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

Tomato seeds need warmth and light to sprout, which is why most growers start them indoors in late winter or early spring rather than direct-seeding outdoors. The exact timing depends on when your last frost date falls in your region — you can find yours by searching "[your city] last frost date" or checking the USDA Plant Hardiness Zone Map. Count backward 6 to 8 weeks from that date, and that is when you should plant your seeds indoors.

Starting indoors gives seedlings a head start so they are sturdy and ready to transplant outside once soil warms up. Seeds started too early will become leggy and weak while waiting for outdoor conditions. Seeds started too late may not mature before fall frost arrives.

Key Takeaways

  • Plant tomato seeds indoors 6 to 8 weeks before your region's last spring frost date, which you can find by searching your city name plus "last frost date".
  • Use seed-starting mix (not garden soil) in small containers with drainage holes, keep soil moist but not waterlogged, and maintain temperatures between 70 and 80 degrees Fahrenheit for germination.
  • Seedlings need 14 to 16 hours of direct light daily from a grow light or sunny window; without enough light, they become tall and spindly.
  • Thin seedlings to one plant per cell once they develop their first true leaves, and transplant to larger containers when roots fill the original pot.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.

Gather containers, soil, and seeds before you start

You need seed-starting mix, small containers with drainage holes, and tomato seeds. Seed-starting mix is a sterile blend of peat moss, coconut coir, or perlite designed to drain well and prevent fungal disease — do not use garden soil or potting soil, which are too dense and can rot seeds. Small containers work best: seed trays with individual cells, recycled yogurt cups with holes punched in the bottom, or commercial seed pots.

Fill containers with moistened seed-starting mix, then plant one or two seeds per cell about one-quarter inch deep. Tomato seeds are small, so press them gently into the soil rather than trying to bury them. Cover the container with a humidity dome or plastic wrap to keep moisture in while seeds germinate.

Maintain warmth and moisture until seedlings emerge

Tomato seeds germinate best at soil temperatures between 70 and 80 degrees Fahrenheit. A heat mat placed under your seed tray speeds germination and keeps temperature consistent. Without a heat mat, place trays in a warm spot — the top of a refrigerator, a south-facing windowsill, or near (but not touching) a heating vent.

Check soil moisture daily. It should feel like a wrung-out sponge — moist but not soggy. Water from below by setting the tray in a shallow pan of water and letting soil absorb it, or mist gently from above. Overwatering causes damping-off, a fungal disease that kills seedlings at the soil line.

Seeds usually sprout in 5 to 10 days at warm temperatures. Once you see green shoots, remove the humidity dome and move the tray to a bright location when ready. Seedlings that sit in darkness will stretch toward light and become weak.

Provide 14 to 16 hours of direct light daily

Seedlings need strong light to grow compact and sturdy. A south-facing window can work if it receives at least 6 hours of direct sun, but a grow light is more reliable. Position the light 2 to 3 inches above the seedlings and raise it as they grow. Run the light for 14 to 16 hours per day using a timer — seedlings need darkness to rest.

Without enough light, seedlings become tall, thin, and pale. This is called "leggy" growth, and leggy seedlings rarely recover even after transplanting outdoors. If your seedlings are stretching, move the light closer or add more hours of light per day.

Keep temperatures between 65 and 75 degrees Fahrenheit during the day. At night, slightly cooler temperatures (around 60 degrees) are fine and actually help seedlings develop stronger stems.

Thin seedlings and transplant to larger pots as they grow

Once seedlings develop their first true leaves — the second set of leaves that look like actual tomato leaves, not the smooth seed leaves — thin them to one plant per cell. Remove the weaker seedling by pinching it off at soil level rather than pulling it out, which disturbs the roots of the remaining plant.

When roots fill the original container (you will see them poking through drainage holes), transplant seedlings to a larger pot, usually a 4-inch pot. Bury the stem deeper than it was growing before — tomatoes can develop roots all along the buried stem, which creates a stronger root system. Use potting soil (not seed-starting mix) for this stage, and keep it consistently moist.

Continue providing 14 to 16 hours of light and maintain temperatures between 65 and 75 degrees. Fertilize every 2 weeks with a diluted liquid fertilizer made for seedlings, following the package instructions.

Harden off seedlings before moving them outdoors

Seedlings grown indoors are tender and will wilt or burn if moved directly into outdoor sun and wind. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing seedlings outside in a sheltered, shaded spot for 1 to 2 hours. Each day, increase the time outdoors and gradually move them into more direct sun. By day 7 or 10, they should be able to stay outside all day and night.

Begin hardening off about a week before your last frost date, so seedlings are ready to plant in the garden once soil has warmed and frost danger has passed. Harden off seedlings only when daytime temperatures stay above 50 degrees Fahrenheit.

Transplant seedlings into the garden after the last frost

Once seedlings are hardened off and your last frost date has passed, plant them in the garden. Choose a spot with at least 6 to 8 hours of direct sun. Dig a hole slightly deeper than the seedling's root ball, bury the stem up to the first true leaves (tomatoes develop roots along buried stems), and water thoroughly.

Space plants 24 to 36 inches apart depending on the variety. Determinate varieties (bush types) need less space; indeterminate varieties (vining types) need more. Water regularly and mulch around the base to retain moisture and regulate soil temperature.

Frequently Asked Questions

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

A south-facing window with at least 6 hours of direct sun can work, but seedlings often become leggy because windowsill light is weaker than a grow light. If you use a window, rotate the tray daily so seedlings grow straight, and keep the window as warm as possible. A grow light is more reliable and costs less than replacing weak seedlings.

What does damping-off look like, and how do I prevent it?

Damping-off appears as a thin, dark line at the base of the seedling where it meets soil, and the seedling collapses. It is caused by fungal disease in wet soil. Prevent it by using sterile seed-starting mix, watering from below rather than overhead, ensuring good air circulation with a small fan, and not overwatering. If seedlings show signs of damping-off, remove affected plants when ready and reduce watering.

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

Seedlings are ready when they have at least two sets of true leaves, are 3 to 4 inches tall, and have been hardened off for 7 to 10 days. The most important factor is that your last frost date has passed and soil temperature is at least 60 degrees Fahrenheit. Planting too early into cold soil will stunt growth or kill seedlings.

Can I reuse seed-starting mix from last year?

Reusing seed-starting mix risks spreading fungal disease to new seedlings. Fresh seed-starting mix is inexpensive and sterile, so it is worth buying new each year. If you must reuse old mix, sterilize it by baking it in a 180-degree oven for 30 minutes, then let it cool completely before use.

What if I started seeds too early and they are getting too big?

Seedlings started too early become overgrown and root-bound before outdoor conditions are ready. Transplant them into larger pots to buy time, or pinch off the growing tip to make them bushier rather than taller. Keep them under lights and in cool conditions (around 60 degrees at night) to slow growth. If frost date is still weeks away, this usually works.