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

Tomato seeds need warmth, moisture, and light to sprout, and they grow fastest when started indoors under controlled conditions rather than directly in the garden. You will sow seeds in a seed-starting mix (not garden soil), keep them at 70 to 80 degrees Fahrenheit, and move them under lights once they sprout. Most gardeners start seeds in late winter or early spring so seedlings are ready to transplant outside when soil temperatures reach 60 degrees and frost danger has passed.

The timeline matters because tomato seedlings take 6 to 8 weeks to reach transplant size — roughly 6 to 8 inches tall with their first true leaves. If you start too early, seedlings become leggy and weak while waiting for warm soil. If you start too late, you will have small plants when the planting window opens, which delays your harvest.

Key Takeaways

  • Sow seeds in a sterile seed-starting mix indoors, keep soil moist but not waterlogged, and maintain temperatures between 70 and 80 degrees Fahrenheit for germination.
  • Move seedlings under grow lights or in a bright south-facing window as soon as the first leaves appear, and keep lights 2 to 3 inches above the plants.
  • Water seedlings from below by setting pots in a tray of water rather than overhead, which prevents damping-off disease and keeps stems strong.
  • Thin seedlings to one plant per cell or pot once they have their first true leaves, and fertilize weekly with diluted liquid fertilizer once they develop their second set of leaves.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.

Gather seeds, containers, and soil before you start

You will need tomato seeds (any variety), small containers with drainage holes (seed trays, peat pots, or recycled yogurt cups work), a seed-starting mix (a light, sterile blend of peat moss or coco coir and perlite — not potting soil or garden soil), and a heat source if your home is cooler than 70 degrees. A seedling heat mat designed for seed starting costs $20 to $40 and maintains consistent warmth under trays.

Fill containers with moistened seed-starting mix, pressing it gently so it holds together but is not compacted. The mix should feel like a wrung-out sponge — damp but not soggy. Tomato seeds are small, so sow 2 to 3 seeds per cell or pot, pressing them lightly into the soil surface. Cover seeds with a thin layer of mix (about 1/8 inch) or leave them uncovered; tomato seeds germinate in either case, though a light covering helps retain moisture.

Keep soil warm and moist until seedlings emerge

Place containers in a warm location — ideally 70 to 80 degrees Fahrenheit — and cover them with a humidity dome or plastic wrap to hold moisture. Check soil daily; it should stay moist but never waterlogged. Soggy soil causes seeds to rot before they sprout. Tomato seeds typically germinate in 5 to 10 days at the right temperature; germination slows dramatically below 70 degrees and may not happen at all below 60 degrees.

Once you see the first green shoots, remove the humidity dome or plastic and move containers when ready to bright light. Seedlings that emerge in darkness become pale and stretched as they search for light, a condition called etiolation. This damage is hard to reverse, so timing the move to light is critical.

Provide bright light and keep seedlings from stretching

Seedlings need 14 to 16 hours of bright light per day. A south-facing window works if it receives direct sun most of the day, but grow lights (LED or fluorescent) are more reliable and prevent the weak, pale growth that happens in dim light. Position lights 2 to 3 inches above the seedling tops and raise them as plants grow. If seedlings start to lean toward a window, rotate trays daily so they grow straight.

Keep the room temperature between 65 and 75 degrees during the day and slightly cooler at night. Cooler nights (around 60 degrees) encourage stockier, stronger stems. If your seedlings are tall and spindly despite good light, they are either too warm, too crowded, or not getting enough light — move lights closer or add more hours of light per day.

Water from below and thin seedlings to one per pot

Water seedlings by setting pots in a tray of water and letting them absorb moisture from the bottom for 10 to 15 minutes, then removing them to drain. This method keeps foliage dry and prevents damping-off, a fungal disease that kills seedlings at the soil line. Overhead watering with a spray bottle or can wets leaves and creates conditions damping-off thrives in.

Once seedlings have their first true leaves (the second set of leaves that appear, which look like actual tomato leaves rather than the smooth seed leaves), thin them to one plant per cell or pot. Snip extra seedlings at soil level with scissors rather than pulling them out, which disturbs roots of the remaining plant. Crowded seedlings compete for light and nutrients and produce weak transplants.

Feed seedlings weekly once they develop true leaves

Seed-starting mix contains almost no nutrients, so seedlings need fertilizer once they use up what is stored in the seed itself. Start feeding when the first true leaves appear, using a diluted liquid fertilizer mixed to half strength. Feed once a week by adding fertilizer to the water tray, or follow the product label if you prefer overhead process.

Tomato seedlings do not need heavy feeding — too much nitrogen produces soft, weak growth. A balanced fertilizer (equal parts nitrogen, phosphorus, and potassium) or a formula slightly higher in phosphorus and potassium supports sturdy stems and root development. Stop feeding about a week before transplanting outdoors.

Harden off seedlings before moving them outside

Seedlings grown indoors are tender and will wilt or die 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 in a sheltered, shaded spot outdoors for 2 to 3 hours, then bring them back inside. Each day, increase outdoor time and gradually move them into brighter light and more wind exposure.

By day 7 to 10, seedlings should spend the full day and night outdoors in dappled shade or morning sun. Once they have adjusted, they are ready to transplant into the garden. Harden off seedlings only when nighttime temperatures stay above 50 degrees; cold soil and air will stunt growth even if seedlings look tough.

Transplant seedlings into the garden when soil is warm

Tomato seedlings are ready to plant in the garden when they are 6 to 8 inches tall, have several sets of true leaves, and soil temperature is at least 60 degrees (ideally 65 to 70 degrees). Dig a hole slightly deeper than the seedling's root ball and bury the stem up to the lowest leaves; tomatoes will root along the buried stem and develop a stronger root system. Space plants 24 to 36 inches apart depending on variety.

Water transplants thoroughly after planting and keep soil consistently moist for the first week as roots establish. Mulch around plants with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature. Once plants are growing steadily, reduce watering frequency but water deeply when you do water, encouraging deep root growth.

Frequently Asked Questions

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

Yes, but you will harvest later. Direct seeding works in warm climates where soil reaches 70 degrees by late spring, but in most regions the growing season is too short. Starting indoors gives you a 6 to 8 week head start, which means fruit ripens before fall frost arrives.

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

Damping-off appears as a thin, dark, water-soaked spot at the base of the seedling stem, causing it to collapse. Prevent it by watering from below, ensuring good air circulation with a small fan, and using sterile seed-starting mix. Remove any seedling that shows signs of damping-off when ready to protect others.

Why are my seedlings tall and pale even under lights?

Lights are too far away, seedlings are too warm, or they are not getting enough hours of light per day. Move lights to 2 to 3 inches above the tops, lower room temperature to 65 to 70 degrees, and increase light duration to 14 to 16 hours daily. Pale color usually improves within a few days of better light.

Can I reuse soil from last year's seedlings?

No. Old seed-starting mix may harbor damping-off fungi and other pathogens. Use fresh, sterile seed-starting mix each year. You can reuse potting soil for outdoor containers or amend it into garden beds, but not for starting new seeds.

How long can seedlings stay in their starter pots before transplanting?

Seedlings can stay in starter pots for 2 to 3 weeks after reaching transplant size if necessary, but they perform better when moved to the garden promptly. If you must wait, move them to larger pots with potting soil to prevent root-bound stress, and continue feeding weekly.