Start seeds 6 to 8 weeks before your last spring frost date

The right time to start tomato seeds indoors depends on when frost stops in your area. Count backward 6 to 8 weeks from your last spring frost date — that is your seed-starting window. If your last frost arrives on May 15, you would start seeds around March 20 to April 1. Starting too early means seedlings get leggy and weak while waiting for transplant time; starting too late means small plants that take longer to produce fruit.

Find your exact frost date by searching "[your city] last spring frost date" or checking the USDA Hardiness Zone Map. Local garden centers and cooperative extension offices also have this information. Once you know the date, mark it on a calendar and count back — this single number drives your entire tomato timeline.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which varies by location and changes year to year.
  • Seedlings need 14 to 16 hours of light daily from a grow light or sunny window, or they will stretch and weaken.
  • Move seedlings to larger containers when they develop their first true leaves (the second set that appears), not when they sprout.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • Transplant into the garden only after the last frost date has passed and soil temperature reaches at least 60°F.

Why timing matters: the frost date rule

Tomatoes are warm-season plants that die if exposed to frost. Planting them too early means they sit in cold soil and may rot before they grow. Planting them too late means you lose weeks of the growing season when tomatoes produce the most fruit. The 6 to 8 week window balances these risks by timing seedlings to reach transplant size exactly when outdoor conditions are safe.

The range accounts for tomato variety — determinate types (bush tomatoes that fruit all at once) mature faster and can start at 6 weeks, while indeterminate types (vining tomatoes that produce all season) benefit from the full 8 weeks. Check your seed packet for the variety type if you are unsure.

Setting up light and warmth for seedlings

Tomato seeds germinate in 5 to 10 days at soil temperature between 70°F and 80°F. Use a heat mat under your seed trays to maintain warmth — a seedling heat mat costs $15 to $40 and speeds germination significantly. Without one, seeds still sprout but take longer, especially if your house is cool. Once seedlings emerge, remove the heat mat and move trays to a light source.

Seedlings need 14 to 16 hours of light daily. A south-facing window works if it receives direct sun all day, but most home windows do not provide enough light. A basic two-bulb fluorescent shop light ($20 to $30) hung 2 to 3 inches above seedlings is far more reliable. Position the light on a timer set to turn on at 6 a.m. and off at 10 p.m. Adjust the light height as seedlings grow so it stays 2 to 3 inches above the tops of the plants. Without adequate light, seedlings stretch toward the window and become tall, thin, and weak.

Watering and moving seedlings to larger pots

Keep soil moist but not waterlogged — soggy soil causes damping off, a fungal disease that kills seedlings at the soil line. Water from below by setting trays in a shallow pan of water for 10 to 15 minutes, then remove them and let excess drain. This method prevents fungal issues and teaches roots to grow downward. Check soil daily; it should feel like a wrung-out sponge.

When seedlings develop their first true leaves — the second set of leaves that appears after the initial sprout leaves — move them to individual 4-inch pots. True leaves look like actual tomato leaves, while the first leaves are smooth and oval. This is usually 3 to 4 weeks after germination. Bury the stem up to the lowest leaves when potting up; tomatoes will root along the buried stem and develop a stronger root system. Continue watering from below and maintain the light schedule.

Hardening off: preparing seedlings for outdoor conditions

Seedlings grown indoors are tender and will wilt or burn if moved directly outside. Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing them in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase time outside and gradually move them into more sun. By day 7 to 10, they should spend the full day and night outside.

Watch the weather forecast during hardening off — if frost is predicted, bring seedlings back inside. A sudden cold snap can kill tender plants. Hardening off is not optional; it prevents transplant shock and gives seedlings time to develop thicker stems and waxy leaf coatings that protect them outdoors.

Transplanting into the garden at the right moment

Wait until two conditions are met: the last frost date has passed, and soil temperature is at least 60°F. Check soil temperature with a soil thermometer ($8 to $15) pushed 4 inches into the ground. Soil warms slowly in spring, and cold soil slows growth and invites disease. Planting into 50°F soil is worse than waiting two more weeks for 65°F soil.

Seedlings are ready to transplant when they are 6 to 8 inches tall and have 4 to 6 true leaves. Bury the stem up to the lowest leaves again — this encourages a deep root system. Space plants 24 to 36 inches apart depending on variety (check the seed packet). Water well after planting and keep soil consistently moist for the first two weeks while roots establish.

Common mistakes to avoid

Starting too early is the most common error. Seedlings started in January for a May frost date become overgrown and root-bound by transplant time, and they never recover fully. Stick to the 6 to 8 week window. Another frequent mistake is insufficient light — seedlings that stretch and fall over are light-starved, not ready to transplant. A grow light is worth the investment.

Overwatering kills more seedlings than underwatering. Soggy soil causes damping off and root rot. Water when the top of the soil feels dry to your finger, and always drain excess water. Finally, do not skip hardening off. Seedlings moved directly from a warm, still indoor environment to outdoor sun, wind, and temperature swings will wilt and may not recover, even if they survive.

Frequently Asked Questions

Can I start seeds directly in the garden instead of indoors?

You can, but it delays harvest by 4 to 6 weeks because seeds germinate slowly in cool spring soil. Starting indoors lets you control temperature and light, producing stronger plants that fruit earlier. Direct seeding works better in warm climates where soil reaches 70°F by late spring.

What if I missed the ideal start date?

Start seeds anyway — late seedlings still produce fruit, just later in the season. A seedling started 2 weeks late will catch up faster than you expect once it is in warm garden soil. Do not skip the hardening off step, which matters more than perfect timing.

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

Seedlings are ready when they are 6 to 8 inches tall, have 4 to 6 true leaves, and have been hardened off. If they are still small at your frost date, transplant them anyway — they will grow faster in warm soil than waiting indoors. Pot-bound seedlings (roots circling the soil) should go out when ready.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination from 10 days to 5 days and is helpful but not required. Seeds germinate without one if your house stays above 65°F, just more slowly. A heat mat is most useful if you start seeds in a cool basement or garage.

What happens if frost hits after I transplant?

Tomato plants die at 32°F. If frost is forecast after transplanting, cover plants with blankets, row covers, or buckets overnight. Remove covers the next morning. If plants freeze, they are lost — this is why waiting for the last frost date matters.