You can start tomato seeds indoors 6 to 8 weeks before your last spring frost, then move the seedlings outside when soil warms

Starting tomatoes from seed is cheaper than buying transplants and gives you access to varieties you won't find at a garden center. The process takes about three months from seed to transplant-ready seedling, but most of that time is just waiting. The actual hands-on work—planting, watering, and moving seedlings to stronger light—takes only a few minutes a week.

The key is timing. Tomato seeds germinate in warm soil (around 70°F), sprout in 5 to 10 days, and need strong light as soon as they emerge. Seedlings are ready to move outside when they have their first true leaves (the second set of leaves that appear) and soil temperatures stay above 50°F at night. Starting too early means you'll have leggy, weak plants waiting for warm weather; starting too late means you'll miss peak growing season.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find through your local cooperative extension office or by searching your zip code online.
  • Use a seed-starting mix (not garden soil), keep it moist but not waterlogged, and provide 14 to 16 hours of direct light daily once seedlings emerge.
  • Thin seedlings to one plant per cell or pot when they develop their first true leaves, or transplant the strongest seedling into a larger container.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.
  • Transplant seedlings into the garden only after nighttime soil temperatures stay above 50°F and daytime air temperatures are consistently above 60°F.

Gather seeds, containers, and soil before you start

You need tomato seeds (any variety), small containers with drainage holes (seed trays, cell packs, or recycled yogurt cups work), seed-starting mix (a sterile blend of peat moss, coconut coir, or perlite—not garden soil), and a way to provide light. A sunny south-facing windowsill works if you have one; a grow light on a timer is more reliable and costs $20 to $60.

Fill containers with moistened seed-starting mix, leaving about half an inch at the top. Plant one or two seeds per cell, pressing them about a quarter-inch into the soil. Tomato seeds are small, so don't bury them deep. Cover the tray loosely with a humidity dome or plastic wrap to keep moisture in while seeds germinate.

Place the tray in a warm spot—a heat mat under the tray speeds germination, but a warm shelf or cabinet works too. Check daily and remove the cover as soon as you see green shoots. Once seedlings emerge, light becomes critical. Without it, they'll stretch toward the window and become thin and weak.

Provide light and water as seedlings grow

Move seedlings to bright light when ready after they sprout. If you're using a windowsill, rotate the tray daily so plants grow straight instead of leaning toward the glass. A grow light should be 2 to 3 inches above the seedlings and on for 14 to 16 hours daily. Raise the light as plants grow so it stays at that distance.

Water when the top of the soil feels dry to your finger, but don't let the tray sit in standing water. Overwatering causes damping-off, a fungal disease that kills seedlings at the soil line. Use room-temperature water and water from below if possible—set the tray in a larger container of water for 10 minutes, then drain it. This keeps foliage dry and reduces disease.

Keep the room temperature between 65°F and 75°F during the day. Seedlings don't need fertilizer yet if you're using seed-starting mix, which has some nutrients built in. Once seedlings develop their first true leaves (the second set, which look like actual tomato leaves), you can begin feeding with a diluted liquid fertilizer every two weeks.

Thin or transplant seedlings when they're crowded

If you planted two seeds per cell and both sprouted, remove the weaker seedling by snipping it at soil level with scissors—don't pull it out, which can damage the roots of the keeper plant. If you planted one seed per cell and seedlings are growing well, you can leave them as is. If they're getting leggy or you want larger plants before moving them outside, transplant the strongest seedling into a 4-inch pot filled with seed-starting mix.

Handle seedlings by their leaves, not their stems, which bruise easily. Make a hole in the new soil, set the seedling in at the same depth it was growing (tomatoes can be planted deeper than they were in the seed tray, and they'll develop roots along the buried stem), and gently firm soil around it. Water after transplanting and return to light.

Harden off seedlings before moving them outside

Seedlings grown indoors are tender and will wilt or burn if moved directly into sun and wind. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing the tray in a sheltered spot outdoors (a porch or against a wall) for 2 to 3 hours on the first day. Bring them back inside.

Each day, increase outdoor time by an hour or two and move them to a slightly more exposed spot. By day 7 or 8, they should be outside all day. On day 9 or 10, leave them out overnight if temperatures stay above 50°F. This process toughens the leaves and stems so they can handle garden conditions.

Plant seedlings in the garden at the right time

Tomatoes are frost-sensitive and will die if exposed to temperatures below 50°F. Wait until nighttime lows stay above 50°F and daytime highs are consistently above 60°F before planting. Check your local weather forecast for the next two weeks; if a cold snap is predicted, wait a few more days.

Choose a spot with at least 6 to 8 hours of direct sun daily. Dig a hole slightly deeper than the root ball, set the seedling in, and firm soil around it. You can bury the stem deeper than it was growing in the pot—tomatoes will develop roots along the buried stem and anchor better. Water well after planting and keep soil consistently moist for the first week while roots establish.

Troubleshoot common seedling problems

Leggy seedlings (tall and thin with large gaps between leaves) mean insufficient light. Move the light source closer or provide more hours of light daily. Damping-off (seedlings collapse at soil level) is a fungal disease caused by overwatering or poor air circulation. Improve drainage, water less often, and increase air movement with a small fan on low speed.

Pale or yellowing leaves usually mean seedlings are ready for fertilizer or have outgrown their containers. Transplant into larger pots or begin feeding with diluted liquid fertilizer. Slow growth can mean the room is too cold; tomato seedlings grow fastest between 70°F and 75°F during the day and 60°F to 65°F at night.

Frequently Asked Questions

Can I start tomato seeds on a windowsill without a grow light?

A south-facing windowsill works if you rotate the tray daily and live in a sunny climate, but seedlings often become leggy because natural light is weaker than a grow light. A grow light is more reliable and costs less than buying transplants. If you use a windowsill, expect some stretching and plan to bury the stem deeper when transplanting.

How deep should I plant tomato seeds?

Plant seeds about a quarter-inch deep in seed-starting mix. Tomato seeds are small and don't need to be buried deeply. If you plant them too deep, they may not have enough energy to push through the soil surface.

What's the difference between seed-starting mix and potting soil?

Seed-starting mix is sterile, lightweight, and drains quickly—ideal for seeds and young seedlings. Potting soil is heavier, contains fertilizer, and holds more moisture, making it better for established plants. Using potting soil for seeds can lead to overwatering and damping-off disease.

Can I reuse seed-starting mix from last year?

Reusing old seed-starting mix risks introducing disease to new seedlings. Fresh mix is inexpensive and worth the cost. If you must reuse it, sterilize it by baking it in a 180°F oven for 30 minutes in a covered container, then let it cool completely before use.

What if my last frost date has already passed and I haven't started seeds?

Buy transplants from a garden center or nursery instead. Starting seeds now will produce seedlings too late in the season to mature before fall frost. Transplants are more expensive but will give you a full growing season and a better harvest.