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

Fresh tomato seeds need warmth, moisture, and light to sprout. If you live where frost arrives late, start seeds indoors under grow lights or on a sunny windowsill 6 to 8 weeks before your last frost date. If your growing season is long and soil stays warm year-round, you can plant seeds directly into garden beds or containers once soil temperature reaches 60°F — though 70°F or warmer produces faster germination. The method you choose depends on your climate and how much control you want over seedling strength before transplanting.

Tomato seeds are small and need consistent conditions to sprout reliably. Whether you start them indoors or outdoors, the basic requirements stay the same: soil that drains well, steady moisture without waterlogging, and warmth. Most varieties sprout within 5 to 10 days at 70°F to 80°F, though germination slows below 60°F.

Key Takeaways

  • Indoor seeds need a seed-starting mix (not garden soil), containers with drainage holes, and consistent moisture kept at 70°F to 80°F until sprouts appear.
  • Direct sowing outdoors works in warm climates but requires soil at least 60°F and protection from heavy rain or wind that can wash seeds away.
  • Seedlings need 14 to 16 hours of direct light daily once they sprout, or they will become tall and weak.
  • Transplant indoor seedlings into larger pots once they develop their first true leaves, and harden them off outdoors for 7 to 10 days before moving them to the garden.

What you need to start seeds indoors

Seed-starting mix is lighter and drains faster than garden soil, which prevents rot and fungal disease in seedlings. Buy a commercial mix (brands like Miracle-Gro Seed Starting Mix or Burpee are widely available) or make your own by mixing equal parts peat moss or coco coir, perlite, and vermiculite. Fill small containers — seed trays, cell packs, or recycled yogurt cups with drainage holes drilled in the bottom — to about half an inch below the rim.

You will also need a heat source. A seedling heat mat (around $20 to $40) keeps soil at the ideal 70°F to 80°F and cuts germination time in half. Without one, place trays on top of a refrigerator, water heater, or warm shelf indoors. Once sprouts emerge, remove the heat mat and move seedlings under lights.

Light is critical. A sunny south-facing windowsill works if you have one, but grow lights (LED shop lights from a hardware store cost $20 to $50) are more reliable. Position lights 2 to 3 inches above seedlings and keep them on for 14 to 16 hours daily. Raise the lights as seedlings grow so they stay close without burning the leaves.

Planting seeds indoors step by step

Fill containers with moist seed-starting mix and press it down gently so it is firm but not compacted. Tomato seeds are tiny, so place one or two seeds on top of the soil in each cell or container, then cover with a thin layer of mix — about one-quarter inch deep. Press the soil gently so seeds make contact with the mix below.

Water the containers from below by setting them in a tray of water for a few minutes, or mist the surface gently with a spray bottle. The soil should feel like a wrung-out sponge: moist throughout but not soggy. Cover the tray with a humidity dome or plastic wrap to hold moisture while seeds germinate, then place it on your heat mat or warm spot.

Check daily and remove the cover as soon as sprouts appear — usually within 5 to 10 days at 70°F to 80°F. Move the tray when ready under grow lights or to a bright windowsill. Keep the soil moist but not waterlogged. Once seedlings develop their first true leaves (the second set of leaves that look like actual tomato leaves), thin out the weaker seedling in each cell by snipping it off at soil level with scissors rather than pulling it out, which can disturb roots.

Transplanting seedlings into larger pots

When seedlings are 2 to 3 inches tall and have at least two sets of true leaves, move them into 4-inch pots filled with potting soil. Tomato seedlings can be planted deeper than they were growing — bury the stem up to the first true leaves, and roots will form along the buried stem, creating a stronger plant. Water gently after transplanting and keep the soil consistently moist.

Continue providing 14 to 16 hours of light daily and maintain temperatures between 65°F and 75°F. Feed seedlings every two weeks with a diluted liquid fertilizer (half-strength all-purpose fertilizer works) once they have developed four true leaves. This prevents nutrient deficiency and keeps growth steady.

Hardening off seedlings before moving them outdoors

Indoor seedlings are tender and will wilt or die if moved directly into outdoor sun and wind. Start hardening them off 7 to 10 days before your last frost date by placing them outside in a sheltered, shaded spot for 1 to 2 hours. Each day, gradually increase their time outside and expose them to more direct sun, until by day 7 or 10 they can spend a full day in sun and wind.

Watch the weather forecast during this period. If frost threatens, bring seedlings back indoors. Once nighttime temperatures stay above 50°F consistently and seedlings have been hardened off, they are ready to plant in the garden.

Direct sowing seeds outdoors

If you live in a warm climate or prefer to skip the indoor step, you can plant seeds directly into garden beds or containers once soil temperature reaches at least 60°F — ideally 70°F or warmer. Prepare the bed by mixing in compost or aged manure to improve drainage and add nutrients. Create small holes about one-quarter inch deep, place one or two seeds in each hole, and cover lightly with soil.

Water gently and keep the soil consistently moist until sprouts appear. Thin seedlings once they have two true leaves, leaving the strongest plant in each spot. Direct-sown seedlings will be ready to produce fruit later than indoor-started plants, but they often develop stronger root systems because they are not transplanted.

Protect young seedlings from heavy rain, which can wash them away, and from strong wind, which can break tender stems. A row cover or shade cloth draped loosely over the bed offers protection while still allowing light and water through.

Common problems and how to avoid them

Damping off — a fungal disease that causes seedlings to collapse at the soil line — happens when soil stays too wet or air circulation is poor. Prevent it by using fresh seed-starting mix, watering from below rather than overhead, removing humidity domes once sprouts appear, and running a small fan nearby to keep air moving.

Leggy seedlings (tall and thin with large gaps between leaves) result from insufficient light. Move lights closer or add more hours of light daily. If seedlings are already leggy, bury them deeper when transplanting into larger pots — the buried stem will develop roots and create a sturdier plant.

Slow germination or no sprouts usually means soil temperature is too cold. Check that your heat mat is working or move the tray to a warmer location. Seeds older than three years germinate less reliably, so use fresh seed if possible.

Frequently Asked Questions

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

Garden soil compacts easily and holds too much moisture, which causes rot and fungal disease in seedlings. Seed-starting mix is designed to drain quickly while staying moist. If you cannot find commercial mix, make your own with peat moss, perlite, and vermiculite in equal parts.

How deep should I plant tomato seeds?

Plant seeds about one-quarter inch deep — roughly the thickness of a pencil tip. Too deep and they will not have enough energy to push through soil. Too shallow and they dry out quickly.

What temperature do tomato seeds need to sprout?

Tomato seeds germinate fastest at 70°F to 80°F and will sprout at temperatures as low as 60°F, though it takes much longer. Below 50°F, germination is unreliable. A heat mat keeps soil in the ideal range indoors.

When should I move seedlings under grow lights?

Move seedlings under lights as soon as the first sprout breaks through the soil surface. Waiting even a day or two causes them to stretch toward available light and become leggy. Keep lights on for 14 to 16 hours daily.

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

Yes, if your growing season is long enough and soil stays warm. Direct sowing works best in warm climates where soil reaches 70°F by late spring. In cooler regions, starting indoors gives you a head start and longer harvest season.