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

Growing tomatoes from seed begins indoors, not in the garden. You start seeds in containers under lights or on a sunny windowsill about 6 to 8 weeks before the last frost date in your area. This timing gives seedlings enough time to develop strong roots and a few true leaves before you move them outside, where they can harden off and then go into the ground.

The reason for starting early is straightforward: tomato seeds are slow. From planting to a transplant-ready seedling takes 6 to 8 weeks. If you wait until the soil warms up to plant directly in the garden, you will not harvest until late summer or fall. Starting indoors lets you move seedlings outside as soon as conditions allow, which means fruit earlier in the season.

Key Takeaways

  • Tomato seeds need warm soil (70 to 80 degrees Fahrenheit), moist seed-starting mix, and light to germinate and grow strong seedlings.
  • Plant seeds about one-quarter inch deep in containers with drainage holes, keep the soil consistently moist, and provide 14 to 16 hours of light daily once seedlings emerge.
  • Thin seedlings to one per cell or pot once 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 after the last frost date.
  • Bury the stem up to the lowest leaves when transplanting into the garden, because tomatoes will root along the buried stem and develop a stronger root system.

Fill containers with seed-starting mix and plant seeds one-quarter inch deep

Use a seed-starting mix, not garden soil or potting soil. Seed-starting mix is lighter and fluffier, which helps seeds germinate and seedlings develop without rot. Fill small containers—cell packs, peat pots, or recycled yogurt cups with drainage holes—to the rim with moist seed-starting mix.

Plant one or two tomato seeds per cell, pressing them into the mix about one-quarter inch deep. Cover lightly with more mix. Tomato seeds are tiny, so do not bury them deep; they need light to germinate. Water gently so the mix stays moist but not waterlogged. Label each container with the variety name and the date you planted.

Provide warmth and moisture until seedlings emerge

Tomato seeds germinate best in warm soil—between 70 and 80 degrees Fahrenheit. If your home is cooler, place the containers on a heat mat set to 75 degrees. Check the soil daily; it should feel moist like a wrung-out sponge, not soggy. Cover the containers loosely with a clear plastic dome or plastic wrap to hold in moisture and warmth.

Seeds usually sprout in 5 to 10 days. Once you see green shoots, remove the cover and move the containers to a light source when ready. Seedlings that do not get light right away will stretch and become weak and spindly.

Provide 14 to 16 hours of light daily and keep soil consistently moist

Seedlings need strong light to grow compact and sturdy. A south-facing windowsill works if it gets at least 6 hours of direct sun, but grow lights are more reliable. Position lights 2 to 3 inches above the seedlings and raise them as the plants grow. Run lights for 14 to 16 hours daily; a timer makes this automatic.

Water when the top of the soil feels dry to the touch. Consistent moisture matters more than frequent watering. Overwatering causes damping off, a fungal disease that kills seedlings at the soil line. Underwatering stresses seedlings and slows growth. Water from below by setting containers in a tray of water and letting them absorb moisture from the bottom, or water gently at the soil line.

Thin seedlings to one per container once true leaves appear

When seedlings develop their first true leaves—the second set of leaves that look like tiny tomato leaves, not the round seed leaves—thin them to one per cell. If you planted two seeds per cell and both sprouted, snip off the weaker seedling at soil level with scissors rather than pulling it out, which can damage the roots of the keeper seedling.

Alternatively, once seedlings have two sets of true leaves, you can transplant the strongest one into a larger container filled with potting soil. This gives roots more room to grow. Bury the stem up to the lowest leaves; tomatoes will root along the buried stem and develop a stronger root system.

Harden off seedlings over 7 to 10 days before planting outside

Seedlings grown indoors are tender. Moving them directly from a warm, still, sheltered environment to the garden will shock them. Hardening off means gradually exposing seedlings to outdoor conditions—wind, sun, and temperature swings—so they toughen up.

Start 7 to 10 days before your last frost date. On day one, place seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours. Each day, increase the time outside and gradually move them into more sun. By day 7 or 8, seedlings should spend the full day outside. If frost threatens, bring them back indoors. Once nighttime temperatures stay above 50 degrees Fahrenheit and the soil has warmed, seedlings are ready to plant in the garden.

Plant seedlings in the garden after the last frost date

Tomatoes cannot tolerate frost. Wait until after your area's last frost date to plant seedlings in the garden. Space them 24 to 36 inches apart in a spot that gets at least 6 to 8 hours of direct sun daily.

Dig a hole slightly deeper than the root ball and bury the stem up to the lowest leaves. Remove the lower leaves before planting so they do not touch the soil. This buried stem will develop roots along its length, creating a stronger, deeper root system that is more resilient to drought and wind. Water well after planting and keep the soil consistently moist for the first two weeks while roots establish.

Frequently Asked Questions

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

You can, but you will harvest much later. Direct seeding works only in warm climates where the growing season is long and soil stays warm. In most regions, starting indoors gives you a head start and fruit weeks earlier. Direct seeding also wastes seeds because germination is slower and less reliable in cool soil.

What should I do if seedlings become tall and spindly?

Spindly seedlings mean they are not getting enough light. Move lights closer or add more hours of light per day. If seedlings are already stretched, bury them deeper when you transplant into larger containers or into the garden. Tomatoes will root along the buried stem and recover.

How often should I fertilize seedlings?

Seed-starting mix contains few nutrients, so seedlings need feeding once they develop their first true leaves. Use a diluted liquid fertilizer—half strength—every two weeks. Stop fertilizing once seedlings are hardened off and planted in the garden, where garden soil or potting soil provides nutrients.

Can I reuse seed-starting mix from last year?

It is safer to use fresh seed-starting mix each year. Old mix may harbor fungal spores that cause damping off. If you must reuse it, sterilize it by baking it in a 180-degree oven for 30 minutes in a covered container, then let it cool completely before use.

What if I miss the hardening-off window and have to plant seedlings directly outside?

Plant them in late afternoon or on a cloudy day to reduce transplant shock. Water deeply and keep soil consistently moist for the first week. Shade seedlings with a row cover or cloth for a few days if strong sun is forecast. They will recover, but growth will be slower than if you had hardened them off gradually.