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

Tomato seeds can go into the ground two ways: started indoors in containers under lights, or planted directly into garden soil once the soil temperature reaches 60°F or higher. Indoor starting gives you a head start on the growing season and lets you control conditions, but it requires space and lights. Direct sowing is simpler if you have a short window between your last frost and first fall frost. Most home gardeners in cooler climates start seeds indoors; those in warm climates often direct sow.

The timing matters because tomato seeds germinate best in warm soil—between 70°F and 80°F—and seedlings need 12 to 16 hours of light daily to grow stocky and strong. If you start too early, seedlings become leggy and weak before transplant time. If you start too late, you miss the peak growing season. Check your local frost dates (your county extension office has them) and count backward 6 to 8 weeks to find your start date.

Key Takeaways

  • Indoor seeds need a seed-starting mix (not garden soil), containers with drainage holes, and consistent moisture—not waterlogged soil.
  • Seedlings require 12 to 16 hours of light daily from a grow light or fluorescent fixture placed 2 to 3 inches above the leaves.
  • Direct sowing outdoors works only after soil reaches 60°F and all frost danger has passed, typically 1 to 2 weeks after your last frost date.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.
  • Plant seeds ¼ inch deep indoors or outdoors; germination takes 5 to 10 days in warm soil.

Planting seeds indoors in containers

Fill small containers—seed trays, peat pots, or recycled yogurt cups with drainage holes—with seed-starting mix, not garden soil or potting soil. Seed-starting mix is lighter and fluffier, which lets roots push through easily and prevents damping off, a fungal disease that kills seedlings at the soil line. Moisten the mix before you plant so it is damp but not soggy.

Plant one or two seeds per cell or pot, pushing them ¼ inch into the mix. Cover lightly with more mix, mist with water, and place the containers in a warm spot—a heat mat under the tray speeds germination. You do not need light yet; seeds germinate in darkness. Once sprouts appear (usually 5 to 10 days), remove any cover and move the containers under lights when ready. If you wait, seedlings stretch toward the light and become weak.

Position grow lights or fluorescent shop lights 2 to 3 inches above the seedling leaves and keep them on for 12 to 16 hours daily. As seedlings grow, raise the lights to maintain that distance. Water from below by setting containers in a tray of water and letting them absorb moisture from the bottom, or water gently from above without wetting the leaves. The soil should be moist, not waterlogged.

Direct sowing seeds outdoors in the garden

Wait until soil temperature reaches at least 60°F—ideally 65°F or warmer—and all danger of frost has passed. You can check soil temperature with a soil thermometer pushed 4 inches into the ground in the morning. Plant seeds ¼ inch deep in rows or hills, spacing them 2 to 3 inches apart. Water gently and keep the soil consistently moist until seedlings emerge.

Once seedlings have two true leaves (the second set that appears, which look like tomato leaves rather than the smooth seed leaves), thin them by removing the weaker seedlings so the remaining plants are 12 to 18 inches apart. This gives each plant room to grow without competing for water and nutrients. Direct-sown seedlings are sturdier than indoor-started ones because they are exposed to wind and sun from the start, so they do not need hardening off.

Hardening off indoor seedlings before transplanting

Seedlings started indoors are tender and will wilt or burn if moved straight into full sun and wind. Harden off seedlings over 7 to 10 days by gradually exposing them to outdoor conditions. Start by placing them in a sheltered, shaded spot outdoors for 2 to 3 hours, then bring them back inside. Each day, increase the time outdoors and move them into brighter light.

By day 7 or 8, seedlings should spend the full day outside in dappled shade or morning sun. On the final days, expose them to a few hours of direct sun if possible. Water more often during hardening off because containers dry out faster outdoors. Once seedlings have hardened off and your soil is warm enough, transplant them into the garden at the depth they were growing in their containers—or deeper, burying the stem up to the first true leaves, which encourages a stronger root system.

Soil preparation and spacing in the garden

Tomatoes grow best in soil rich in organic matter with good drainage. Mix compost or aged manure into the top 8 to 10 inches of soil before planting. Space transplants 24 to 36 inches apart depending on the variety—determinate (bush) types need less space, indeterminate (vining) types need more. Space rows 3 to 4 feet apart so you can move between them to water, prune, and harvest.

Plant transplants at the same depth they were in their containers, or deeper. Tomato stems buried underground develop additional roots along the buried portion, creating a stronger root system. Water transplants thoroughly after planting and keep soil consistently moist—about 1 to 2 inches per week from rain or irrigation. Mulch around plants with 2 to 3 inches of straw or shredded leaves to retain moisture and regulate soil temperature.

Common mistakes when planting tomato seeds

Using regular garden soil or potting soil instead of seed-starting mix often leads to poor germination and damping off. Garden soil is too dense and may carry disease; potting soil is too rich in nutrients for tiny seeds. Planting seeds too deep (more than ½ inch) prevents them from reaching the surface. Planting too early indoors creates leggy seedlings that are weak by transplant time, and planting too late means you miss peak growing season.

Inconsistent watering—letting soil dry out or keeping it waterlogged—stresses seedlings and invites disease. Insufficient light causes seedlings to stretch and weaken. Skipping the hardening-off process causes transplant shock and leaf burn. Planting outdoors before soil is warm enough slows germination and can cause seeds to rot. Burying transplants too shallow leaves roots exposed and vulnerable to drying out.

Frequently Asked Questions

Can I use regular potting soil for tomato seeds?

Regular potting soil works but is not ideal. Seed-starting mix is lighter and fluffier, which helps tiny roots push through and reduces damping off. If you only have potting soil, mix it half and half with perlite or coconut coir to lighten it.

How long do tomato seeds take to sprout?

Tomato seeds germinate in 5 to 10 days in warm soil (70°F to 80°F). Cooler soil slows germination; soil below 60°F may cause seeds to rot instead of sprout. A heat mat speeds the process.

What if my seedlings are tall and spindly?

Spindly seedlings mean insufficient light or lights placed too far away. Move lights closer (2 to 3 inches above leaves) and run them 14 to 16 hours daily. You can also bury the leggy stem deeper when transplanting to encourage root growth along the buried portion.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination but is not required. Seeds will sprout in a warm room (70°F or warmer) without one, just more slowly. A mat is helpful if your home is cool or if you want seeds to sprout in 5 days instead of 10.

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

Yes, if your growing season is long enough and soil is warm. Direct sowing works best in warm climates or if you plant after soil reaches 65°F. In cooler climates, indoor starting gives you a longer harvest window because seedlings mature faster than direct-sown seeds.