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

Tomato seeds need warmth and moisture to sprout, but not soil that stays waterlogged. The fastest route is starting seeds indoors under lights 6 to 8 weeks before your last frost date — this gives seedlings time to grow strong before transplanting. If you live in a warm climate or prefer to skip the indoor step, you can plant seeds directly into garden soil once the soil temperature reaches 60°F, though outdoor seeds take longer to produce fruit.

The choice between indoor and outdoor planting depends on your climate and how soon you want to harvest. Indoor-started seedlings are ready to move outside as small plants, which means fruit arrives earlier. Direct sowing is simpler but works best in regions with a long growing season.

Key Takeaways

  • Indoor seeds need a seed-starting mix, small containers with drainage holes, and consistent moisture — not a sunny windowsill, which produces weak, leggy seedlings.
  • Seeds sprout fastest in soil kept between 70°F and 80°F, which usually requires a heat mat or warm shelf, not just room temperature.
  • Once seedlings develop their first true leaves (the second set), thin them to one plant per cell or pot to prevent crowding and disease.
  • Harden off seedlings by moving them outdoors for a few hours each day for one week before planting them in the garden, so they adjust to wind and direct sun.
  • Direct-sown seeds in the garden need consistent moisture for the first two weeks and should be planted half an inch deep in warm soil.

What you need to start seeds indoors

Seed-starting mix is lighter and drains better than garden soil, which matters because tomato seeds rot in soggy conditions. Use a commercial seed-starting mix (brands like Miracle-Gro Seed Starting Mix or Burpee Seed Starting Mix work) or make your own by mixing equal parts peat moss or coco coir, perlite, and vermiculite. Fill small containers — seed trays, cell packs, or even recycled yogurt cups with drainage holes drilled in the bottom — to about three-quarters full.

You will also need a heat source. A seedling heat mat plugged into a thermostat keeps soil at 70°F to 80°F, the range where tomato seeds sprout fastest. Without one, seeds germinate much more slowly at room temperature. A heat mat costs $20 to $40 and lasts for years. If you do not have one, place seed trays on top of a refrigerator or in a warm closet, though germination will take longer.

Grow lights are essential once seedlings sprout. A sunny windowsill does not provide enough light — seedlings stretch toward the window and become tall, thin, and weak (called "leggy"). A straightforward two-bulb fluorescent shop light ($30 to $50) hung 2 to 3 inches above seedlings works well. Keep lights on for 14 to 16 hours per day.

Planting and germinating seeds indoors

Fill containers with moistened seed-starting mix and plant one tomato seed per cell, pressing it about one-quarter inch into the soil. Tomato seeds are small, so handle them carefully. Cover the tray with a humidity dome or plastic wrap to keep moisture in while seeds germinate.

Place the tray on the heat mat and check soil moisture every day — it should feel like a wrung-out sponge, not dry and not soggy. Seeds usually sprout in 5 to 10 days at 70°F to 80°F. Once you see green shoots, remove the humidity dome and move the tray under grow lights when ready. Lower the temperature to 65°F to 70°F at night if possible, which produces sturdier plants.

Water from below by setting containers in a tray of water for 10 to 15 minutes, then letting them drain. This method keeps the soil moist without splashing water on leaves, which can cause fungal disease. Water whenever the top of the soil feels dry to the touch.

Thinning and transplanting seedlings

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 extra seedlings at soil level with scissors rather than pulling them out, which disturbs roots of the keeper plant. If you planted multiple seeds per cell, you now have only one strong plant per container.

Once seedlings have two sets of true leaves and are 2 to 3 inches tall, you can transplant them into larger containers (4-inch pots work well) filled with potting soil. Bury the stem up to the lowest leaves — tomatoes will root along the buried stem, creating a stronger root system. Keep lights 2 to 3 inches above the tops of plants as they grow.

Fertilize seedlings every two weeks with a diluted liquid fertilizer (half-strength is fine) once they have their first true leaves. Seed-starting mix contains few nutrients, so feeding prevents stunted growth.

Hardening off before transplanting outdoors

About one week before your last frost date, begin hardening off seedlings — gradually exposing them to outdoor conditions so they adjust to wind, direct sun, and temperature swings. On day one, place seedlings in a sheltered, shaded spot outdoors for one to two hours. Each day, increase time outside and gradually move them into more sun.

By day seven, seedlings should spend the full day and night outdoors if temperatures stay above 50°F. If frost threatens, bring them back inside. This process toughens stems and leaves so transplants do not wilt or sunburn when planted in the garden.

Planting seedlings in the garden

Wait until after your last frost date and soil has warmed to at least 60°F before planting seedlings outdoors. Dig a hole slightly deeper than the root ball and bury the stem up to the lowest leaves, just as you did when transplanting to larger pots. Tomatoes root along the buried stem, so deeper planting creates a stronger plant.

Space plants 24 to 36 inches apart depending on the variety — check the seed packet for the specific spacing. Water deeply after planting and keep soil consistently moist for the first two weeks while roots establish. Mulch around the base with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature.

Direct sowing seeds outdoors

If you prefer to skip indoor starting, plant seeds directly in the garden once soil reaches 60°F and all frost danger has passed. Prepare the planting area by mixing compost or aged manure into the top few inches of soil. Plant seeds half an inch deep in clusters of three to four seeds, spacing clusters 24 to 36 inches apart.

Keep soil consistently moist but not waterlogged for the first two weeks while seeds germinate and seedlings emerge. Once seedlings have two true leaves, thin them to one plant per cluster by snipping extras at soil level. Direct-sown tomatoes usually flower and fruit 2 to 3 weeks later than indoor-started seedlings, so this method works best in regions with long, warm growing seasons.

Common problems and how to avoid them

Leggy seedlings — tall, thin plants with large gaps between leaves — result from insufficient light. Move lights closer to seedlings or run them for 16 hours per day. Damping off, a fungal disease that kills seedlings at soil level, happens when soil stays too wet or air does not circulate. Use sterile seed-starting mix, water from below, and run a small fan near seedlings to improve air movement.

Slow germination means soil is too cold. Check that your heat mat is plugged in and set correctly — soil should feel warm to the touch. Wilting after transplanting usually means seedlings were not hardened off enough or planted too shallow. may support the buried stem is in contact with soil and water deeply after planting.

Frequently Asked Questions

Can I use regular garden soil to start tomato seeds?

No. Garden soil compacts when wet, suffocates seeds, and often contains fungi that cause damping off. Seed-starting mix is loose and sterile, which is why it works. You can use garden soil once seedlings are transplanted into larger containers or the garden.

How long do tomato seedlings need grow lights?

Keep lights on for 14 to 16 hours per day from the moment seedlings sprout until they are transplanted outdoors. Once in the garden, they need only natural sunlight — at least six to eight hours per day, though more is better.

What if I miss my last frost date and seedlings are still indoors?

Transplant them into larger pots and keep them under lights indoors until frost danger passes. They will grow taller but remain healthy. Once outdoor temperatures stay above 50°F at night, begin hardening off and plant them outside.

Do I need to pollinate tomato flowers indoors?

No. Tomato flowers are self-pollinating, and the air movement from a small fan is usually enough. If you want to be thorough, gently shake flower clusters with your finger when they open, which mimics wind pollination.

Can I save seeds from store-bought tomatoes to plant next year?

Only if the tomato is an heirloom or open-pollinated variety — the seed packet will say. Seeds from hybrid tomatoes (marked F1 on the packet) will not produce plants identical to the parent, so they are not worth saving. Heirloom seeds stay true to type and can be saved, dried, and stored for years.