Start tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow outdoors after frost danger has passed

The timing depends on whether you're starting from seed indoors or planting seedlings directly outside. Indoor seed starting happens 6 to 8 weeks before your area's last spring frost date — this gives seedlings time to grow strong before transplanting. Direct sowing (planting seeds straight in the ground) happens after all frost risk is gone and soil has warmed to at least 60°F, ideally 70°F or warmer.

Your last spring frost date is the key number. Search "[your city] last frost date" or check the USDA Plant Hardiness Zone Map to find yours. If you live in zone 5, your last frost might be May 15. If you're in zone 9, it might be March 15. Once you know that date, count backward 6 to 8 weeks to find when to start seeds indoors.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your city name plus "last frost date."
  • Transplant seedlings outdoors only after the last frost has passed and soil temperature reaches at least 60°F, ideally 70°F or warmer.
  • Direct sowing (planting seeds straight outside) works only in warm climates or late in the season when frost danger is completely gone.
  • Seedlings started indoors need grow lights or a sunny south-facing window to stay stocky and strong before moving outside.
  • Hardening off — gradually exposing seedlings to outdoor conditions over 7 to 10 days — prevents shock and wilting after transplant.

Finding Your Last Spring Frost Date

Your last spring frost date is the average date when your area experiences its final frost of the season. After that date, nighttime temperatures stay above 32°F. Tomatoes cannot survive frost, so this date is your hard important date for moving seedlings outdoors.

The easiest way to find it is to search your city name and "last frost date" — most gardening sites and your local cooperative extension office publish this. If you know your USDA hardiness zone, you can also estimate: zone 3 is around May 25, zone 5 is around May 15, zone 7 is around April 15, and zone 9 is around March 15. These are rough guides; your actual date may vary by a week or two depending on your exact location.

Once you have the date, mark it on a calendar. Then count backward 6 to 8 weeks. That's when you start seeds indoors. If your last frost is May 15, start seeds around March 20 to April 3.

Starting Seeds Indoors Under Lights

Indoor seed starting gives you a head start in cool climates and lets you choose exactly which varieties to grow. Fill a seed tray or small pots with seed-starting mix (a light, sterile blend, not garden soil). Moisten the mix, plant seeds about 1/4 inch deep, and cover the tray loosely with plastic or a humidity dome.

Tomato seeds germinate best at 70°F to 80°F. Use a heat mat under the tray if your home is cooler. Seeds sprout in 5 to 10 days. As soon as green shoots appear, remove the plastic and move the tray under grow lights or into a very sunny south-facing window. Keep lights 2 to 3 inches above the seedlings and run them for 14 to 16 hours a day. Without enough light, seedlings become tall, thin, and weak.

Water from below by setting pots in a tray of water and letting them soak up moisture from the bottom. This prevents damping-off disease, a fungal problem that kills seedlings at the soil line. Keep the soil moist but not waterlogged. Once seedlings have their first true leaves (the second set of leaves that appear), thin them to one plant per cell or pot, or transplant them into larger individual containers.

Hardening Off Before Moving Outdoors

Seedlings grown indoors are tender. Moving them straight from a warm, still room to outdoor sun and wind will shock them and cause wilting or death. Hardening off — gradually exposing them to outdoor conditions — takes 7 to 10 days and is essential.

Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours on the first day. Bring them back inside. Each day, increase the time outside and gradually move them into more sunlight. By day 7 or 8, they should be outside all day. By day 10, they can stay out overnight if temperatures stay above 50°F. After hardening off, seedlings are ready to transplant into the garden.

Transplanting Seedlings Into the Garden

Transplant seedlings outdoors only after your last frost date has passed and soil temperature is at least 60°F, ideally 70°F or warmer. Cold soil slows growth and can rot roots. Check soil temperature with a soil thermometer or wait until nighttime lows stay consistently above 50°F for a week.

Dig a hole slightly larger than the seedling's root ball. Tomatoes can be planted deeper than they were growing in pots — bury the stem up to the first true leaves, and roots will form along the buried stem, creating a stronger plant. Space plants 24 to 36 inches apart depending on variety. Water well after planting and keep soil consistently moist for the first 2 to 3 weeks while roots establish.

Direct Sowing Seeds Outdoors

Direct sowing (planting seeds straight in the ground) works best in warm climates with long growing seasons or when you're planting a second crop in midsummer. Wait until soil is at least 70°F and all frost danger has passed. Plant seeds 1/4 inch deep, 6 inches apart, in a row or cluster. Keep soil moist until seedlings emerge in 5 to 10 days.

Once seedlings have two true leaves, thin them to 24 to 36 inches apart by removing weaker plants. Direct-sown tomatoes mature later than transplants because they start from seed outdoors, so this method works best if your growing season is long enough. In short-season areas (zones 5 and colder), indoor seed starting is more reliable.

Common Timing Mistakes to Avoid

Starting seeds too early is the most common mistake. Seedlings started 10 or 12 weeks before the last frost become overgrown and root-bound before they can go outside, and they often struggle after transplanting. Stick to 6 to 8 weeks. If you start too late, seedlings won't be large enough to transplant when frost danger passes, and you'll lose weeks of growing time.

Transplanting too early — before the last frost or when soil is still cold — kills seedlings or severely stunts them. Wait for warm soil and warm nighttime temperatures. Hardening off too quickly or skipping it altogether causes wilting and leaf burn. Take the full 7 to 10 days even if it feels slow.

Overwatering seedlings indoors causes damping-off disease and root rot. Water only when the soil surface feels dry to the touch. Underwatering seedlings under lights causes them to dry out quickly, so check them daily.

Frequently Asked Questions

What if I don't know my last frost date?

Search your city name and "last frost date" online, or contact your local cooperative extension office — they have this information for your exact area. You can also estimate based on your USDA hardiness zone, though the actual date may vary by a week or two.

Can I start tomato seeds in late winter if I have grow lights?

Yes, grow lights let you start seeds indoors anytime, but seedlings still can't go outside until after the last frost and when soil is warm. Starting too early just means keeping seedlings indoors longer, which wastes space and light. Stick to 6 to 8 weeks before your frost date for the best timing.

What happens if I transplant seedlings before the last frost?

A frost will kill or severely damage seedlings. Even a light frost (32°F to 35°F) can kill tender tomato plants. Always wait until the last frost date has passed and nighttime temperatures stay above 50°F before moving seedlings outside permanently.

Can I start tomato seeds outdoors in spring instead of indoors?

Direct sowing works in warm climates with long growing seasons, but in cooler areas it delays harvest by several weeks because seeds take longer to germinate and grow than transplants do. Indoor seed starting gives you a 6 to 8 week head start, so you harvest earlier.

Do I need grow lights, or can I use a sunny window?

A very sunny south-facing window works if you have one, but grow lights are more reliable and prevent seedlings from becoming tall and spindly. If using a window, rotate seedlings daily so they grow straight, and keep them as close to the glass as possible.