Start seeds indoors 6 to 8 weeks before your last spring frost

Growing tomatoes from seed means starting them indoors in late winter, then moving them outside after the frost danger passes. You'll sow seeds in a shallow container filled with seed-starting mix (not garden soil), keep them warm and moist, and move them to a sunny window or grow light once they sprout. Most home gardeners start seeds in February or March, depending on when their region's last frost date falls.

The whole process takes about 6 to 8 weeks from seed to a plant ready for the garden. You'll know your seedlings are ready to transplant outside when they're 6 to 8 inches tall, have their first true leaves (the second set that appears), and the soil temperature stays above 60°F at night.

Key Takeaways

  • Sow seeds indoors 6 to 8 weeks before your region's last frost date, which you can find through your local cooperative extension office or a frost date calculator.
  • Use seed-starting mix in a shallow tray or recycled container with drainage holes, keep soil moist but not waterlogged, and maintain a temperature of 70–75°F until sprouts appear.
  • Move seedlings to a sunny south-facing window or under grow lights as soon as the first green shoots emerge, and keep lights 2 to 3 inches above the plants.
  • Thin seedlings to one plant per cell or pot when they develop their first true leaves, and water from below by setting containers in a tray of water rather than pouring from above.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.

Gather your supplies before you sow

You need seed-starting mix, a shallow container with drainage holes, tomato seeds, and a heat source. Seed-starting mix is lighter and fluffier than potting soil—it drains well and doesn't compact around tiny seeds. You can buy it bagged at any garden center, or mix your own from equal parts peat moss (or coconut coir), perlite, and vermiculite.

For containers, use a seed tray, a recycled yogurt cup, an egg carton, or a newspaper pot. Drill or poke drainage holes in the bottom if they don't have them. Fill the container with moistened seed-starting mix—squeeze a handful; it should hold together but not drip water. Sow tomato seeds about a quarter-inch deep, two or three seeds per cell. Label each container with the variety name and the date you sowed.

A heat mat set to 70–75°F speeds germination and keeps soil temperature consistent. If you don't have one, place containers on top of a refrigerator, water heater, or in a warm corner of your home. Tomato seeds germinate in 5 to 10 days at this temperature; they'll take much longer or fail entirely if the soil stays cold.

Water carefully and watch for the first shoots

Keep the soil moist but never soggy. Check the surface daily—if it feels dry to the touch, mist it lightly with a spray bottle or set the whole container in a shallow tray of water and let it soak up moisture from below. Overwatering causes damping-off disease, a fungal infection that kills seedlings at the soil line. If you see this happen, improve air circulation by running a small fan nearby and let the soil dry out slightly between waterings.

Cover the container loosely with a humidity dome or plastic wrap to hold moisture while seeds germinate. Remove the cover as soon as you see the first green shoots—usually within 5 to 10 days. At this point, move the seedlings when ready to bright light.

Provide strong light from the moment seedlings emerge

Seedlings need 14 to 16 hours of bright light each day, starting the moment they break through the soil. A sunny south-facing window works if it gets at least 6 hours of direct sun, but most home windows don't provide enough light. Seedlings grown in dim light become tall, thin, and weak—a condition called "leggy."

A grow light is more reliable. Position it 2 to 3 inches above the seedlings and raise it as they grow, keeping that distance constant. Use a timer to run the light for 14 to 16 hours daily. If you don't have a grow light, place seedlings as close as possible to a bright window and rotate the tray daily so they don't lean toward the light.

Once seedlings are under lights, you can remove the heat mat. Room temperature (65–70°F) is fine now. Keep the soil moist but not wet, and water from below by setting the container in a tray of water for 10 to 15 minutes, then removing it and letting excess drain.

Thin seedlings and pot them up as they grow

When seedlings develop their first true leaves (the second set that looks like actual tomato leaves, not the smooth oval cotyledons), thin them to one plant per cell. Snip the weaker seedlings at soil level with scissors rather than pulling them out—pulling disturbs the roots of the plant you want to keep.

