant.

Hardening off indoor seedlings before transplanting

Seedlings started indoors are tender and will wilt or die if moved straight into the garden. Harden them off by gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing them outside in a sheltered, shaded spot for 2 to 3 hours on the first day. Each day, increase the time outdoors and move them into more direct sun.

By day 7 or 8, seedlings should spend the full day outside. Bring them in at night if temperatures drop below 50°F. Once they've spent a full 24 hours outside without wilting, they're ready to transplant into the garden. This process toughens stems and leaves so they handle wind and sun without shock.

Transplanting seedlings into the garden

Plant hardened-off seedlings in the garden after your last frost date, when soil is warm and all danger of frost has passed. Dig a hole slightly deeper than the seedling's root ball. Tomatoes can be planted deeper than they grew in the pot—bury the stem up to the first true leaves, and roots will form along the buried stem, creating a stronger root system.

Space plants 24 to 36 inches apart depending on the variety. Compact determinate types need less space; sprawling indeterminate varieties need more. Water well after planting and keep soil consistently moist for the first two weeks while roots establish.

Common problems and how to avoid them

Seeds don't sprout: Soil is too cold or too dry. Use a heat mat to maintain 70–75°F and check soil moisture daily—it should feel like a wrung-out sponge. Seeds older than three years may have low germination rates; buy fresh seed if sprouting fails.

Seedlings stretch and fall over: Not enough light. Move the light closer (2 to 3 inches above the tops) or run it longer (14 to 16 hours daily). A small fan running on low speed also strengthens stems by creating gentle air movement.

Seedlings wilt after transplanting: They weren't hardened off enough, or soil dried out after planting. Water when ready and shade plants for a few days if wilting is severe. In the future, extend the hardening-off period and keep soil moist for two weeks after transplanting.

Seedlings turn yellow: Soil is waterlogged or lacks nitrogen. Let soil dry slightly between waterings and avoid standing water. Once seedlings have true leaves, feed them weekly with diluted liquid fertilizer (half the recommended strength).

Frequently Asked Questions

Can I plant tomato seeds directly in the garden instead of starting them indoors?

Yes, if your growing season is long enough. Direct sowing works well in warm climates where frost ends by late April or early May. In cooler areas with shorter seasons, indoor starting gives you a head start so plants mature before fall frost arrives. Direct sowing is simpler but delays harvest by 6 to 8 weeks.

How deep should I plant tomato seeds?

Plant seeds ¼ inch deep in either indoor trays or garden soil. Deeper planting reduces germination because seeds struggle to push through soil; shallower planting dries out too quickly. Press the seed gently into moist soil and cover it lightly.

What temperature do tomato seeds need to sprout?

Seeds germinate best at 70–75°F. They'll sprout more slowly below 60°F and won't sprout at all below 50°F. A heat mat under seed trays maintains consistent warmth. Soil temperature matters more than air temperature, so use a soil thermometer to check.

Do tomato seedlings need light before they sprout?

No. Seeds germinate in darkness and don't need light until they sprout. Once the first green shoot appears, move seedlings into bright light when ready—a grow light or sunny window. Without light, seedlings stretch tall and weak within days.

When can I move seedlings outside permanently?

After hardening them off for 7 to 10 days and once your area's last frost date has passed. Soil should be at least 60°F, ideally 65°F or warmer. Planting too early into cold soil slows growth and can rot roots. Wait until nighttime temperatures stay above 50°F consistently.