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

The timing depends on when frost stops in your area, not on the calendar date. If your last frost arrives on May 15, count back 6 to 8 weeks and start seeds around mid-March. If your last frost is April 1, start seeds in early February. You can find your local frost date by searching "last frost date" plus your zip code — the National Weather Service publishes these for every county.

Starting too early means seedlings get leggy and weak while waiting for warm soil outdoors. Starting too late means you plant small, stressed seedlings into the garden when they should already be sturdy. The 6 to 8 week window gives you seedlings that are ready to harden off and transplant right when soil temperatures reach 60°F and nighttime lows stay above 50°F.

Key Takeaways

  • Count back 6 to 8 weeks from your area's last spring frost date to find your seed-starting date.
  • Tomato seeds need soil temperature between 70°F and 80°F to germinate reliably, which usually means a heat mat indoors.
  • Seedlings need 14 to 16 hours of direct light per day once they sprout, or they will become tall and thin.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into the garden.
  • Transplant into the garden only after nighttime temperatures stay above 50°F and soil has warmed to at least 60°F.

How to calculate your seed-starting date

Your last frost date is the average date when frost stops occurring in spring. After that date, soil warms and nighttime temperatures stay above freezing. Tomatoes cannot survive frost, so you time everything backward from this single date.

Go to the National Weather Service website or search "last frost date [your zip code]" to find the exact date for your location. Write it down. Then count back 42 to 56 days (6 to 8 weeks). That is your seed-starting window. For example, if your last frost date is May 20, start seeds between March 25 and April 8. If your last frost date is April 10, start seeds between February 13 and March 1.

The reason for the range is that different tomato varieties mature at different speeds. Determinate varieties (bush types that produce all fruit at once) can be started at 6 weeks. Indeterminate varieties (vining types that produce all season) benefit from the full 8 weeks. If you are unsure, aim for 7 weeks as a middle ground.

What you need to start seeds indoors

You need seed-starting mix (not garden soil), small containers with drainage holes, a heat mat, and a light source. Seed-starting mix is sterile and drains quickly, which prevents the fungal disease called damping off that kills seedlings in regular potting soil. Use containers at least 2 inches wide and 2 to 3 inches deep — recycled yogurt cups with holes poked in the bottom work fine.

A heat mat is essential because tomato seeds germinate best at soil temperature between 70°F and 80°F. Room temperature (68°F to 72°F) is too cold and seeds will sit dormant or rot. A seedling heat mat costs $20 to $40 and plugs into an outlet under your seed trays. Once seedlings sprout, you can remove the mat.

For light, use a grow light or fluorescent shop light positioned 2 to 3 inches above seedling tops. Windowsill light is almost never strong enough — seedlings will stretch toward the window and become weak and pale. Set the light on a timer for 14 to 16 hours per day. Seedlings need darkness too, so do not leave lights on 24 hours.

The step-by-step process for starting seeds

Fill your containers with moist seed-starting mix. Press the mix down gently so it is firm but not compacted. Place one tomato seed about 1/4 inch deep in each container — tomato seeds are small, so use a toothpick to position them. Cover the seed lightly with mix and mist with water until the surface is damp.

Place containers on the heat mat and cover with a humidity dome or plastic wrap to keep moisture in. Check daily and mist if the surface begins to dry. Tomato seeds usually sprout in 5 to 10 days at 75°F. Once you see the first green shoot, remove the cover when ready and move the container under your grow light.

Lower the light so it sits 2 to 3 inches above the seedling tops. As the seedling grows, raise the light to maintain that distance. Water from below by setting containers in a tray of water for 10 minutes, then removing them to drain. This prevents fungal disease on the leaves. Keep soil consistently moist but not waterlogged.

When seedlings develop their first true leaves (the second set of leaves that look like actual tomato leaves), thin out weaker seedlings or transplant each one into its own 4-inch pot. Bury the stem up to the lowest leaves — tomatoes will root along buried stem and develop a stronger plant.

Hardening off seedlings before transplanting

Hardening off means gradually exposing indoor seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start this process 7 to 10 days before you plan to transplant into the garden. Seedlings that go straight from a sheltered indoor space to full outdoor sun will wilt and may not recover.

On day one, place seedlings in a sheltered spot outdoors (against a wall, out of wind) for 2 to 3 hours in dappled shade. Bring them back inside. On day two, leave them out for 4 to 5 hours. Each day, increase time outdoors and gradually move them into more direct light. By day 7 to 10, seedlings should spend the full day and night outdoors if nighttime temperatures stay above 50°F.

Watch the weather forecast during hardening off. If frost or temperatures below 50°F are predicted, bring seedlings inside. A single cold night can stunt growth or kill tender plants. Once nighttime lows stay consistently above 50°F and soil temperature reaches 60°F, seedlings are ready to transplant into the garden.

Common mistakes to avoid

Starting seeds too early is the most common error. Seedlings started 10 or 12 weeks before the last frost will be overgrown and root-bound by planting time, and they often do not transplant well. Stick to 6 to 8 weeks. The second mistake is insufficient light. Seedlings under weak light become tall, thin, and pale — they will not produce well even after transplanting. Invest in a basic grow light.

Overwatering is another frequent problem. Seedlings need moist soil, not soggy soil. Soggy soil causes damping off and root rot. Water from below by setting pots in a tray of water, or water carefully at soil level. Let the surface dry slightly between waterings. Do not skip the hardening-off step either — seedlings planted directly from indoors to full sun often fail to establish.

Frequently Asked Questions

Can I start tomato seeds on a sunny windowsill without a grow light?

Windowsill light is rarely strong enough. Seedlings will stretch toward the window, become pale and weak, and produce poorly after transplanting. A basic fluorescent shop light costs $15 to $30 and makes a huge difference. If you must use a windowsill, rotate containers daily and expect weaker plants.

What if I missed my seed-starting date?

You can still start seeds a few weeks late, but seedlings will be smaller at transplant time. They will catch up by mid-summer if you plant them into warm soil and give them consistent water and light. Alternatively, buy transplants from a nursery — they are already at the right size for planting.

Do I need a heat mat if my house is warm?

Most homes stay between 68°F and 72°F, which is too cold for reliable tomato seed germination. A heat mat raises soil temperature to the 70°F to 80°F range that tomato seeds need. Without one, seeds germinate slowly or not at all. A mat is inexpensive and worth the investment.

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

Seedlings are ready when they are 6 to 8 inches tall, have at least two sets of true leaves, and have been hardened off for 7 to 10 days. Check that nighttime temperatures stay above 50°F and soil temperature is at least 60°F. If conditions are not right yet, keep seedlings indoors under lights and water regularly.

Can I reuse seed-starting mix from last year?

Reusing old mix risks introducing fungal spores that cause damping off. Fresh seed-starting mix is inexpensive and sterile. Use new mix each year for the best results, especially if seedlings died from disease the previous year.