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

The right time to plant tomato seeds indoors depends on one date: your area's last spring frost. Count backward 6 to 8 weeks from that date, and that is when you start seeds under lights indoors. If your last frost is May 15, you start seeds around March 20 to April 1. If it is April 1, you start around mid-February.

Tomato seedlings need 6 to 8 weeks to grow large enough to transplant outdoors — roughly 6 to 8 inches tall with several sets of true leaves. Starting too early means seedlings become leggy and weak while waiting for warm soil. Starting too late means you transplant small plants into the ground when they are barely ready, which delays fruiting.

Find your last spring frost date by searching "[your city] last frost date" or checking the USDA Hardiness Zone Map. The date varies by 2 to 3 weeks depending on whether you live in a valley, on a hilltop, or near a large body of water, so use your specific location, not your state average.

Key Takeaways

  • Count backward 6 to 8 weeks from your last spring frost date to find your indoor seed-starting date.
  • Seedlings started too early become tall and spindly before transplant time; seedlings started too late are too small when frost danger passes.
  • Your last frost date varies by location within your region, so search for your specific city or zip code rather than your state average.
  • Tomato seeds germinate in 5 to 10 days at 70 to 80 degrees Fahrenheit and need 14 to 16 hours of light daily once sprouted.

How to find your exact last frost date

The USDA Hardiness Zone Map is the standard reference. Go to planthardiness.ars.usda.gov, enter your zip code, and the map shows your zone. From there, search "[your zone] last frost date" — for example, "Zone 5b last frost date" — and you will find the typical date for your area.

Local cooperative extension offices also publish frost dates for their counties. Search "[your county] cooperative extension" and look for their frost date chart. These are often more precise than the national map because they account for local terrain and water features.

If you live in a frost-prone area, ask neighbors or a local garden center what date they use. Gardeners in your exact neighborhood have real experience with how late frost actually arrives in your specific spot, which can differ from the official date by a week or more.

Why the 6 to 8 week window matters

Tomato seedlings grow fastest in their first 4 weeks. By week 6, they have developed true leaves (the second set of leaves that look like actual tomato leaves, not the smooth seed leaves). By week 8, they are ready to harden off — the process of gradually exposing them to outdoor conditions before transplanting.

If you start seeds 10 or 12 weeks early, seedlings sit in pots waiting for transplant time. They become root-bound, stretch toward the light, and may flower indoors before they go in the ground. Flowering indoors wastes energy that should go into root and stem growth. These plants often produce less fruit than seedlings started at the right time.

If you start seeds only 4 weeks before your frost date, seedlings are still small and fragile when they go outside. Cold soil and wind stress them heavily. They may survive, but they grow slowly for weeks while they recover, pushing your first harvest back by 2 to 3 weeks.

Adjusting the timeline for different tomato types

Determinate varieties (bush types that ripen most fruit at once) need the full 6 to 8 weeks because they are compact and mature quickly once planted. Starting them late means they reach full size just as frost arrives in fall, cutting your harvest short.

Indeterminate varieties (vining types that produce all season) are more forgiving. They can start producing fruit even if transplanted smaller, because they keep growing and flowering until frost. You can start these 5 to 6 weeks before your frost date if space or light is tight.

Cherry and heirloom varieties vary widely. Check the seed packet for "days to maturity" — this is the number of days from transplant to first ripe fruit. Add 6 to 8 weeks for seedling growth, then count backward from your first expected frost in fall (usually 6 months after your last spring frost). This tells you the latest date you can start seeds and still harvest before cold weather.

Light and temperature during the 6 to 8 week period

Tomato seeds germinate in darkness at 70 to 80 degrees Fahrenheit. Use a heat mat under seed trays to keep soil warm. Once seedlings emerge (5 to 10 days), remove the heat mat and move trays under grow lights or in a sunny south-facing window.

Seedlings need 14 to 16 hours of light daily. Without enough light, they stretch and become weak. If you use a sunny window, rotate trays every 2 to 3 days so all sides get equal light. If you use grow lights, keep them 2 to 3 inches above the seedlings and raise them as plants grow.

Keep daytime temperatures around 65 to 75 degrees and nighttime temperatures 5 to 10 degrees cooler. Cool nights encourage stocky growth instead of tall, spindly stems. If your seedlings are stretching, lower the temperature or move lights closer.

Hardening off before transplanting outdoors

One week before your transplant date, begin hardening off. Place seedlings outdoors in a sheltered spot (against a wall, under an eave, or in a cold frame) for 1 to 2 hours on the first day. Each day, add 1 to 2 hours of outdoor time and increase sun exposure. By day 7, seedlings should spend the full day outside but come inside at night.

Hardening off toughens leaf tissue and strengthens stems so plants do not wilt or sunburn when they go in the ground permanently. Skip this step and even healthy seedlings may collapse in the first week outdoors.

Transplant into the garden only after your last frost date has passed and soil temperature reaches at least 60 degrees Fahrenheit. Tomatoes planted in cold soil sit dormant for weeks. Wait for warm soil and you will see faster growth and earlier fruit.

What to do if you started seeds at the wrong time

If seedlings are too tall and leggy, bury the stem deeper when you transplant. Tomato stems develop roots all along their length, so planting up to the first true leaves creates a stronger root system and a sturdier plant. This does not fully fix the problem, but it helps.

If seedlings are too small at transplant time, plant them anyway once frost danger passes. Small plants grow slowly at first but usually catch up by mid-summer. You will harvest later, but the season is long enough for most varieties to produce a full crop.

If you started seeds too early and seedlings are flowering indoors, pinch off the flowers before transplanting. This redirects energy back to roots and stems. Once plants are in the ground and established, they will flower again and set fruit normally.

Frequently Asked Questions

Can I start tomato seeds outdoors instead of indoors?

Yes, but only if your growing season is long enough. Direct-seeding outdoors works in zones with 90+ frost-free days and warm soil by late spring. In most of the United States, the season is too short — you would not harvest until October or November. Starting indoors gives you a 6 to 8 week head start, which is why most gardeners do it.

What if I do not know my exact last frost date?

Use the USDA Hardiness Zone Map or ask your local cooperative extension office. If neither is available, ask a local garden center or neighbors what date they use. You can also start seeds 2 weeks later than the official date to be safe — this gives you a smaller window for error.

Can I start seeds earlier if I have grow lights?

Grow lights do not change the frost date, so starting much earlier than 6 to 8 weeks still results in overgrown seedlings waiting to go outside. You can start 1 to 2 weeks earlier if you have excellent lights and cool temperatures, but beyond that, seedlings become too large and leggy.

Do all tomato varieties need the same 6 to 8 weeks?

Most do, but check your seed packet. Some fast-maturing varieties (60 to 70 days to maturity) can start 5 to 6 weeks before your frost date. Slow varieties (85+ days) may need a full 8 weeks or even a head start if your growing season is short.

What happens if I transplant seedlings before the last frost date?

A late frost will kill seedlings or severely damage them. Even a light frost (32 degrees) kills tomato plants. Wait until nighttime temperatures stay above 50 degrees and your last frost date has passed before moving seedlings outside permanently.