Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow outdoors after soil reaches 60°F

The timing depends on whether you start seeds indoors under lights or plant them straight into the garden. Indoor seed starting gives you a head start—seedlings grow under controlled conditions for 6 to 8 weeks, then move outside after the last frost date passes. Direct sowing means planting seeds straight into warm garden soil, which works only in warm climates or late in the season.

Your last spring frost date is the key number. Find it by searching "[your city] last frost date"—your local cooperative extension office publishes this for every county. Count backward 6 to 8 weeks from that date to know when to start seeds indoors. If your last frost is May 15, start seeds indoors around March 20 to April 1. If you live where frost never comes, you can start seeds indoors anytime and transplant when seedlings are 6 to 8 weeks old.

Soil temperature matters more than air temperature for direct sowing. Tomato seeds germinate slowly below 60°F and rot in cold, wet soil. Use a soil thermometer to check garden soil depth of 4 inches—wait until it reads 60°F or higher, ideally 65°F to 70°F. This usually happens 1 to 2 weeks after your last frost date, depending on your region.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find from your local cooperative extension office.
  • Direct sow seeds outdoors only when soil temperature reaches 60°F or higher, checked 4 inches deep with a soil thermometer.
  • Indoor-started seedlings need 12 to 16 hours of light daily and should be hardened off (gradually exposed to outdoor conditions) for 7 to 10 days before transplanting.
  • Seeds planted indoors germinate in 5 to 10 days under warm conditions; direct-sown seeds take 7 to 14 days depending on soil warmth.

Starting Seeds Indoors Under Lights

Fill seed trays or small pots with seed-starting mix—a sterile, lightweight blend of peat moss, coconut coir, or perlite. Regular garden soil is too dense and holds too much moisture, which causes seedling rot. Moisten the mix before filling containers so water distributes evenly.

Plant one seed per cell or pot, pressing it about 1/4 inch deep. Cover with a humidity dome or plastic wrap to keep moisture in. Place trays on a heat mat set to 70°F to 75°F—tomato seeds germinate fastest in warm conditions. Check daily; remove the dome as soon as sprouts appear, usually in 5 to 10 days.

Once seedlings emerge, remove the heat mat and move trays under grow lights or a sunny south-facing window. Lights should sit 2 to 3 inches above seedlings and run 12 to 16 hours daily. Raise the lights as seedlings grow to keep that distance constant. Without enough light, seedlings become tall and spindly. Water from below by setting pots in a tray of water for 10 minutes, then draining—this prevents damping off, a fungal disease that kills seedlings at soil level.

Hardening Off Seedlings Before Moving Outdoors

Hardening off means gradually exposing indoor seedlings to outdoor sun, wind, and temperature swings over 7 to 10 days. Seedlings grown under lights are tender and will wilt or burn if moved directly outside.

Start 7 to 10 days before your last frost date. On day one, place seedlings in a shaded, sheltered spot outdoors for 2 to 3 hours. Each day, increase time outdoors and move them into more direct sun. By day 7 or 8, seedlings should spend the full day outside. Bring them in at night if temperatures drop below 50°F. By day 10, they can stay out overnight if nighttime lows stay above 50°F.

Water seedlings well during hardening off—outdoor air dries soil faster than indoor conditions. Check soil moisture daily and water when the top 1/2 inch feels dry. Stop fertilizing during this period; seedlings need to toughen up, not grow soft new leaves.

Direct Sowing Seeds in the Garden

Direct sowing works best in warm climates where soil stays above 60°F by late spring, or in regions with a long growing season. Plant seeds 1/4 inch deep in warm, moist soil. Space seeds 2 to 3 inches apart in rows or clusters; thin seedlings to 12 to 24 inches apart once they have two true leaves (the second set of leaves that appear).

Keep soil consistently moist but not waterlogged for the first 2 to 3 weeks while seeds germinate and seedlings establish. Once seedlings are 4 to 6 inches tall, water deeply but less often to encourage deep root growth. Direct-sown seedlings are naturally hardened off by outdoor conditions, so they usually grow stockier than indoor-started plants.

