Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow them outdoors after soil warms to 60°F

The timing depends on where you live and whether you start seeds indoors or plant them straight in the ground. Most home gardeners start seeds indoors under lights in late winter, then move seedlings outside after the frost danger passes. If you live in a warm climate or want to skip the indoor step, you can plant seeds directly in garden soil once the soil temperature stays above 60°F and nighttime air temperatures stay above 50°F.

Your last spring frost date is the key number. This is the average date when frost stops occurring in your area. Count backward 6 to 8 weeks from that date to find when to start seeds indoors. For example, if your last frost date is May 15, start seeds indoors around March 20 to April 1. You can find your frost date by searching your zip code on the National Weather Service website or asking at a local garden center.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your zip code online.
  • Tomato seedlings need 12 to 16 hours of light per day indoors, either from a grow light or a sunny south-facing window.
  • Move seedlings outdoors only after nighttime temperatures stay above 50°F and soil warms to at least 60°F, usually 2 to 3 weeks after your last frost date.
  • Direct sowing seeds outdoors works in warm climates but takes longer to produce fruit than starting indoors.
  • Tomatoes planted too early outdoors will sit dormant in cold soil and may rot before they sprout.

How to count backward from your frost date

Your last spring frost date is printed on most local gardening calendars and is available free from the National Weather Service. Search "last frost date" plus your city name, or call your county extension office. Once you have the date, count back 42 to 56 days (6 to 8 weeks). Mark that date on a calendar—that is when to plant seeds in trays indoors.

If you miss the window by a week or two, you can still start seeds, but your plants will be smaller when they go outside and will produce fruit later in the season. If you start too early, seedlings become leggy (tall and thin) and weak while waiting for warm soil outdoors.

Starting seeds indoors under lights

Fill seed trays or small pots with seed-starting mix (a light, sterile blend sold at garden centers). Plant one seed per cell, cover lightly with mix, and water until damp. Place trays under a grow light or on a sunny windowsill. Tomato seeds germinate in 5 to 10 days at room temperature (68 to 75°F) and do not need heat mats, though a mat speeds germination slightly.

Once seedlings emerge, they need 12 to 16 hours of light per day. A grow light hung 2 to 3 inches above the trays works best; a sunny south-facing window works if you have one. Without enough light, seedlings stretch toward the window and become weak. Water when the soil surface feels dry, but do not let trays sit in standing water. At 2 to 3 weeks old, when seedlings have their first true leaves (the second set of leaves that appear), thin to the strongest plant per cell or transplant into larger pots.

Hardening off seedlings before moving them outside

Seedlings grown indoors are tender and will wilt or die if moved directly into outdoor sun and wind. Spend 7 to 10 days hardening them off—gradually exposing them to outdoor conditions. Start by placing trays in a sheltered spot outdoors for 1 to 2 hours on day one, then increase time and exposure each day. By day 7 or 8, seedlings can stay outside all day. By day 10, they can stay out overnight if temperatures stay above 50°F.

Watch the weather forecast during hardening off. If frost threatens, bring seedlings back indoors. If a heat wave arrives, move them to afternoon shade to prevent wilting.

Transplanting seedlings into the garden

Plant seedlings outdoors after your last frost date has passed and soil has warmed to at least 60°F. Nighttime temperatures should stay above 50°F. Dig a hole slightly deeper than the seedling's root ball, bury the stem up to the first true leaves (tomatoes root along buried stems), and water well. Space plants 24 to 36 inches apart depending on variety.

If you have a long growing season, you can plant a second round of seeds indoors 4 to 6 weeks after the first planting for a fall harvest. This works best in areas where the first frost does not arrive until October or November.

Direct sowing seeds outdoors

In warm climates or if you prefer not to start seeds indoors, you can plant seeds directly in garden soil once soil temperature reaches 60°F and stays there. Plant seeds half an inch deep, 6 inches apart, then thin seedlings to 24 to 36 inches apart once they have two true leaves. Direct-sown tomatoes take longer to produce fruit—usually 70 to 85 days from planting to first harvest instead of 50 to 70 days for transplants—because they spend their first weeks in the ground rather than growing under lights.

Direct sowing works well in southern regions where the growing season is long. In northern areas, it often does not leave enough time for fruit to mature before frost arrives.

Common timing mistakes and how to avoid them

Planting seeds too early indoors creates leggy, weak seedlings that struggle after transplanting. Planting outdoors too early kills seeds in cold soil or causes them to rot. Hardening off too quickly—moving seedlings straight from indoors to full sun—causes wilting and leaf damage. Waiting too long to transplant means seedlings become root-bound in their trays and take longer to recover in the garden.

The safest approach is to follow your frost date, not the calendar. A late frost in May can kill transplants that went out in April. A warm spell in March does not mean soil is warm enough yet. Soil thermometers cost a few dollars and remove the guesswork—wait until soil reads 60°F before planting outdoors.

Frequently Asked Questions

Can I plant tomato seeds in February?

Only if your last frost date is in late April or later. Seedlings started in February will be overgrown and weak by the time soil warms enough to plant them outside. Count back 6 to 8 weeks from your frost date instead of picking a calendar date.

What if I start seeds too late?

You can still plant them, but fruit will ripen later in the season. In short growing seasons, late-planted tomatoes may not finish ripening before frost. If you miss the ideal window by more than a few weeks, direct sowing outdoors may be faster than starting indoors.

Do tomato seeds need light to germinate?

No. Tomato seeds germinate in darkness and do not need light until after they sprout. Once seedlings emerge, they need 12 to 16 hours of light per day to grow strong. A grow light or sunny window works; without light, seedlings become tall and spindly.

Can I plant seedlings outside if there's still frost at night?

No. Tomatoes die at temperatures below 50°F. Wait until nighttime lows stay above 50°F consistently, usually 1 to 2 weeks after your last frost date. If an unexpected frost is forecast after you plant, cover seedlings with cloth or move pots indoors.