Start seeds indoors 6 to 8 weeks before your last spring frost, or direct sow them outdoors after soil warms

Tomato seeds need warmth, moisture, and light to sprout—conditions you can create indoors under grow lights or on a sunny windowsill, or outdoors once soil reaches 60°F. Starting indoors gives you a head start on the growing season and lets you control the environment. Direct sowing outdoors works if you live in a warm climate or have a long growing season, but seeds take longer to germinate and seedlings are more vulnerable to pests and weather.

The choice depends on where you live. If your frost-free season is shorter than 70 days, start indoors. If you have 80+ frost-free days and warm soil by late spring, you can direct sow. Most home gardeners start indoors because it produces stronger plants and gives you more control over timing.

Key Takeaways

  • Tomato seeds germinate best at 70–80°F and sprout in 5 to 10 days when kept consistently moist.
  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find through your local cooperative extension office.
  • Seedlings need 14 to 16 hours of direct light per day from a grow light or sunny south-facing window to stay stocky and strong.
  • Thin seedlings to one plant per cell or pot once they develop their first true leaves, or transplant them into larger containers to prevent crowding.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.

Gather seeds, soil, containers, and light before you start

You will need tomato seeds (buy from a seed catalog or packet), seed-starting mix (a sterile, lightweight blend of peat moss, coconut coir, or perlite—not garden soil), small containers with drainage holes, and a heat source. Containers can be seed trays, cell packs, peat pots, or recycled yogurt cups with holes poked in the bottom. Fill them with moist seed-starting mix.

Light is critical. A sunny south-facing windowsill works if you have at least 6 hours of direct sun, but grow lights (LED or fluorescent) are more reliable. Position lights 2 to 3 inches above the seedlings and raise them as plants grow. A heating mat under the containers speeds germination but is not required if your home stays 70–80°F.

Gather everything before you sow so you are not scrambling for supplies once seeds sprout. Tomato seeds stay viable for 4 to 6 years if stored in a cool, dry place, so you can buy extras and save them.

Sow seeds at the right depth and keep soil consistently moist

Fill containers with moist seed-starting mix and press it gently so it is firm but not compacted. Place one or two tomato seeds in each cell or pot, then cover with 1/4 inch of mix. Do not bury them deeper—tomato seeds need light to germinate, and burying them slows sprouting or prevents it altogether.

Mist the soil surface with water until it is damp, then cover the container with a humidity dome or plastic wrap to trap moisture. Check daily and mist again if the surface starts to dry. Seeds germinate in 5 to 10 days at 70–80°F. Once the first green shoot appears, remove the cover and move the container under lights when ready. Seedlings that stay in darkness become pale and weak.

If you are direct sowing outdoors, wait until soil temperature reaches 60°F (use a soil thermometer to check). Plant seeds 1/4 inch deep, 6 inches apart, and water gently. Thin seedlings to 12 inches apart once they have two true leaves. Direct-sown seeds take 7 to 14 days to sprout depending on soil warmth.

Provide 14 to 16 hours of light daily and keep seedlings warm

Once seedlings emerge, light becomes the limiting factor. Position grow lights 2 to 3 inches above the tops of seedlings and run them for 14 to 16 hours per day. Raise the lights as plants grow so the distance stays constant. Without enough light, seedlings become leggy—tall, thin, and pale—and will not transplant well.

Keep soil temperature between 65–75°F during the day and slightly cooler at night. A heating mat helps, but most homes stay warm enough indoors. Water when the top of the soil feels dry to the touch, but do not let containers sit in standing water. Overwatering causes damping-off disease, a fungal infection that kills seedlings at the soil line.

Once seedlings have their first true leaves (the second set of leaves that appear), you can begin fertilizing with a diluted liquid fertilizer every 7 to 10 days. Seed-starting mix contains few nutrients, so feeding helps seedlings grow strong.

Thin or transplant seedlings when they develop true leaves

Tomato seedlings grow quickly and need room. When seedlings have their first true leaves—the second set, which look like actual tomato leaves—thin them to one plant per cell. If you planted two seeds per cell and both sprouted, snip off the weaker one at soil level with scissors rather than pulling it out, which disturbs the roots of the survivor.

If seedlings become crowded or leggy, transplant them into larger containers (4-inch pots work well). Bury the stem up to the lowest leaves—tomatoes can root along buried stems and develop a stronger root system. Use the same seed-starting mix and keep the soil moist but not waterlogged.

