Start 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 germinate best in warm soil between 70°F and 80°F, which is why most home growers start them indoors under lights rather than waiting for outdoor soil to warm. You can start seeds in small containers filled with seed-starting mix, keep them moist and warm, and move seedlings to larger pots as they grow. If you live in a warm climate or prefer to skip the indoor step, you can plant seeds directly into garden soil once nighttime temperatures stay above 50°F, though they will fruit later than transplants.

The indoor method gives you stronger plants, lets you control the growing environment, and means you can harden off seedlings before transplanting—a process that toughens them for outdoor conditions. Direct sowing is simpler and requires no equipment, but it works best in regions with long, warm growing seasons.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last frost date using seed-starting mix, a warm location, and grow lights or a sunny window.
  • Keep soil moist but not waterlogged, and maintain temperatures between 70°F and 80°F for germination in 5 to 10 days.
  • Transplant seedlings to larger containers once they develop their first true leaves, and move them under lights to prevent leggy, weak growth.
  • Harden off seedlings by placing them outdoors in shade for a few hours, gradually increasing sun and time over 7 to 10 days before planting in the garden.
  • For direct sowing outdoors, plant seeds ½ inch deep in warm soil after all frost danger has passed, and thin seedlings to 24 inches apart once they have two true leaves.

Gather your supplies before you start

For indoor seed starting, you will need seed-starting mix (a sterile, lightweight blend designed for seeds—not garden soil), small containers with drainage holes, a heat mat or warm location, and grow lights or a very sunny south-facing window. Seed-starting mix is crucial because garden soil compacts, holds too much moisture, and can introduce fungi that kill young seedlings. Small yogurt cups, cell trays, or peat pots all work; just drill or poke drainage holes in the bottom if they do not have them.

A heat mat keeps soil at the ideal temperature without relying on your home's warmth, which often fluctuates. If you do not have one, place containers on top of the refrigerator or near (not touching) a heat source. Grow lights should be positioned 2 to 3 inches above seedlings and run for 14 to 16 hours daily; a straightforward shop light with LED or fluorescent bulbs works well. For direct outdoor sowing, you need only seeds, a garden bed or containers, and a way to keep soil moist until germination.

Fill containers and sow seeds indoors

Fill your containers with moistened seed-starting mix—wet it thoroughly before filling so the mix absorbs water evenly. Plant one or two tomato seeds per cell or small pot, pressing them ½ inch into the mix. Tomato seeds are small, so handle them gently and do not bury them deeper than ½ inch or they may not germinate. Cover the container loosely with a humidity dome or plastic wrap to trap moisture, then place it on your heat mat or warm location.

Check the soil daily and mist it if the surface begins to dry. Do not let it become waterlogged—soggy soil causes damping off, a fungal disease that kills seedlings at the soil line. Seeds should sprout in 5 to 10 days at 70°F to 80°F. Once you see green shoots, remove the cover and move containers under grow lights when ready. If you wait, seedlings will stretch toward the window and become leggy and weak.

Care for seedlings until they are ready to transplant

Keep grow lights 2 to 3 inches above the tops of seedlings, raising them as plants grow. Run lights for 14 to 16 hours daily and turn them off for 8 to 10 hours so seedlings can rest. Water from below by setting containers in a tray of water for 10 to 15 minutes, then removing them to drain—this prevents fungal issues and keeps foliage dry. Keep soil consistently moist but never soggy.

Once seedlings develop their first true leaves (the second set of leaves that appear, which look like tiny tomato leaves), thin them by removing the weaker seedling if you planted two per cell. When seedlings are 2 to 3 inches tall with at least two sets of true leaves, transplant them into larger containers—4-inch pots work well. Bury them deeper than they were growing, up to their first true leaves; tomatoes will root along the buried stem and develop a stronger root system. Continue under lights, watering when the top of the soil feels dry to the touch.

Harden off seedlings before moving them outdoors

About 7 to 10 days 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, shady spot outdoors for 2 to 3 hours, then bring them back inside. Each day, increase the time outside and gradually move them into dappled shade, then partial sun. By day 7 or 8, they should tolerate several hours of direct morning sun.

Watch the weather forecast during hardening off. If frost threatens, bring seedlings indoors when ready. Once nighttime temperatures stay above 50°F and seedlings have been in full sun for at least a day, they are ready to plant in the garden.

Plant seedlings or direct-sown seeds in the garden

Choose a location with at least 6 to 8 hours of direct sun daily. Dig a hole slightly larger than the root ball of your seedling and bury it up to the first true leaves, just as you did when transplanting to larger pots. Space plants 24 to 36 inches apart to allow air circulation and reduce disease. Water gently after planting and keep soil consistently moist for the first two weeks while roots establish.

For direct sowing outdoors, wait until soil temperature reaches 60°F (a soil thermometer takes the guesswork out). Plant seeds ½ inch deep in groups of 3 to 4 seeds spaced 2 inches apart. Keep soil moist until seedlings emerge in 7 to 14 days. Once they develop two true leaves, thin them to one plant every 24 inches by snipping off extras at soil level rather than pulling them, which can disturb the roots of the keeper plant.

Avoid common mistakes at each stage

The most common error is planting seeds too deep or in regular garden soil instead of seed-starting mix. Seeds buried deeper than ½ inch often fail to germinate, and garden soil compacts and stays too wet. Another frequent mistake is moving seedlings under lights too late—if they stretch toward a window for more than a day or two, they become permanently weak and leggy, even after lights are added.

Overwatering kills more seedlings than underwatering. Soil should be moist, not soggy; if it stays wet, fungi thrive. Skipping the hardening-off step causes transplant shock and wilting when seedlings move outdoors. Finally, planting seedlings outdoors before frost danger has truly passed kills them—check your local frost date and wait until nighttime temperatures stay above 50°F consistently.

Frequently Asked Questions

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

Regular potting soil is too heavy and holds too much moisture for seeds. Seed-starting mix is lighter, drains better, and is sterile, which prevents damping off. If you must use potting soil, mix it 50/50 with perlite or coconut coir to improve drainage, though seed-starting mix is inexpensive and worth buying.

What if my seedlings are tall and spindly?

Leggy seedlings mean they are not getting enough light. Move grow lights closer (2 to 3 inches above the tops) and run them 14 to 16 hours daily. If you are using a window, it likely does not provide enough light—add a grow light. You can also bury leggy seedlings deeper when transplanting to larger pots; they will root along the buried stem and recover somewhat.

How do I know when soil is warm enough to direct sow?

Use a soil thermometer pushed 2 inches into the garden bed. Tomato seeds need soil at least 60°F to germinate reliably, though 70°F or warmer is ideal. Soil warms fastest in raised beds and in dark mulch. If you plant too early in cold soil, seeds may rot before sprouting.

Do I need grow lights if I have a sunny window?

A south-facing window can work if it receives at least 12 to 14 hours of direct sun daily, but most home windows do not provide enough light. Seedlings grown in windows alone often become leggy. Grow lights are inexpensive and reliable, and they let you start seeds in any room regardless of natural light.

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

Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes produce seeds that will not grow true to type—the plants may look and taste different from the parent. Heirloom seeds stay true. Scoop seeds and pulp into a jar, ferment them for 2 to 3 days to remove the gel coating, rinse, dry completely on a plate, and store in a cool, dry place.