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

Growing tomatoes from seed means starting them indoors in late winter or early spring, then moving them outside once the soil warms up. You do not need special equipment — a sunny windowsill, seed-starting soil, and small containers with drainage holes are enough. The goal is to have sturdy seedlings ready to transplant into the garden right around the time frost danger passes in your area.

The timing matters because tomato seeds germinate best in warm soil (around 70°F) and seedlings need 6 to 8 weeks of growth before they are strong enough to survive outdoors. Starting too early means they get leggy and weak waiting for warm weather. Starting too late means you will not have mature plants ready to flower when the season peaks.

Key Takeaways

  • Fill small containers with seed-starting soil, plant one seed per cell about a quarter-inch deep, and keep the soil moist but not waterlogged until sprouts appear.
  • Seedlings need 12 to 16 hours of bright light daily — a sunny south-facing window or grow light works, but a dark shelf will produce weak, stretched plants.
  • Once seedlings have two sets of true leaves, thin them to one plant per cell 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.
  • Plant seedlings deeper than they grew in their containers — tomatoes can root along buried stems and develop stronger root systems.

Fill containers with seed-starting soil and plant one seed per cell

Use seed-starting soil, not garden soil or potting mix. Seed-starting soil is lighter and fluffier, which lets roots push through easily and reduces the risk of damping off (a fungal disease that kills seedlings). Fill small containers — yogurt cups, cell packs, or seed trays — to about half an inch from the rim.

Water the soil until it is evenly moist, then poke a hole about a quarter-inch deep with your finger or a pencil. Drop one seed into each hole and cover it lightly. Tomato seeds are small but do not need to be buried deep. Label each container with the variety name and the date you planted it, because seedlings all look the same for the first few weeks.

Place the containers in a warm spot — on top of a refrigerator, a heat mat, or a sunny windowsill works. Tomato seeds germinate fastest at around 70°F. Once sprouts appear (usually 5 to 10 days), move the containers to bright light when ready, or the seedlings will stretch and weaken trying to reach the light source.

Provide 12 to 16 hours of bright light daily

Seedlings need strong, direct light as soon as they sprout. A south-facing windowsill in spring can work if it gets at least 6 hours of direct sun, but most indoor windows do not provide enough light. Seedlings grown in dim light become leggy — tall, thin, and weak — because they stretch toward whatever light is available.

A grow light or shop light with LED or fluorescent bulbs is more reliable. Position the light 2 to 3 inches above the seedlings and raise it as they grow. Leave the light on for 12 to 16 hours daily. You can use a straightforward timer to turn it on and off automatically, which removes the guesswork.

Water the seedlings when the top of the soil feels dry to the touch. Overwatering is the most common mistake — soggy soil invites damping off and root rot. The soil should be moist, not wet. Water from below by setting containers in a tray of water and letting them soak it up, or water gently at the soil line and avoid wetting the leaves.

Thin seedlings to one plant per cell once they have true leaves

After the first leaves appear, wait a few days for the true leaves to emerge — these are the second set of leaves and they look like actual tomato leaves, not the smooth oval seed leaves. Once you see true leaves, thin the seedlings so only one plant grows per cell.

If you planted multiple seeds per cell, pinch off the weaker seedlings at soil level with your fingernails or small scissors. Do not pull them out, because pulling disturbs the roots of the plant you want to keep. If you planted one seed per cell and it sprouted, you are done with this step.

If your seedlings are crowded or growing slowly, transplant them into larger containers — 4-inch pots or cell packs with more space. Bury them deeper than they were growing before. Tomatoes can develop roots along any buried stem, so burying them up to their first true leaves creates a stronger root system.

Fertilize seedlings every 2 weeks with diluted liquid fertilizer

Seed-starting soil contains very few nutrients, so seedlings need feeding once they develop true leaves. Use a balanced liquid fertilizer diluted to half strength — something labeled 10-10-10 or similar. Feed every 2 weeks by mixing the fertilizer into your watering can and watering as usual.

Stop fertilizing about a week before you move seedlings outdoors. This helps them adjust to outdoor conditions without a surge of soft new growth that sunburn easily. If seedlings look pale or yellow, they may need feeding sooner, but overfeeding produces weak, leggy plants just as easily as underfeeding.

Harden off seedlings before moving them to the garden

Hardening off means gradually exposing seedlings to outdoor sun, wind, and temperature swings over 7 to 10 days. Seedlings grown indoors are tender and will wilt or sunburn if you move them straight into full sun. Start by placing them in a shaded, sheltered spot outdoors for an hour or two, then bring them back inside.

Each day, increase their outdoor time and move them into brighter light. By day 7 or 8, they should spend the full day outside in dappled sun or morning sun. If nighttime temperatures drop below 50°F, bring them inside at night. Once nighttime lows stay above 50°F consistently, they can stay out overnight.

Watch the weather forecast before you start hardening off. A sudden cold snap or heavy rain can set seedlings back, so wait for a stretch of mild weather if possible.

Plant seedlings in the garden when soil is warm and frost danger has passed

Plant seedlings outdoors once nighttime temperatures stay above 50°F and your last spring frost date has passed. Check your local frost date by searching your zip code and "last frost date" — this date varies by region and can shift year to year.

Choose a spot with at least 6 to 8 hours of direct sun daily. Dig a hole slightly deeper than the seedling's root ball and bury the stem up to the first true leaves or even higher — tomatoes develop roots along buried stems and will grow a stronger root system. Space plants 24 to 36 inches apart depending on the variety.

Water the seedling in well and keep the soil consistently moist for the first week while it establishes. Mulch around the base with 2 to 3 inches of straw or shredded leaves to retain moisture and regulate soil temperature.

Frequently Asked Questions

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

Yes, but only if your growing season is long enough. Direct seeding works in warm climates where the last frost date is in March or earlier, because seeds need warm soil to germinate and seedlings need time to grow before cold returns. In most of the United States, starting indoors gives you a head start and ensures mature plants ready to flower when summer heat arrives.

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

Damping off is a fungal disease that causes seedlings to collapse at the soil line and die. Prevent it by using fresh seed-starting soil, not reused soil; watering from below instead of overhead; ensuring good air circulation with a small fan; and avoiding overwatering. If seedlings start to collapse, remove them when ready and let the remaining soil dry out slightly.

Why are my seedlings tall and spindly?

Seedlings stretch toward light when they do not get enough brightness. Move them closer to a grow light, increase the hours of light to 14 to 16 per day, or place them in a sunnier window. If they are already leggy, you can bury them deeper when you transplant them into larger containers — the buried stem will develop roots and create a sturdier plant.

How often should I water seedlings?

Water when the top of the soil feels dry to the touch, usually every 2 to 3 days depending on temperature, humidity, and container size. Seedlings in small cells dry out faster than those in larger pots. The soil should be moist but never waterlogged — if water sits on the surface, you are watering too much.

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

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 the same as the parent plant. For heirloom varieties, let a ripe tomato fully mature on the vine, scoop out the seeds and pulp into a jar, let them ferment for 2 to 3 days, rinse them, and dry them completely on a plate before storing in a cool, dry place.