Seedlings are ready to transplant into the garden when they are 6 to 8 inches tall, have several sets of true leaves, and have developed a sturdy stem. This usually takes 6 to 8 weeks from sowing.

Harden off seedlings before moving them outdoors

Hardening off means gradually exposing seedlings to outdoor conditions—wind, direct sun, and temperature swings—so they adjust without wilting or sunburning. Start 7 to 10 days before planting day. Place seedlings in a sheltered spot outdoors for 1 to 2 hours on the first day, then bring them back inside. Each day, increase outdoor time by 1 to 2 hours and expose them to a bit more direct sun and wind.

By the end of the hardening-off period, seedlings should spend a full day and night outdoors. If frost is forecast, bring them inside. Once nighttime temperatures stay above 50°F and soil has warmed to 60°F or higher, seedlings are ready to plant in the garden.

Harden off even seedlings grown indoors on a windowsill. The transition from controlled indoor conditions to outdoor weather is a shock, and skipping this step often results in stunted growth or plant death.

Plant seedlings in the garden at the right spacing and depth

Dig a hole slightly larger than the root ball and plant seedlings at the same depth they were growing in their container—or deeper. Like transplanting, burying the stem up to the lowest leaves encourages a stronger root system. Space seedlings 24 to 36 inches apart depending on the variety; determinate (bush) types need less space than indeterminate (vining) types.

Water gently after planting and keep soil consistently moist for the first 2 to 3 weeks while roots establish. Once plants are growing vigorously, water deeply once or twice per week depending on rainfall and heat. Mulch around plants with 2 to 3 inches of compost or straw to retain moisture and regulate soil temperature.

Tomatoes begin flowering 4 to 6 weeks after transplanting and produce fruit 6 to 8 weeks after that, depending on variety and growing conditions. Indeterminate varieties continue producing until frost; determinate varieties produce most of their fruit in a concentrated window.

Common mistakes that slow growth or kill seedlings

Overwatering is the most common error. Seedlings need moist soil, not soggy soil. If containers do not have drainage holes, roots rot. If you water daily without checking soil moisture, you are likely overwatering. Let the top 1/4 inch dry between waterings.

Insufficient light produces leggy seedlings that do not transplant well. A sunny windowsill is rarely enough; grow lights are more reliable. Position them close to seedlings and run them 14 to 16 hours daily.

Skipping hardening off or planting too early exposes tender seedlings to cold soil and frost. Wait until soil reaches 60°F and nighttime temperatures stay above 50°F. If you plant early and frost is forecast, cover plants with row cloth or bring potted plants indoors.

Damping-off disease kills seedlings at the soil line. It thrives in cold, wet conditions. Use sterile seed-starting mix, may support containers have drainage, and do not overwater. If seedlings start to collapse, remove affected plants when ready and improve air circulation with a small fan.

Frequently Asked Questions

Can I save seeds from my tomato plants to grow next year?

Yes, but only from determinate or heirloom varieties. Hybrid tomatoes produce seeds that do not grow true to type—the plants will not be identical to the parent. Scoop seeds from ripe fruit, ferment them in water for 2 to 3 days to remove the gel coating, rinse, dry completely on a plate, and store in a cool, dry place. They remain viable for 4 to 6 years.

What is the difference between true leaves and seed leaves?

Seed leaves (cotyledons) are the first leaves that emerge from the seed and look oval or rounded. True leaves appear next and look like actual tomato leaves—lobed and textured. Seedlings are ready to thin or transplant once they have their first set of true leaves, usually 2 to 3 weeks after germination.

Do I need a heating mat to start tomato seeds?

No. Heating mats speed germination by keeping soil at 70–80°F, but seeds will germinate in a warm room without one—it just takes a few extra days. If your home stays below 65°F, a heating mat is worth the cost. Turn it off once seedlings emerge; they do not need extra heat after that.

Why are my seedlings tall and spindly?

Insufficient light is the main cause. Move grow lights closer to seedlings or increase light duration to 16 hours per day. Overcrowding also causes legginess because seedlings compete for light. Thin or transplant seedlings to give each one more space. Temperature that is too warm can also stretch seedlings; keep it between 65–75°F during the day.

Can I start tomato seeds outdoors in containers instead of in the ground?

Yes. Fill containers with potting mix (not seed-starting mix, which dries out too quickly), sow seeds 1/4 inch deep, and keep soil moist. Thin seedlings to one per container once they have true leaves. Transplant into the garden or larger containers when seedlings are 6 to 8 inches tall. Container-grown seedlings need consistent watering because containers dry faster than garden soil.