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 on the growing season and works in any climate — you germinate seeds under controlled conditions, then move seedlings outside once frost danger passes. Direct sowing means planting seeds straight into warm garden soil, which is simpler but delays your harvest by several weeks.

To find your frost dates, search your zip code on the National Weather Service website or ask at your local cooperative extension office. Once you know your last spring frost date, count backward 6 to 8 weeks to determine when to start seeds indoors. For example, if your last frost is May 15, start seeds around March 20 to April 1. If you prefer direct sowing, wait until soil temperature reaches at least 60°F — ideally 65°F or warmer — which usually happens 1 to 2 weeks after your last frost date.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your zip code on the National Weather Service website.
  • Seedlings need 12 to 16 hours of light daily under grow lights or a sunny south-facing window to stay stocky and strong.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.
  • Direct sow seeds outdoors only after soil temperature reaches 60°F, which is usually 1 to 2 weeks after your last frost date.
  • Tomato seeds germinate best in soil between 70°F and 80°F and sprout in 5 to 10 days under those conditions.

Starting Seeds Indoors Under Lights

Fill seed trays or small pots with seed-starting mix — a sterile, lightweight blend of peat moss, coconut coir, and perlite that drains quickly and prevents damping off (a fungal disease that kills seedlings). Moisten the mix before filling containers so it absorbs water evenly. Plant one seed per cell or pot, press it lightly into the soil, and cover with a thin layer of mix. Tomato seeds are small and don't need to be buried deep — a quarter-inch of soil is enough.

Place trays in a warm spot between 70°F and 80°F. A heat mat under the trays speeds germination to 5 to 10 days. Once seedlings emerge, remove the heat mat and move trays under grow lights positioned 2 to 3 inches above the tops of the plants. Adjust the lights as seedlings grow so they stay at that distance. Run lights for 12 to 16 hours daily — a timer makes this automatic. Without enough light, seedlings become tall and spindly instead of compact and sturdy.

Water seedlings from below by setting trays in a shallow pan of water for 10 to 15 minutes, then removing them to drain. This prevents the soil surface from staying wet, which invites fungal disease. Once seedlings develop their first true leaves (the second set of leaves that appear), you can begin fertilizing with a diluted liquid fertilizer every 7 to 10 days.

Hardening Off Before Transplanting

Hardening off means gradually exposing indoor seedlings to outdoor sun, wind, and temperature swings so they toughen up before permanent planting. Skip this step and seedlings will wilt, turn purple, or die when moved directly from the controlled indoor environment to the garden.

Begin hardening off 7 to 10 days before your last frost date. On day one, place seedlings in a sheltered spot outdoors with dappled shade for 2 to 3 hours, then bring them back inside. Each day, increase outdoor time and move them into brighter light. By day 7 or 8, seedlings should spend the full day outside in partial sun. By day 10, they can handle full sun and stay outside overnight if temperatures stay above 50°F. If frost is forecast during hardening off, bring all seedlings indoors.

Watch soil moisture closely during hardening off — outdoor air dries pots much faster than indoor air. Check pots daily and water when the top of the soil feels dry to the touch.

Transplanting Seedlings Into the Garden

Transplant seedlings into the garden after your last frost date has passed and soil has warmed to at least 60°F. Dig a hole slightly deeper than the seedling's root ball and set the plant in so the lowest leaves are just above soil level. Tomatoes can be planted deeper than they grew in their pot — they'll develop roots along the buried stem, creating a stronger root system. Space plants 24 to 36 inches apart depending on the variety.

Water thoroughly after planting and keep soil consistently moist (not waterlogged) for the first 2 to 3 weeks while roots establish. A 2-inch layer of mulch around each plant helps retain moisture and keeps soil temperature steady. Avoid mulching right against the stem, which can trap moisture and cause rot.

Direct Sowing Seeds Outdoors

Direct sowing works best in warm climates where soil reaches 60°F early in spring, or in regions with a long growing season. Plant seeds directly into garden soil after the last frost date and once soil temperature has warmed. Prepare the planting area by mixing in compost or aged manure to improve drainage and add nutrients.

