Start seeds indoors 6 to 8 weeks before your last spring frost, or direct sow outdoors after frost danger passes

The timing for tomato seeds depends on where you live and whether you start them indoors or plant them straight in the ground. Indoor seed starting — the most common method — means planting seeds in containers under lights 6 to 8 weeks before your area's last frost date. This gives seedlings time to grow strong before transplanting them outside. Direct sowing — planting seeds straight into garden soil — works only in warm climates and happens after all frost risk is gone.

Your last frost date is the key number. It is the average date of the final frost in spring for your specific location. Seeds started indoors before that date will be ready to move outside right when soil warms up. If you start too early, seedlings become leggy and weak while waiting for warm weather. If you start too late, you lose weeks of growing season and may not harvest before fall frost arrives.

Find your last frost date by searching "[your city] last frost date" or checking the USDA Plant Hardiness Zone Map. Once you have that date, count backward 6 to 8 weeks. That is when you plant seeds indoors.

Key Takeaways

  • Indoor seed starting happens 6 to 8 weeks before your last spring frost date, which you can find by searching your city name plus "last frost date".
  • Seedlings need 12 to 16 hours of light daily under grow lights or in a bright window to stay compact and healthy.
  • Direct sowing outdoors works only after all frost danger has passed and soil temperature reaches at least 60°F, preferably 70°F or warmer.
  • Transplant indoor seedlings outside when they are 6 to 8 inches tall and have their first true leaves, hardening them off over 7 to 10 days first.
  • Different tomato varieties mature at different speeds — check seed packets for days to maturity and count backward from your first fall frost date to confirm you have enough time.

How to find your last frost date and count backward

Your last frost date is printed on seed packets and gardening websites specific to your region. The USDA Plant Hardiness Zone Map lets you enter your zip code and shows your zone, which corresponds to frost dates. Local cooperative extension offices also publish frost date calendars for their counties.

Once you have the date, mark it on a calendar and count back 6 weeks for early varieties (like cherry tomatoes) or 8 weeks for larger slicing tomatoes. Write that date down — it is your seed-starting day. For example, if your last frost date is May 15, count back 8 weeks to land on March 20. Start seeds indoors around March 20.

This timing assumes you will transplant seedlings outside when they reach 6 to 8 inches tall, which usually takes 6 to 8 weeks from planting. Seedlings grow faster or slower depending on light, temperature, and variety, so the date is a target, not a hard important date.

Indoor seed starting: light, temperature, and moisture

Seeds germinate best in warm soil — between 70°F and 80°F. Use a heat mat under seed trays if your home is cooler. Once seedlings sprout, they need 12 to 16 hours of light daily from grow lights positioned 2 to 3 inches above the leaves. Without enough light, seedlings stretch tall and thin (called "leggy") and will not transplant well.

Keep soil moist but not waterlogged. Check daily and water when the top of the soil 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 shallow pan of water and letting soil absorb it from underneath.

Once seedlings have their first true leaves (the second set of leaves that appear), thin them to one plant per cell or pot. Crowded seedlings compete for light and air, making them weak. If you started multiple seeds in one cell, snip off the weaker seedlings at soil level with scissors rather than pulling them out, which disturbs roots of the plant you want to keep.

Hardening off: preparing seedlings for outdoor conditions

Seedlings grown indoors are tender and will wilt or die if moved directly outside. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase time outside and gradually move them into more sunlight.

By day 7 or 8, seedlings should spend the full day outside in direct sun and return indoors at night. By day 10, they can stay outside overnight if temperatures stay above 50°F. This process toughens the leaves and stems so they handle wind, sun, and temperature swings without shock.

Do not skip hardening off. Seedlings moved directly from a warm, still indoor space to outdoor sun and wind will stop growing or die back, setting you weeks behind.

Direct sowing outdoors: when soil is warm enough

Direct sowing works in warm climates where the growing season is long and soil warms quickly. Plant seeds directly in garden soil only after your last frost date has passed and soil temperature reaches at least 60°F, ideally 70°F or warmer. Use a soil thermometer to check — do not guess by air temperature.

Plant seeds 1/4 inch deep and 2 to 3 inches apart. Keep soil consistently moist until seedlings emerge, which takes 5 to 10 days in warm soil. Once seedlings have two true leaves, thin them to 12 to 24 inches apart depending on variety. Direct-sown plants usually catch up to transplanted seedlings by mid-summer and may even flower earlier because their roots were never disturbed.

The downside of direct sowing is that you lose 6 to 8 weeks of growing time compared to starting indoors. In short-season areas (where first fall frost comes in September), direct sowing may not give you enough time to harvest before cold weather arrives.

Matching variety maturity to your growing season

Tomato varieties mature at different speeds. Cherry tomatoes often mature in 60 to 70 days. Large slicing tomatoes like 'Brandywine' take 75 to 85 days or longer. Check the seed packet for the variety's days to maturity, then count backward from your first fall frost date to confirm you have enough time.

For example, if your first fall frost is October 15 and a tomato variety needs 80 days to mature, count back 80 days to land on August 26. That is the latest date you can transplant seedlings outdoors and still harvest before frost. If you are starting seeds indoors 6 to 8 weeks before your last spring frost (say, May 15), you will transplant around mid-July, giving you plenty of time.

In short-season areas, choose early varieties and start seeds indoors rather than direct sowing. In long-season areas, you have flexibility with both timing and variety choice.

Adjusting timing for your climate

Gardeners in cool climates (zones 3 to 5) must start seeds indoors because the outdoor growing season is short. Start seeds in late March or early April, transplant in late May or early June, and harvest before September frost. Waiting to direct sow means missing the season entirely.

Gardeners in warm climates (zones 8 to 10) can direct sow in spring and often grow a second crop in late summer for fall harvest. Some warm-climate gardeners skip spring planting and start seeds in June or July for a fall and winter harvest when temperatures cool and disease pressure drops.

Gardeners in moderate climates (zones 6 to 7) have the most flexibility. You can start seeds indoors in March, direct sow in May, or even do both — some indoors for early harvest and some direct-sown for continuous production.

Frequently Asked Questions

What if I start seeds too early?

Seedlings become tall, thin, and weak while waiting for warm weather to transplant. They may also outgrow their containers and become root-bound. If you start too early, you can pot seedlings up into larger containers to give roots more room, but it is better to time planting to your frost date from the start.

Can I start seeds outdoors in a cold frame or greenhouse?

Yes. A cold frame or unheated greenhouse extends the season and protects seedlings from frost and wind while they harden off naturally. This works well in moderate climates but requires monitoring temperature and venting on warm days to prevent overheating.

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. True leaves are the second set that appears — the first leaves (cotyledons) are smooth and oval. Wait until after your last frost date and soil has warmed before moving them outside permanently.

What if my area has a very short growing season?

Start seeds indoors 8 weeks before your last frost date and choose early-maturing varieties (60 to 70 days). Transplant as soon as soil is warm enough, use black plastic mulch to warm soil further, and consider growing in containers you can move to a sunny spot or cover with frost cloth if an early frost threatens.

Can I save seeds from my tomatoes to plant next year?

Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes produce seeds that do not grow true to type. Let a ripe tomato fully mature on the plant, scoop out seeds and pulp into a jar, ferment for 2 to 3 days, rinse, dry completely on a plate, and store in a cool, dry place. Plant them the following year using the same timing as store-bought seeds.