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

The timing depends on whether you're starting seeds indoors under lights or planting them straight into the garden. Indoor seed starting gives you a head start—seedlings are ready to transplant outside right when the soil warms up. Direct sowing (planting seeds straight in the ground) is simpler but means a later harvest, since tomato seeds won't germinate in cold soil.

Your last spring frost date is the key number. Find it by searching "[your city] last frost date"—your local cooperative extension office publishes this for every county. Count backward 6 to 8 weeks from that date, and that's when to start seeds indoors. If your last frost is May 15, for example, start seeds indoors around March 20 to April 1.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find from your local cooperative extension office.
  • Tomato seeds need soil at least 60°F to germinate; they germinate best at 70–80°F and will rot in cold, wet soil.
  • Seedlings started indoors need 14 to 16 hours of light daily from a grow light or bright window to stay stocky and strong.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • Direct-sown seeds can go in the ground after the last frost date and soil has warmed, but they will produce fruit 2 to 3 weeks later than transplants.

How to calculate your seed-starting date

Your last spring frost date is the average date of the final frost in your area. After that date, nighttime temperatures stay above freezing. Tomato seedlings and transplants cannot survive frost, so this date marks the earliest you can safely move them outdoors.

To find your date, visit your state's cooperative extension website and search for your county. The University of Minnesota, University of Wisconsin, and similar land-grant universities maintain these records for every region. You can also search "last frost date [your city]" online—most results will show the date clearly.

Once you have the date, count back 6 to 8 weeks. Most tomato varieties are ready to transplant at 6 weeks; slower-growing varieties or those you want larger may need 8 weeks. Mark that date on a calendar—that's your indoor seed-starting day.

Indoor seed starting: light, warmth, and timing

Tomato seeds germinate best in soil between 70°F and 80°F. They will sprout in cooler soil, but more slowly and with higher risk of rot. A seed-starting heat mat (a waterproof heating pad placed under seed trays) keeps soil at the right temperature and speeds germination to 5 to 10 days. Without one, germination takes 10 to 14 days in a warm room.

Once seedlings emerge, remove the heat mat and move trays to a bright location. Seedlings need 14 to 16 hours of light daily to grow stocky and strong. A south-facing window works if it receives direct sun most of the day; otherwise, use a grow light (LED or fluorescent) positioned 2 to 3 inches above the seedlings. Raise the light as they grow so it stays that distance away.

Water seedlings when the soil surface feels dry to your finger. Use room-temperature water and water from below (by setting the tray in a shallow pan of water) rather than overhead, which can spread fungal disease. Keep air moving gently with a small fan to strengthen stems.

Hardening off: preparing seedlings for outdoor conditions

Hardening off means gradually exposing seedlings to outdoor sun, wind, and temperature swings so they toughen up before transplanting. This step takes 7 to 10 days and prevents transplant shock or sunburn.

Start 7 to 10 days before your last frost date. On day one, place seedlings outdoors in a sheltered, shaded spot for 2 to 3 hours. Each day, increase time outdoors and gradually move them into more direct sun. By day 7 or 8, they should be outside all day. If nighttime temperatures will stay above 50°F, leave them out overnight. If frost is forecast, bring them back inside.

During hardening off, seedlings may look stressed or slow their growth—this is normal. They're building stronger stems and leaves to handle outdoor conditions.

Transplanting seedlings into the garden

Transplant seedlings outdoors after your last spring frost date and when soil temperature reaches at least 60°F (ideally 65°F or warmer). Soil that's too cold slows root growth and increases disease risk. You can check soil temperature with a soil thermometer, or wait until nighttime lows stay consistently above 50°F, which usually means soil is warm enough.

Dig a hole slightly deeper than the seedling's root ball. Tomatoes can be planted deeper than they were growing in the pot—bury the stem up to the lowest leaves, and roots will form along the buried stem, creating a stronger root system. Space plants 24 to 36 inches apart depending on variety (check the seed packet). Water well after planting.

Direct sowing seeds outdoors

If you prefer not to start seeds indoors, you can plant seeds directly in the garden after the last frost date and when soil reaches 60°F. Plant seeds ½ inch deep, 6 inches apart. Thin seedlings to final spacing (24 to 36 inches) once they have two true leaves.

Direct-sown tomatoes will produce fruit 2 to 3 weeks later than transplants, since they start from seed rather than a head-start seedling. This method works well in warm climates with long growing seasons but may not give enough time for fruit to ripen in short-season areas.

Adjusting timing for your climate

In short-season areas (northern regions, high elevation), start seeds indoors earlier—aim for the full 8 weeks before the last frost date—so plants are large enough to produce fruit before fall frost arrives. In long-season areas (warm climates, low elevation), you have more flexibility; 6 weeks is usually enough.

If you live in a very warm climate where frost rarely occurs, you can start seeds indoors in late winter (January or February) and transplant in early spring, or start a second crop in midsummer for a fall harvest. Check with your local cooperative extension for variety recommendations and timing for your specific region.

Frequently Asked Questions

What if I miss my seed-starting date?

You can still plant seeds indoors up to 4 weeks before the last frost date, though seedlings will be smaller at transplant time. They will still grow and produce, just slightly later. Alternatively, direct-sow seeds outdoors after the last frost date—they take longer to produce fruit but require no indoor setup.

Can I start seeds earlier to get bigger plants?

Starting seeds more than 8 weeks early usually results in leggy, overgrown seedlings that are harder to transplant and more prone to disease. Stick to the 6 to 8 week window for stocky, healthy plants ready to go in the ground right when conditions are right.

What if my seedlings get too tall and spindly?

Spindly seedlings mean insufficient light. Move the grow light closer (2 to 3 inches above the tops) or add more hours of light per day. A small fan also strengthens stems. If seedlings are already too tall, you can bury them deeper when transplanting—tomatoes will root along the buried stem.

Do I need a heat mat to start tomato seeds?

A heat mat speeds germination and is helpful but not required. Seeds will germinate in a warm room (70°F or above) in 10 to 14 days without one. A heat mat cuts that to 5 to 10 days and is worth the cost if you're starting many seeds or live in a cool climate.

Can I transplant seedlings before the last frost date if I use row covers?

Row covers (lightweight fabric) provide some frost protection but are not reliable for tomatoes, which are very frost-sensitive. Wait until after the last frost date and soil is warm. The few weeks you gain by transplanting early are not worth the risk of losing plants to a late frost.