Plant tomato seeds indoors 6 to 8 weeks before your last spring frost

The timing depends on when frost stops in your area, not on a calendar date. If your last frost arrives on May 15, start seeds indoors around March 20. If it arrives June 1, start around April 15. You can find your local frost date by searching "[your city] last frost date" — the National Weather Service publishes these for every region.

Indoors, tomato seeds germinate in 5 to 10 days under warm conditions (70 to 80 degrees Fahrenheit). They grow into seedlings over the next 6 to 8 weeks. By the time soil outdoors is warm enough to transplant them, they will be 6 to 8 inches tall with their first true leaves — ready to go in the ground.

If you miss the indoor window, you can still buy seedlings from a garden center and plant them directly outdoors after the last frost date. This costs more per plant but saves the indoor setup.

Key Takeaways

  • Count backward 6 to 8 weeks from your area's last spring frost date to find when to start seeds indoors.
  • Tomato seeds need warmth (70 to 80 degrees) and moisture to germinate, which is easier to control indoors than in cold soil.
  • Transplant seedlings outdoors only after the last frost date, when soil temperature reaches at least 60 degrees.
  • Buying seedlings from a garden center skips the indoor step if you start too late or lack indoor space.
  • Hardening off — moving seedlings outside for a few hours each day — takes 7 to 10 days before planting them permanently.

Why start indoors instead of direct seeding

Tomato seeds will not germinate in cold soil. If you plant them directly outdoors before the last frost, they sit dormant and often rot. Starting them indoors gives you a head start: seedlings are already growing when soil warms up, so they produce fruit weeks earlier than direct-seeded plants.

Indoor seedlings also let you control light, moisture, and temperature precisely. Outdoors, a late cold snap or heavy rain can kill young seeds. Indoors, you control both. This is why most tomato growers start seeds indoors rather than waiting for outdoor soil to warm.

How to find your last frost date

Your last frost date is the average date of the final frost in spring for your location. After that date, soil stays warm enough for tomatoes. Search the National Weather Service website or type "[your city] last frost date" into a search engine — you will find it in seconds.

If you live in a cold climate, your last frost might be June 1 or later. If you live in a warm climate, it might be March 15 or earlier. The date changes by region, so do not use a neighbor's date or a general zone map. Your local frost date is the one that matters.

Once you have the date, subtract 6 to 8 weeks. That is your indoor planting window. Mark it on a calendar so you do not forget.

Indoor seed-starting setup and timing

You need a warm spot (70 to 80 degrees), moisture, and light. A seed-starting tray with soil mix, a heat mat (optional but helpful), and a grow light or sunny windowsill will work. Keep soil moist but not waterlogged — overwatering kills more seedlings than underwatering.

Once seeds sprout, move them into light when ready. Without light, seedlings stretch and become weak. A south-facing window or a grow light 2 to 3 inches above the seedlings works. Leave the light on for 14 to 16 hours a day.

When seedlings develop their first true leaves (the second set of leaves that appear), you can move them into individual pots. This gives roots more room to grow. Keep them warm and moist until transplant time.

Hardening off before moving seedlings outdoors

Seedlings grown indoors are tender. Moving them straight from a warm, still room to outdoor wind and sun will shock them and stunt their growth or kill them. Hardening off — gradually exposing them to outdoor conditions — takes 7 to 10 days.

Start by placing seedlings outside in a sheltered spot for 1 to 2 hours on the first day. Increase the time by an hour or two each day. By day 7 or 8, they can stay outside overnight if temperatures stay above 50 degrees. By day 10, they are ready to plant in the ground.

Do this hardening off after your last frost date has passed and soil has warmed to at least 60 degrees. If a late frost is forecast while seedlings are outside, bring them back indoors.

Transplanting seedlings into the garden

Plant seedlings outdoors after the last frost date when soil is at least 60 degrees. Dig a hole slightly deeper than the root ball and bury the stem up to the first true leaves — tomatoes will root along the buried stem, creating a stronger plant.

Space plants 24 to 36 inches apart depending on the variety. Determinate (bush) types need less space; indeterminate (vining) types need more. Water well after planting and keep soil consistently moist for the first two weeks while roots establish.

If you planted indoors too early and seedlings are already flowering or fruiting before transplant time, pinch off the flowers. Let the plant focus on roots and leaves first. Flowers will come back once the plant is in the ground and established.

Buying seedlings instead of starting from seed

If you miss the indoor window or do not have space for seed trays, buying seedlings from a garden center or nursery is a straightforward option. Look for stocky, dark green plants with visible roots at the drainage holes. Avoid tall, pale, or leggy seedlings — they are stressed and will not perform as well.

Seedlings are usually ready to plant outdoors when ready after purchase, but still harden them off for a few days if they were grown indoors. Plant them the same way as seedlings you started yourself: bury the stem up to the first true leaves and space them 24 to 36 inches apart.

Buying seedlings costs more per plant than starting from seed, but you skip the indoor setup and the 6 to 8 week wait. This is a reasonable trade-off if you start late or have limited indoor space.

Frequently Asked Questions

Can I plant tomato seeds directly in the ground?

You can, but only after the last frost date when soil is warm (at least 60 degrees). Cold soil will cause seeds to rot. Direct seeding outdoors works, but plants will produce fruit 3 to 4 weeks later than seedlings started indoors. Most growers start indoors for an earlier harvest.

What if I plant seeds indoors too early?

Seedlings will be large and may start flowering before the last frost date arrives. Pinch off the flowers and keep seedlings in a cool spot (around 60 degrees) to slow growth. Once you can transplant them outdoors, they will adjust and produce normally.

Do I need a grow light?

A south-facing window with 6 to 8 hours of direct sun works if you have one. Without enough light, seedlings become tall and weak. A basic grow light costs $20 to $50 and gives you more control. It is not required, but it produces sturdier seedlings.

What temperature kills tomato seedlings?

Frost (32 degrees or below) kills seedlings. Temperatures below 50 degrees slow growth and stress plants. Keep seedlings indoors until after the last frost date, then harden them off gradually before planting outdoors.

Can I plant tomatoes in summer instead of spring?

In warm climates with a long growing season, you can start a second crop in mid-summer for a fall harvest. Count backward 6 to 8 weeks from your first fall frost date. In cold climates, the growing season is too short for a second crop to mature before winter.