The trade-off: direct sowing saves the work of indoor lighting and hardening off, but you lose 6 to 8 weeks of growing time. In short-season regions, this delay means fewer tomatoes before frost arrives. In warm climates with long seasons, the timing difference matters less.

Soil Preparation and Planting Depth for Transplants

Whether you grew seedlings indoors or direct sowed, tomatoes benefit from rich soil. Mix 2 to 3 inches of compost or aged manure into the top 8 to 10 inches of garden soil before planting. Tomatoes are heavy feeders and need steady nutrients throughout the season.

When transplanting indoor-started seedlings, plant them deeper than they grew in their pots. Bury the stem up to the lowest true leaves—tomatoes will root along the buried stem, creating a stronger root system. This is one of the few times deeper planting helps rather than harms. Space plants 24 to 36 inches apart depending on variety; determinate (bush) types need less space, indeterminate (vining) types need more.

Water transplants thoroughly after planting and keep soil consistently moist for the first 2 to 3 weeks while roots establish. Once plants are growing vigorously, water deeply but less often to encourage deep roots and reduce disease pressure.

Common Mistakes to Avoid When Planting

Planting too early indoors creates overgrown seedlings that become root-bound and stressed before transplant time. If you start seeds more than 8 weeks before your last frost, seedlings will be too large and leggy by transplant day. Mark your calendar 6 to 8 weeks before your frost date and stick to it.

Skipping the hardening-off period causes wilting, leaf burn, or death when tender seedlings hit outdoor sun and wind. Even if it feels like extra work, this step takes only 10 days and prevents losing your entire crop.

Planting in cold soil causes seeds to rot before they germinate. If you direct sow and soil is below 60°F, seeds sit in wet soil for weeks without sprouting. Wait for warmth; there is no advantage to planting early in cold ground.

Using garden soil instead of seed-starting mix compacts around seedlings and holds too much water, leading to damping off. Seed-starting mix is inexpensive and worth the investment.

Timing by Region and Climate

Timing varies widely by location. In cold climates with short seasons (zones 3 to 5), start seeds indoors in late March or early April and transplant after the last frost in May. In moderate climates (zones 6 to 7), start seeds in mid-March and transplant in late April or early May. In warm climates (zones 8 to 10), you can start seeds in late winter for spring planting, or again in midsummer for a fall crop.

Check your local cooperative extension website for your specific last frost date and average first fall frost date. Knowing both dates tells you how long your growing season is and whether you have time for a second planting in summer for fall harvest.

Frequently Asked Questions

Can I plant tomato seeds directly outside instead of starting them indoors?

Yes, if your soil is warm enough (60°F or higher) and you have a long growing season. Direct sowing saves time and equipment, but you lose 6 to 8 weeks of growth compared to transplants, which means fewer tomatoes before frost. In short-season regions, indoor starting is usually worth the effort.

What if I start seeds indoors too early and they get too big?

Overgrown seedlings become root-bound and stressed. You can pinch off the top growth to slow them down, or bury them deeper when transplanting to use the extra stem. Next year, count backward from your frost date more carefully and start later.

How do I know if my seedlings are ready to transplant outside?

Seedlings are ready when they are 6 to 8 weeks old, have at least two sets of true leaves, and your last frost date has passed. They should be stocky and green, not tall and spindly. Harden them off for 7 to 10 days before planting in the garden.

Can I plant tomato seeds in fall instead of spring?

In warm climates where frost does not arrive until late fall or winter, yes. Start seeds in summer for transplanting in late summer or early fall, so plants mature before cold weather. In cold climates, fall planting does not work because seedlings will not have time to produce fruit before frost.

What temperature should soil be for direct sowing tomato seeds?

Soil should be 60°F or warmer, ideally 65°F to 70°F. Use a soil thermometer pushed 4 inches deep to check. Seeds planted in soil below 60°F germinate slowly and often rot before sprouting. Wait for warmth rather than planting early in cold ground.