Plant seeds half an inch deep in small mounds or rows, spacing them 6 inches apart. Water gently so seeds stay in contact with soil. Seedlings emerge in 5 to 10 days if soil stays between 70°F and 80°F. Once they develop their first true leaves, thin them to their final spacing of 24 to 36 inches by removing the weaker seedlings.

Direct sowing delays your harvest by 3 to 4 weeks compared to transplanting, since seeds take time to germinate and seedlings must grow larger before producing flowers. In short-season areas, this delay may mean missing peak harvest before fall frost arrives.

Adjusting Timing for Your Climate

Cool-climate gardeners in zones 3 and 4 must start seeds indoors in late February or early March to get a harvest before frost. Waiting until soil warms outdoors would mean planting so late that plants flower only in fall, when temperatures drop and fruit stops ripening. Starting indoors gives you 2 to 3 months of growth before transplanting, which is enough time to set fruit before the first fall frost.

Warm-climate gardeners in zones 9 and 10 can direct sow in spring and often grow a second crop in late summer for a fall harvest. Soil warms quickly in these regions, so seeds germinate fast and seedlings grow rapidly. Some gardeners in very warm areas start seeds in late summer (July or August) for a fall and winter crop, since tomatoes struggle in extreme heat.

If you live in a region with unpredictable spring weather — late freezes or cold snaps — starting seeds indoors gives you control. You can hold seedlings under lights until frost danger truly passes, rather than gambling on an early outdoor planting.

Common Mistakes to Avoid

Planting too early indoors is the most common error. Seedlings started more than 8 weeks before the last frost become overgrown, root-bound, and stressed by the time they can go outside. They often flower and set fruit while still in pots, which stunts growth. Count backward from your frost date and stick to the 6 to 8 week window.

Insufficient light creates tall, weak seedlings that flop over. If you don't have grow lights, a sunny south-facing window works, but rotate trays daily so plants don't lean toward the light. Seedlings need 12 to 16 hours of light — a regular household bulb won't provide enough intensity, so invest in an inexpensive LED grow light if possible.

Skipping hardening off causes transplant shock and wilting. Even if it feels tedious, the 7 to 10 day process is essential. Overwatering seedlings leads to damping off and root rot. Let the soil surface dry slightly between waterings and always use sterile seed-starting mix, not garden soil.

Frequently Asked Questions

Can I start tomato seeds in winter indoors?

Yes, but only if you have grow lights. Seeds germinate and seedlings grow year-round under lights, so you can start seeds in January or February for a spring transplant. Without lights, seedlings become leggy and weak. If you're starting very early (before February), be prepared to pot up seedlings into larger containers as they grow, since they'll outgrow their starting trays before outdoor planting time arrives.

What if my last frost date is very late?

If your last frost isn't until June, starting seeds 6 to 8 weeks earlier means germinating them in late March or April. Seedlings will be large and possibly flowering by transplant time, which is normal. You can pot them up into larger containers to give roots more room. Alternatively, delay starting seeds until late April or early May so seedlings are smaller at transplant time.

Can I plant tomato seeds in fall for a winter crop?

This works only in warm climates (zones 9 and 10) where winter temperatures stay above freezing. Start seeds in late July or August so seedlings mature and flower in fall and early winter, when temperatures are mild. In cold climates, tomatoes won't survive winter outdoors and won't ripen fruit in short days and cold temperatures.

How do I know if my soil is warm enough for direct sowing?

Use a soil thermometer pushed 4 inches into the ground in the morning. Tomato seeds germinate best at 70°F to 80°F but will sprout slowly at 60°F. If soil is below 60°F, seeds may rot before germinating. Wait a week or two and check again — soil warms quickly once spring arrives.

Should I soak tomato seeds before planting?

Soaking isn't necessary for tomato seeds, which have thin coats that absorb water easily. It won't hurt, but it doesn't speed germination noticeably. If you do soak, use room-temperature water for no more than 24 hours, then plant when ready. Oversaturated seeds can rot.