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

The timing for tomato seeds depends on when frost stops in your area. Count backward from your last frost date — the day when freezing temperatures typically end for the season — and plant seeds indoors 6 to 8 weeks before that date. This gives seedlings time to grow strong enough to survive transplanting outdoors.

Your last frost date varies by location. If you live in a cold climate like Minnesota or upstate New York, that date might be May 15 or later. In warmer zones like Florida or Southern California, frost may end in February or March. You can find your specific date by searching "last frost date" plus your zip code, or by checking the USDA Plant Hardiness Zone Map.

Starting seeds indoors rather than direct-seeding outdoors gives you a head start on the growing season. Tomatoes need 70 to 85 days from transplant to first ripe fruit, so an early indoor start means you harvest sooner.

Key Takeaways

  • Plant seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your zip code online.
  • Seedlings need 12 to 16 hours of light daily under grow lights or a sunny window to stay short and sturdy.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • Move seedlings outdoors after the last frost date, when soil temperature reaches at least 60°F and ideally 70°F or warmer.
  • In short-season climates, start seeds earlier or choose fast-maturing varieties that produce fruit in 60 to 70 days instead of 80 or more.

How to calculate your planting date

Find your last frost date first. Search online for "last frost date" and your zip code, or visit the National Weather Service website and enter your location. Write down the date — for example, May 20 or April 10.

Count backward 6 to 8 weeks from that date. If your last frost is May 20, count back 8 weeks to early April. That is when you start seeds indoors. If your last frost is April 10, start seeds in late February. A calendar or straightforward subtraction works; you can also use an online seed-starting calculator.

This timing assumes you will transplant seedlings outdoors after the frost date passes. If you plan to keep seedlings in containers on a porch or patio rather than in the ground, you can start them a week or two earlier since you control the temperature.

Light and temperature for indoor seedlings

Seedlings started indoors need strong light to grow short and stocky. A sunny south-facing window works if it gets at least 12 hours of direct sun daily, but most indoor spaces do not provide enough light. Grow lights — fluorescent or LED fixtures positioned 2 to 3 inches above the seedlings — are more reliable. Run them for 12 to 16 hours per day.

Keep soil warm while seeds germinate. Tomato seeds sprout fastest at 70 to 80°F. A heat mat under seed trays speeds germination from 7 to 10 days down to 5 to 7 days. Once seedlings emerge, you can lower the temperature slightly to 65 to 70°F during the day and 55 to 65°F at night.

Water seedlings when the soil surface feels dry to the touch. Overwatering causes damping-off, a fungal disease that kills seedlings at the soil line. Use a spray bottle or water from below by setting trays in a pan of water and letting soil wick it up.

Hardening off before moving outdoors

Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start this process 7 to 10 days before your last frost date. On day one, place seedlings in a sheltered spot outdoors for 1 to 2 hours in dappled shade. Bring them back inside.

Each day, increase time outdoors and expose them to more direct sun. By day 7 or 8, seedlings should spend the full day outside. If temperatures drop below 50°F at night, bring them inside or cover them. By day 10, they are ready to stay outdoors overnight if temperatures stay above 50°F.

Seedlings that skip hardening off often wilt or stop growing when transplanted, because the shock of wind and intense sun is too sudden. The gradual transition prevents this setback.

Transplanting into the garden after the last frost

Wait until after your last frost date to plant seedlings in the ground. Even a light frost kills tomato plants. Check the forecast for the week you plan to transplant; if frost is predicted, wait a few more days.

Soil temperature matters as much as air temperature. Tomatoes grow slowly in cold soil and may rot before they root. Soil should be at least 60°F, ideally 70°F or warmer. You can measure it with a soil thermometer, or wait until nighttime temperatures stay above 55°F for several nights in a row — a sign that soil has warmed.

Dig a hole slightly deeper than the seedling's root ball. Tomatoes can be planted deeper than they grew in the pot; they will develop roots along the buried stem. Space plants 24 to 36 inches apart depending on the variety. Water well after planting.

Adjusting timing for short growing seasons

If you live in a climate with a short summer — fewer than 100 frost-free days — starting seeds 8 weeks early may not be enough. Choose early-maturing varieties that produce fruit in 60 to 70 days, such as 'Early Girl', 'Siberia', or 'Glacier'. These ripen faster than standard varieties that need 80 or more days.

You can also start seeds even earlier, 10 to 12 weeks before the last frost, if you have grow lights and space. Larger seedlings transplant better and begin producing sooner. Just be careful not to let them get too tall and leggy before moving them outdoors.

In very short seasons, some gardeners use black plastic mulch or row covers to warm the soil and air around plants, which can add 2 to 3 weeks to the effective growing season.

Common timing mistakes and how to avoid them

Starting seeds too early is the most common error. Seedlings started 12 or more weeks before the last frost become overgrown and root-bound before they can go outside. They transplant poorly and often produce less fruit. Stick to 6 to 8 weeks unless you have a very short season.

Planting outdoors before the last frost date is the second mistake. A single frost night kills seedlings or stunts them severely. If you are eager to plant early, use row covers or cloches to protect plants on cold nights, or wait for the frost date to pass.

Skipping the hardening-off step causes wilting and slow growth. Even if seedlings look strong indoors, they need time to adjust to outdoor conditions. The 7 to 10 days spent hardening off pays back in faster growth and earlier harvests.

Frequently Asked Questions

Can I direct-seed tomatoes outdoors instead of starting them indoors?

You can, but it delays harvest by 6 to 8 weeks. Direct seeding works in long-season climates where you have 100+ frost-free days. In shorter seasons, you will not have time for fruit to ripen before fall frost. Indoor starting is the standard method for most gardeners.

What if I miss my planting window and it is already late spring?

You can still plant seedlings after the last frost date, even if you started them later than ideal. Look for stocky seedlings at garden centers rather than tall, leggy ones. Plant them deeper than usual and give them consistent water and warmth. They will catch up, though you may harvest later.

Do I need grow lights, or will a windowsill work?

A bright south-facing window with 12+ hours of direct sun daily can work, but most indoor spaces do not provide enough light. Seedlings grown in dim light become tall and weak. Grow lights are inexpensive and reliable; a basic fluorescent fixture costs $20 to $40 and lasts for years.

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

Seedlings are ready when they have 2 to 3 sets of true leaves (the leaves that look like tomato leaves, not the first round seed leaves) and are 4 to 6 inches tall. They should be stocky, not tall and spindly. If they are leggy, they need more light before transplanting.

What happens if soil is too cold when I plant seedlings?

Cold soil slows root growth and can cause seedlings to sit in place for weeks without growing. They may also rot before roots establish. Wait for soil to warm to at least 60°F, or use black plastic mulch to warm it faster. Patience at this stage pays off in stronger plants.