Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow them outdoors after soil warms to 60°F

Tomato seeds need warmth and moisture to sprout, but they do not need light until they break through the soil. Most home gardeners start seeds indoors in late winter or early spring so the seedlings are ready to transplant when outdoor soil is warm enough. If you live in a warm climate or want to skip the indoor step, you can plant seeds directly in the garden once the soil temperature stays above 60°F — usually 1 to 2 weeks after your last frost date.

The choice between indoor and outdoor planting depends on your climate and how much time you have. Indoor starting gives you stronger, larger plants by transplant time and lets you control the growing environment. Direct sowing is simpler and requires no equipment, but seeds may rot in cool, wet soil and germination is slower.

Key Takeaways

  • Start seeds indoors in seed-starting mix (not garden soil) in a warm spot around 70°F, keeping the soil moist but not waterlogged.
  • Seedlings need 14 to 16 hours of bright light daily once they sprout, either from a grow light or a sunny south-facing window.
  • Thin seedlings to one plant per cell or pot when they develop their first true leaves, or they will compete for light and nutrients.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.
  • For direct sowing outdoors, plant seeds half an inch deep in warm soil and keep the area consistently moist until sprouts appear.

Starting seeds indoors in containers

Fill a seed tray, cell pack, or small pots with seed-starting mix — a light, sterile blend of peat moss, coconut coir, or perlite designed to drain well and prevent fungal disease. Do not use garden soil or potting mix, which compact and retain too much water around delicate seeds. Moisten the mix before planting so it is damp but not soggy.

Plant one or two seeds per cell, pressing them about half an inch into the soil. Tomato seeds are small, so you can also sprinkle them on the surface and cover lightly with a thin layer of mix. Mist the surface gently with a spray bottle so you do not wash seeds away. Cover the tray with a plastic dome, plastic wrap, or a clear plastic bag to hold moisture and warmth while seeds germinate.

Place the tray in a warm spot — ideally 70 to 75°F — such as the top of a refrigerator, a heat mat, or a warm shelf away from drafts. Tomato seeds germinate in 5 to 10 days at this temperature. Check the soil daily and mist if the surface begins to dry, but do not let water pool. Once sprouts appear, remove the cover and move the tray to a light source when ready.

Providing light and managing seedling growth

Seedlings need 14 to 16 hours of bright light daily to grow stocky and strong. A sunny south-facing window works if the light is direct and unobstructed, but most home growers use a grow light — a fluorescent or LED fixture positioned 2 to 3 inches above the seedlings. Raise the light as the plants grow so it stays at that distance. Without enough light, seedlings become tall, thin, and weak (a condition called "leggy"), and they will not transplant well.

Keep the soil moist but not waterlogged. Water from below by setting pots in a tray of water and letting them absorb moisture from the bottom, or water gently at soil level with a small watering can. Overwatering causes damping-off, a fungal disease that kills seedlings at the soil line.

When 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. Snip away the weaker seedling with scissors rather than pulling it out, which can disturb the roots of the one you are keeping. If you planted only one seed per cell, skip this step.

Hardening off seedlings before transplanting

Indoor seedlings are tender and will wilt or burn if moved directly into outdoor sun and wind. Hardening off — gradually exposing them to outdoor conditions — takes 7 to 10 days and is essential for success. Begin when seedlings are 2 to 3 inches tall and your outdoor nighttime temperatures are above 50°F.

On day one, place seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase the time outdoors and gradually move them into more sun and wind. By day 7 or 8, they should be outside all day. Leave them outside overnight only when nighttime temperatures stay above 50°F. If a cold night is forecast, bring them back inside.

During hardening off, water more often because outdoor air dries soil faster than indoor air. Watch the forecast and do not rush the process — a sudden move to full sun or cold wind can stunt or kill seedlings that have not acclimated.

Direct sowing seeds outdoors

If you prefer to skip indoor starting, plant seeds directly in the garden after the soil has warmed to at least 60°F and all danger of frost has passed. Check your local frost date — your county extension office or a frost date calculator online can tell you when it is safe to plant in your area.

Prepare a planting spot with loose, well-draining soil mixed with compost or aged manure. Plant seeds half an inch deep, spacing them 1 to 2 inches apart. Water gently so the soil is moist but not muddy. Keep the soil consistently moist — not soggy — until seedlings emerge, which usually takes 5 to 10 days in warm soil.

Once seedlings have two true leaves, thin them to one plant every 12 to 18 inches, depending on the variety. Snip away the extras with scissors so you do not disturb the roots of the plants you are keeping. Direct-sown seedlings will be smaller and later to flower than indoor-started plants, but they often grow into healthy, productive plants.

Transplanting seedlings into the garden

Harden-off seedlings are ready to transplant when they are 6 to 8 inches tall and have several sets of true leaves. Dig a hole slightly larger than the root ball and plant the seedling at the same depth it was growing in its pot — or even deeper, burying the stem up to the first true leaves. Tomatoes will root along the buried stem, creating a stronger root system.

Space plants 24 to 36 inches apart, depending on the variety and how you plan to support them. Water well after planting and keep the soil consistently moist for the first week while roots establish. Mulch around the base with 2 to 3 inches of compost or straw to retain moisture and regulate soil temperature.

Common problems and how to avoid them

Seeds do not sprout: Soil was too cold, too wet, or too dry. Tomato seeds need warmth (70°F or above) and consistent moisture. Check that your heat source is working and that you are misting regularly without waterlogging the soil.

Seedlings are tall and thin: Not enough light. Move the grow light closer or place seedlings in a brighter window. Leggy seedlings can be buried deeper when transplanted, but prevention is easier than rescue.

Seedlings wilt or die at soil level: Damping-off fungal disease, caused by overwatering or poor air circulation. Use sterile seed-starting mix, water from below, and may support air can move around seedlings. Do not reuse soil from the previous year without sterilizing it.

Seedlings wilt after transplanting: Roots are drying out or the plant is in shock from hardening off too quickly. Water when ready and provide afternoon shade for a few days. If you skipped hardening off, move the plant to a sheltered spot and gradually expose it to more sun over several days.

Frequently Asked Questions

Can I use regular potting soil instead of seed-starting mix?

Regular potting mix is heavier and retains more water than seed-starting mix, which increases the risk of damping-off and root rot. Seed-starting mix is inexpensive and worth the small extra cost. If you must use potting mix, lighten it by mixing in equal parts perlite or coconut coir.

What if I do not have a grow light?

A sunny south-facing window can work if the light is direct and unobstructed for at least 14 hours daily. Rotate the tray every day so seedlings grow straight instead of leaning toward the light. If seedlings become leggy, a grow light will improve results next year — they are inexpensive and reusable.

How long can I keep seedlings indoors before transplanting?

Seedlings can stay indoors for 8 to 10 weeks, but they grow best when transplanted as soon as outdoor conditions allow. Seedlings older than 10 weeks may become root-bound or overgrown, and they are harder to harden off. If your last frost date is still weeks away, move seedlings to larger pots to give roots more room.

Do I need to fertilize seedlings?

Seed-starting mix contains no nutrients, so seedlings need fertilizer once they develop true leaves. Use a diluted liquid fertilizer (half strength) every 1 to 2 weeks, or use a slow-release fertilizer mixed into the soil at planting. Stop fertilizing a week before transplanting outdoors.

Can I save seeds from my tomato plants?

Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes produce seeds that do not grow true to type — the plants will not be identical to the parent. Let a ripe tomato sit at room temperature for a few days, scoop out the seeds and gel into a jar, add a little water, and let it ferment for 2 to 3 days. Rinse, dry on a paper towel, and store in a cool, dry place.