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

The timing depends on when frost stops in your area, not on the calendar. If your last frost date is May 15, you would start seeds indoors around mid-March. If it's June 1, you'd start around early April. Seeds started too early will outgrow their containers and become leggy and weak before transplant time. Seeds started too late won't be mature enough to produce fruit before fall frost arrives.

You can find your last frost date by searching "[your city] last frost date" or checking the USDA Hardiness Zone Map. The date is the average date of the last freeze in spring—not a may provide, but a reliable guide for planting decisions. Write it down; you'll use it for other plants too.

Key Takeaways

  • Count backward 6 to 8 weeks from your last spring frost date to find your indoor seed-starting date.
  • Tomato seedlings need 14 to 16 hours of light daily and soil between 70 and 80 degrees Fahrenheit to germinate and grow strong.
  • Seedlings are ready to move outdoors when they have their first true leaves (the second set) and the soil outdoors has warmed to at least 60 degrees.
  • Hardening off—gradually exposing seedlings to outdoor conditions over 7 to 10 days—prevents transplant shock and sunburn.

Why 6 to 8 weeks is the right window

Tomato seedlings reach transplant size in about 6 to 8 weeks from germination. At that point, they have several true leaves, a sturdy stem, and roots that fill the container without circling it endlessly. If you start earlier, the seedlings will be ready to go outside before the soil has warmed enough, and they'll sit in pots getting rootbound and stressed. If you start later, they won't be mature enough to set fruit before the season ends.

The 6 to 8 week window also accounts for the fact that not every seed germinates on the same day. Some sprout in 5 days, others take 10. Starting a batch gives you a range of seedling ages, and the slower ones will catch up by transplant time.

What your seedlings need to grow strong indoors

Tomato seeds germinate best in soil between 70 and 80 degrees Fahrenheit. A heating mat under your seed trays speeds germination and keeps the soil warm. Once seedlings emerge, they need 14 to 16 hours of bright light daily—a sunny windowsill is rarely enough. Seedlings grown in low light stretch toward the light source and become tall, thin, and weak. A shop light with standard fluorescent or LED bulbs, positioned 2 to 3 inches above the seedlings and raised as they grow, produces stocky, healthy plants.

Water the soil when the top feels dry to the touch, but don't let it stay soggy. Overwatering causes damping-off, a fungal disease that kills seedlings at the soil line. Use a spray bottle or water from below by setting the tray in a larger container of water and letting the soil wick it up.

Once seedlings have their first true leaves (the second set—the first pair are seed leaves and look different), you can start feeding them with a diluted liquid fertilizer every two weeks. Seed-starting mix has no nutrients, so seedlings need supplemental feeding to stay healthy.

Hardening off before moving seedlings outdoors

Seedlings grown indoors are tender. Moving them directly from a warm, still, protected environment to wind, direct sun, and temperature swings will shock them and may kill them. Hardening off is the process of gradually acclimating them over 7 to 10 days.

Start by placing seedlings outdoors in a sheltered, shaded spot for 1 to 2 hours on the first day. Each day, increase the time outdoors and gradually move them into more direct light. By day 7 or 8, they should be in full sun for most of the day. Bring them inside at night until nighttime temperatures stay above 50 degrees Fahrenheit consistently. This slow transition toughens the stems and leaves and prepares the plants for outdoor conditions.

Soil temperature matters as much as air temperature

Even if daytime air temperatures are warm, transplanting into cold soil will stall growth and invite root rot. Tomatoes prefer soil at least 60 degrees Fahrenheit, but 65 to 70 degrees is better. You can check soil temperature with a soil thermometer pushed 3 inches into the ground in the morning.

If your last frost date has passed but soil is still cold, wait another week or two. Covering the soil with black plastic or landscape fabric for a week or two before planting warms it faster. Planting too early into cold soil wastes the work you put into starting seeds indoors.

Adjusting timing for your growing season length

If you live in a short-season area where frost arrives early in fall, you may need to start seeds earlier than the standard 6 to 8 weeks. Count backward from both your last spring frost date and your first fall frost date to see how many days you have to grow. Determinate varieties (bush types that ripen most fruit at once) need less time than indeterminate varieties (vining types that produce all season). If your season is short, choose determinate varieties and start seeds at the earlier end of the window.

In long-season areas, you can start seeds later or even direct-sow them outdoors once soil warms. Starting indoors still gives you a head start on fruit production, but you have more flexibility.

Common mistakes that delay or damage seedlings

Starting seeds too early is the most common error. Seedlings outgrow their containers, become rootbound, and decline in quality before transplant time arrives. They also take up space under lights for weeks longer than necessary. If you've already started seeds too early, you can pot them up into larger containers to buy time, but it's better to plan correctly from the start.

Starting seeds in regular garden soil instead of seed-starting mix causes compaction and damping-off. Seed-starting mix is light, sterile, and drains well. It's inexpensive and worth using.

Transplanting into the garden without hardening off causes wilting, leaf scorch, and sometimes death. The hardening-off step takes time but prevents losses and produces stronger plants.

Frequently Asked Questions

Can I start tomato seeds outdoors instead of indoors?

Yes, if your growing season is long enough. Direct-sowing seeds outdoors works in areas with 90+ frost-free days, but you'll harvest later than seedlings started indoors. Starting indoors gives you a 6 to 8 week head start on fruit production, which matters in shorter seasons.

What if I started seeds too early and they're already too big?

Pot them up into larger containers (4-inch pots) to give roots more room. Keep them under lights and continue watering and feeding. They'll be larger at transplant time, but they'll still grow and produce fruit. Avoid planting them deeper than they were in the original pot—tomatoes can root along the buried stem, but extremely deep planting can cause rot.

Do I need grow lights or will a windowsill work?

A south-facing windowsill in bright sun can work, but seedlings often stretch toward the light and become weak. Grow lights produce sturdier plants and give you more control. A basic shop light with fluorescent or LED bulbs costs $20 to $40 and lasts for years.

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

Seedlings are ready when they have several true leaves (not just the seed leaves), a sturdy stem about the thickness of a pencil, and roots visible at the drainage holes. The soil outdoors should be at least 60 degrees, and you should have completed hardening off over 7 to 10 days.

What if my last frost date is very late and I don't have space for seedlings?

Direct-sow seeds outdoors once soil warms, or buy transplants from a nursery. Nursery seedlings are already hardened off and ready to plant, which saves space and time. You'll pay more per plant but avoid the indoor setup entirely.