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

Tomato seeds germinate fastest in warm soil — between 70 and 80 degrees Fahrenheit. If you live where frost arrives late, start seeds indoors under lights in late winter so seedlings are ready to transplant when outdoor soil reaches 60 degrees. If your growing season is long and soil warms early, you can sow seeds directly into garden beds or containers after the last frost date has passed and soil temperature stays above 60 degrees.

The choice between indoor and outdoor sowing depends on your climate and how much control you want over seedling strength. Indoor-started seedlings are sturdier and flower earlier, but require lights and space. Direct sowing is simpler but gives you less time to harden off plants before they face full sun and wind.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last frost date under grow lights or on a sunny windowsill, keeping soil between 70 and 80 degrees Fahrenheit.
  • Use seed-starting mix (not garden soil) in small containers with drainage holes, and keep the mix moist but not waterlogged until seedlings emerge in 5 to 10 days.
  • Move seedlings under lights as soon as they sprout, positioning lights 2 to 3 inches above the tops of plants and running them 14 to 16 hours daily.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into garden beds or larger containers.
  • Direct sow seeds outdoors after your last frost date in warm soil, planting them one-quarter inch deep and thinning seedlings to 24 inches apart once they have two true leaves.

Indoor seed starting: containers, soil, and moisture

Fill small containers — seed trays, peat pots, or recycled yogurt cups with drainage holes drilled in the bottom — with seed-starting mix, not garden soil. Seed-starting mix is lighter and holds moisture more evenly than garden soil, which can compact and suffocate seeds. Moisten the mix before planting so it is damp throughout but not soggy.

Plant one or two seeds per cell or pot, pressing them into the soil surface and covering them with one-quarter inch of mix. Tomato seeds are small and do not need to be buried deep. Cover the containers with a humidity dome or plastic wrap to hold moisture while seeds germinate. Once seedlings emerge — usually in 5 to 10 days — remove the cover when ready so air can circulate and prevent mold.

Keep the soil moist but not waterlogged. Water from below by setting containers in a tray of water and letting them absorb moisture from the bottom, or water gently from above with a spray bottle. Overwatering is the most common cause of damping-off disease, which kills seedlings at the soil line.

Lighting and temperature for strong indoor seedlings

Move seedlings under lights as soon as the first green shoots appear. A sunny south-facing windowsill works if you have one, but grow lights — fluorescent shop lights or LED panels — give more consistent results. Position lights 2 to 3 inches above the tops of seedlings and raise them as plants grow. Run lights for 14 to 16 hours daily, using a timer to keep the schedule consistent.

Keep the growing area between 65 and 75 degrees Fahrenheit during the day. At night, temperatures can drop to 55 to 60 degrees, which actually strengthens seedlings. If your seedlings are tall and spindly, they are not getting enough light — move lights closer or run them longer.

Once seedlings develop their first true leaves (the second set of leaves that appear, which look like actual tomato leaves), thin them so only the strongest seedling remains in each pot. Pinch off the weaker one at soil level rather than pulling it out, which can damage roots of the survivor.

Fertilizing and transplanting indoor seedlings

Seed-starting mix contains no nutrients, so seedlings need feeding once they develop true leaves. Mix a diluted liquid fertilizer — half the strength recommended on the bottle — and feed every 7 to 10 days. Stop fertilizing one week before transplanting outdoors.

When seedlings are 3 to 4 inches tall and have at least two sets of true leaves, transplant them into larger containers — 4-inch pots or cell packs — filled with potting soil. Bury the stem deeper than it was in the seed tray; tomatoes will root along the buried stem and develop a stronger root system. Water gently after transplanting and keep soil consistently moist for the first week.

Hardening off: preparing indoor seedlings for outdoor conditions

Indoor seedlings are tender and will wilt or burn if moved directly into full sun and wind. Harden off plants over 7 to 10 days by gradually exposing them to outdoor conditions. Start by placing seedlings in a sheltered spot outdoors for 1 to 2 hours on a calm day, then bring them back inside. Each day, increase the time outdoors and the amount of direct sun they receive.

By day 7 or 8, seedlings should spend the full day and night outdoors in a protected location. On the final day or two before transplanting, place them in full sun for several hours. If frost threatens during hardening off, bring seedlings back inside when ready.

Direct sowing seeds outdoors

If you prefer to sow seeds directly into the garden, wait until soil temperature reaches 60 degrees Fahrenheit and all danger of frost has passed. Plant seeds one-quarter inch deep in rows or clusters, spacing them 2 to 3 inches apart. Water gently so soil stays moist until seedlings emerge.

Once seedlings have two true leaves, thin them to 24 inches apart by pinching off the weaker plants. Direct-sown seedlings will flower and fruit later than indoor-started ones, but they are hardier and require no transplanting shock. In short growing seasons, direct sowing may not give you a harvest before frost arrives, so indoor starting is often the better choice.

Transplanting seedlings into the garden or containers

Harden-off indoor seedlings are ready to transplant when they are 6 to 8 inches tall, have a thick stem, and have been exposed to outdoor conditions for at least a week. Dig a hole slightly deeper than the root ball and bury the stem up to the first true leaves; tomatoes develop roots along the buried stem. Space plants 24 to 36 inches apart depending on the variety — determinate (bush) types need less space than indeterminate (vining) types.

Water thoroughly after transplanting and keep soil consistently moist for the first two weeks while roots establish. Mulch around the base of plants with 2 to 3 inches of straw or shredded leaves to hold moisture and regulate soil temperature. Avoid mulching directly against the stem, which can trap moisture and encourage rot.

Common problems and how to avoid them

Seedlings that are tall and pale are not getting enough light — move lights closer or run them longer. Seedlings that wilt suddenly despite moist soil likely have damping-off disease, caused by overwatering or poor air circulation. Remove affected seedlings and improve air flow by running a small fan on low speed near the growing area.

If seedlings develop yellow lower leaves after transplanting, they may be nitrogen-hungry. Feed with diluted liquid fertilizer every 7 to 10 days until plants are established. If seedlings stop growing after hardening off, they may be too cold — soil below 60 degrees slows root growth. Wait a few more days for soil to warm before transplanting.

Frequently Asked Questions

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

Garden soil compacts easily and holds too much moisture, which causes seeds to rot and seedlings to develop damping-off disease. Seed-starting mix is light, drains well, and costs only a few dollars per bag. It is worth the small investment for much higher germination rates.

What if I do not have grow lights?

A sunny south-facing windowsill can work if you rotate seedlings daily so they grow straight rather than leaning toward the light. Seedlings will be slower to grow and may be leggy, but they will survive. Grow lights are inexpensive — basic fluorescent shop lights cost $20 to $40 — and give better results if you plan to start seeds every year.

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

Soil thermometers cost $5 to $10 at garden centers and take the guesswork out. Soil should be at least 60 degrees Fahrenheit, though 70 degrees or warmer gives faster germination. If you do not have a thermometer, wait until nighttime temperatures stay above 50 degrees for a week — soil will be warm enough by then.

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

Yes, but only from heirloom and 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 saved from healthy plants and stored in a cool, dry place will germinate the following year.

What should I do if seedlings get too tall before transplanting time?

Bury the stem deeper when transplanting — tomatoes root along the buried stem and will develop a stronger root system. You can also pinch off the top growing tip to encourage branching, though this delays flowering by a week or two. Improve light intensity and air circulation to prevent legginess in future years.