When seedlings are 2 to 3 inches tall, you can pot them up into larger containers—4-inch pots work well. Bury the stem up to the lowest leaves; tomatoes will root along the buried stem and develop a stronger root system. Use potting soil (not seed-starting mix) for this step. Water gently after potting and keep the soil consistently moist.

Continue providing 14 to 16 hours of light daily and maintain room temperature around 65–70°F. Feed seedlings every two weeks with a diluted liquid fertilizer made for seedlings, following the package directions. This replaces the nutrients they would get from soil in nature.

Harden off seedlings before moving them outside

Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start this process 7 to 10 days before your planned transplant date. On day one, place seedlings in a sheltered spot outdoors in dappled shade for 2 to 3 hours. Bring them back inside.

Each day, increase outdoor time by an hour and gradually move them into more direct sun. By day 7 or 8, seedlings should spend the full day outside. If nighttime temperatures drop below 50°F, bring them in. By day 10, they can stay outside overnight if temperatures stay above 50°F. This gradual transition prevents sunburn (pale or papery leaves) and wilting.

Plant seedlings in the garden when soil warms

Transplant seedlings into the garden when they're 6 to 8 inches tall, have several true leaves, and nighttime soil temperature stays above 60°F. Dig a hole slightly deeper than the root ball and bury the stem up to the lowest leaves—just as you did when potting up. Tomatoes develop roots all along the buried stem, creating a stronger plant.

Space plants 24 to 36 inches apart depending on the variety. Determinate (bush) types need less space; indeterminate (vining) types need more. Water gently after planting and keep soil consistently moist for the first two weeks while roots establish. Mulch around the base with 2 to 3 inches of straw or shredded leaves to retain moisture and regulate soil temperature.

Common mistakes to avoid

The most common error is starting seeds too early. If you sow 12 weeks before your frost date instead of 6 to 8 weeks, seedlings become overgrown and root-bound before transplant time. They'll be stressed and slower to recover in the garden. Check your local frost date first, then count backward.

Insufficient light is the second major problem. Seedlings grown in dim conditions stretch toward available light and become too tall and fragile to handle. If you notice this happening, move lights closer and run them longer, or move seedlings to a brighter window when ready.

Overwatering kills more seedlings than underwatering. Soggy soil invites damping-off disease and root rot. Let the soil surface dry slightly between waterings, and always use containers with drainage holes. If you see wilting despite moist soil, reduce watering and improve air circulation with a small fan.

Frequently Asked Questions

Can I start tomato seeds directly in the garden instead of indoors?

You can, but you'll get fewer plants and a later harvest. Direct seeding works only in warm climates where soil stays above 60°F by late spring. In most regions, starting indoors gives you a head start of 6 to 8 weeks and lets you choose the strongest seedlings to transplant.

What's the difference between determinate and indeterminate tomatoes?

Determinate varieties grow as compact bushes, produce most fruit at once, and stop growing at a set height—good for canning and small spaces. Indeterminate varieties grow as vines, produce fruit throughout the season, and need staking or caging. Both start from seed the same way; the difference shows up once they're in the garden.

Why are my seedlings tall and spindly?

They're not getting enough light. Move them closer to a window, add a grow light, or run your current light for longer each day. You can also pot up leggy seedlings deeper, burying the stem up to the lowest leaves to create a stronger root system.

How long can I keep seedlings indoors before planting them outside?

Seedlings can stay indoors for 8 to 10 weeks without problems if you keep them under lights and feed them regularly. Beyond that, they become root-bound and stressed. If your frost date hasn't arrived, pot them up into larger containers or move them to a cold frame to slow growth.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination and keeps soil temperature consistent, but it's not required. Tomato seeds will germinate at room temperature (65–70°F), just more slowly—10 to 14 days instead of 5 to 10. If your home is cool, a heat mat is worth the investment.