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

Indoor seed starting gives tomato plants a head start before outdoor planting season. You sow seeds in containers under lights, keep them warm and moist, and move them outdoors once they develop true leaves and the soil has warmed. The timing matters: if you start too early, seedlings become leggy and weak waiting for transplant day; if you start too late, you miss the growing season.

The process takes about 6 to 8 weeks from seed to transplant-ready seedling. Look up your local last spring frost date—your county extension office publishes this—then count backward. If your frost date is May 15, start seeds around mid-March. Starting indoors lets you grow varieties that need a longer season than your climate naturally provides.

Key Takeaways

  • Start seeds 6 to 8 weeks before your last spring frost date, which you can find through your county extension office.
  • Use seed-starting mix (not garden soil), keep it consistently moist, and provide 14 to 16 hours of light daily with a grow light or bright windowsill.
  • Seedlings need warmth: 70 to 80 degrees Fahrenheit during the day, slightly cooler at night, which a heat mat can provide.
  • Thin seedlings to one plant per cell once they sprout, and move them to larger containers when they develop their first true leaves.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.

Gather seeds, containers, and soil before you begin

You need tomato seeds (any variety), small containers with drainage holes, seed-starting mix, and a light source. Containers can be seed trays, cell packs, yogurt cups with holes drilled in the bottom, or recycled plastic takeout containers—anything that drains and holds soil. Do not use garden soil or potting soil meant for mature plants; seed-starting mix is lighter and fluffier, which lets roots push through easily and reduces disease.

A grow light is the most reliable option. A basic two-bulb fluorescent shop light from a hardware store costs $20 to $40 and works as well as expensive LED panels. If you use a sunny windowsill instead, rotate containers daily so seedlings grow straight instead of leaning toward the light. A heat mat under your containers keeps soil at 70 to 80 degrees Fahrenheit, which speeds germination; it is optional but helpful, especially in cool basements or early spring.

Fill containers and sow seeds at the right depth

Fill containers with moist seed-starting mix—wet it before filling so it absorbs water evenly. Tomato seeds are tiny, so sow 2 or 3 seeds per cell or small container, pressing them gently into the soil surface. Cover them with a thin layer of mix, about 1/8 inch deep. Tomato seeds need light to germinate, so do not bury them.

Mist the surface gently with water from a spray bottle so you do not wash seeds away. Cover the containers with a humidity dome or plastic wrap to keep moisture in while seeds germinate. Place them on a heat mat set to 75 to 80 degrees Fahrenheit. Seeds usually sprout in 5 to 10 days. Once you see green shoots, remove the cover and move containers under lights.

Provide light and keep seedlings from getting leggy

Position grow lights 2 to 3 inches above seedlings and run them for 14 to 16 hours daily. As plants grow, raise the light to maintain that distance—if lights sit too high, seedlings stretch and become thin and weak. A straightforward timer on the light outlet makes this automatic: set it to turn on at 6 a.m. and off at 8 p.m., or whatever 14-hour window fits your schedule.

Thin seedlings once they sprout their first true leaves (the second set of leaves that appear, which look like actual tomato leaves). Snip away extra seedlings with scissors rather than pulling them out, which disturbs roots of the keeper plant. Leave one strong seedling per cell. This prevents crowding and competition for light and water.

Water consistently and thin to one plant per container

Keep seed-starting mix moist but not waterlogged. Water from below by setting containers in a tray of water and letting them soak up moisture from the drainage holes, or water gently from above with a spray bottle. Soggy soil causes damping-off disease, which kills seedlings at the soil line. If the mix dries out completely, seedlings wilt and may not recover.

Once seedlings develop their first true leaves, they are ready to move to larger containers if they are crowded. Fill 4-inch pots with potting soil (regular potting soil is fine now, not seed-starting mix) and transplant one seedling per pot. Bury the stem deeper than it was growing before—tomatoes can develop roots all along the buried stem, which strengthens the plant. Keep lights at the same 2 to 3 inch distance above the new containers.

Harden off seedlings before moving them outdoors

Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start 7 to 10 days before your planned transplant date. On day one, place seedlings in a sheltered spot outdoors in shade for 1 to 2 hours. Each day, increase time outside and gradually move them into dappled light, then partial sun.

By day 7 or 8, seedlings should spend the full day outside in a spot that gets several hours of sun. Bring them inside at night if temperatures drop below 50 degrees Fahrenheit. On the final days, leave them out overnight if nighttime temperatures stay above 50 degrees. This toughens the leaves and stems so they do not wilt or sunburn when planted in the garden.

Transplant seedlings into the garden when soil is warm

Tomatoes need soil that has warmed to at least 60 degrees Fahrenheit, ideally 65 to 70 degrees. Planting into cold soil slows growth and stresses plants. You can check soil temperature with a soil thermometer from a garden center, or wait until nighttime temperatures stay consistently above 50 degrees Fahrenheit for a week—that usually means soil has warmed enough.

Dig a hole slightly deeper than the root ball and bury the stem up to the lowest true leaves. Tomatoes develop roots along buried stems, which creates a stronger root system. Space plants 24 to 36 inches apart depending on variety. Water deeply after planting and keep soil consistently moist for the first two weeks while roots establish.

Frequently Asked Questions

Can I use regular potting soil to start seeds?

Seed-starting mix works better because it is lighter and fluffier, which helps tiny roots push through easily. Regular potting soil is denser and can compact, making germination harder. If you only have potting soil, it will work, but mix it half-and-half with perlite or coconut coir to lighten it.

What if my seedlings are tall and skinny?

Leggy seedlings mean they are not getting enough light or are too warm. Move lights closer (2 to 3 inches above the tops), run them longer (14 to 16 hours daily), or lower the temperature slightly. You can also bury leggy seedlings deeper when transplanting—they will develop roots along the buried stem and recover.

Do I need a heat mat?

A heat mat speeds germination and helps in cool spaces, but it is not required. Seeds will germinate at room temperature (around 70 degrees), just more slowly. If your basement or growing area stays below 65 degrees, a heat mat is worth the investment.

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

Seedlings are ready when they have developed several sets of true leaves, are 4 to 6 inches tall, and have been hardened off for 7 to 10 days. Check that nighttime outdoor temperatures stay above 50 degrees Fahrenheit and soil has warmed to at least 60 degrees before planting.

Can I reuse seed-starting mix from last year?

Reusing old mix risks spreading disease to new seeds. Fresh seed-starting mix is inexpensive and sterile. If you must reuse, bake it in a 180-degree oven for 30 minutes to kill pathogens, then let it cool completely before using.