Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or directly in the garden after soil warms to 60°F

The timing depends on whether you start seeds indoors under lights or sow them straight into garden soil. Indoor seeds need a head start because seedlings take 6 to 8 weeks to grow large enough to transplant outside. Direct seeding into warm soil is faster but only works after frost danger has passed and the ground has warmed up. Your local frost date is the key number—find it by searching "[your city] last frost date" or checking the USDA Plant Hardiness Zone Map for your zip code.

Tomatoes are frost-sensitive plants. A single frost will kill seedlings and young transplants. Even soil that feels warm to your hand may still be too cold for seeds to germinate reliably. Soil thermometers cost $5 to $10 and take the guesswork out of direct seeding.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your city name plus "last frost date."
  • Transplant seedlings outdoors only after the last frost date has passed and nighttime temperatures stay above 50°F.
  • Direct sow seeds into garden soil only when soil temperature reaches 60°F or higher, measured with a soil thermometer.
  • Indoor seedlings need 12 to 16 hours of light per day from a grow light or bright window to grow stocky and strong.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.

Finding Your Last Frost Date

Your last frost date is the average date of the final frost in spring for your location. It varies by region—a gardener in Minnesota may have a last frost date in late May, while someone in Georgia might have one in mid-April. Search "[your city name] last frost date" online, or use the USDA Plant Hardiness Zone Map by entering your zip code. Many state university extension offices also publish frost date calendars for their regions.

This date is not a may provide—frost can arrive after it, or the season can stay warm. But it gives you a reliable target. Count backward 6 to 8 weeks from that date to find when to start seeds indoors. If your last frost date is May 15, for example, start seeds indoors around March 20 to April 1.

Starting Seeds Indoors Under Lights

Indoor seed starting gives you larger, stronger transplants ready to produce fruit earlier in the season. Fill seed trays or small pots with seed-starting mix (a sterile, lightweight blend), moisten it, and plant one seed per cell about 1/4 inch deep. Keep the soil consistently moist but not waterlogged. Seeds germinate in 5 to 10 days at room temperature (68°F to 75°F is ideal).

Once seedlings sprout, move them under grow lights when ready. Position the lights 2 to 3 inches above the seedlings and run them for 12 to 16 hours per day. Without adequate light, seedlings become tall, thin, and weak—a condition called "leggy." A basic fluorescent or LED grow light costs $20 to $50 and makes a large difference. Keep the room temperature between 65°F and 75°F during the day.

Water seedlings from below by setting the tray in a shallow pan of water for 10 to 15 minutes, then draining it. This prevents damping off, a fungal disease that kills seedlings at soil level. Once seedlings develop their first true leaves (the second set to appear), begin feeding them weekly with diluted liquid fertilizer at half strength.

Hardening Off Seedlings Before Transplanting

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

By day 7 or 8, seedlings should spend the full day outside. Bring them in at night if temperatures drop below 50°F. On the final days, leave them out overnight if nighttime temperatures stay above 50°F. This process thickens the stems and prepares the leaves for direct sun, so transplants establish faster and produce sooner.

Transplanting Seedlings Into the Garden

Plant hardened-off seedlings outdoors only after your last frost date has passed and nighttime temperatures consistently stay above 50°F. Soil temperature should be at least 60°F—use a soil thermometer to check. Dig a hole slightly deeper than the seedling's root ball and bury the stem up to the first true leaves; tomatoes will root along the buried stem and develop a stronger root system.

Space plants 24 to 36 inches apart depending on the variety. Determinate (bush) types need less space; indeterminate (vining) types need more. Water gently after planting and keep soil consistently moist for the first week while roots establish. Mulch around the base with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature.

Direct Seeding Tomatoes Into the Garden

Direct seeding saves the cost of lights and seed trays but requires patience and warm soil. Wait until soil temperature reaches 60°F—ideally 65°F or warmer for faster germination. Plant seeds 1/4 inch deep in groups of 2 to 3 seeds, spaced 24 to 36 inches apart. Thin seedlings to one per spot once they develop their first true leaves.

Keep soil consistently moist until seedlings are 3 to 4 inches tall. Direct-seeded plants typically flower and fruit 1 to 2 weeks later than transplants, but they often develop stronger root systems because they are never disturbed. This method works best in regions with long growing seasons or when you are willing to wait longer for the first harvest.

Adjusting Timing for Your Growing Season

If your growing season is short (fewer than 70 frost-free days), start seeds indoors earlier—8 to 10 weeks before your last frost date—to give plants time to mature before fall frost arrives. If your season is long, you can start seeds later or direct seed and still harvest before cold weather. Check the seed packet for the variety's days to maturity, which tells you how many days from transplanting to first ripe fruit.

In very hot climates, you may also start a second crop of seeds in mid-summer for a fall harvest. Plant these seeds indoors in June or July so transplants are ready to go into the garden in August, after the worst summer heat passes. This extends your tomato season into cooler fall months when plants often produce more reliably.

Frequently Asked Questions

What if I miss my planting window?

You can still plant later in the season, though you will harvest fewer tomatoes. Start seeds indoors up to 10 weeks before your first fall frost date instead. Many regions have a second planting window in mid-summer for a fall crop. Check your seed packet for days to maturity to see if you have enough time.

Can I plant tomato seeds directly outdoors in spring?

Yes, but only after soil warms to 60°F and frost danger has passed. Direct seeding takes longer to produce fruit than transplants—usually 1 to 2 weeks longer—because seeds need time to germinate and seedlings need time to grow. This method works best in warm climates with long growing seasons.

Do I need grow lights to start seeds indoors?

A bright south-facing window can work if you have one, but seedlings often become leggy without supplemental light. A basic fluorescent or LED grow light is inexpensive and produces much stronger seedlings. If you use a window, rotate trays daily so seedlings grow evenly.

What temperature kills tomato seedlings?

Frost (32°F or below) kills seedlings and young transplants. Even temperatures below 50°F slow growth and can damage leaves. Wait until nighttime temperatures stay consistently above 50°F before moving seedlings outdoors permanently.

How deep should I plant tomato seeds?

Plant seeds 1/4 inch deep indoors or outdoors. Planting too deep delays germination or prevents it entirely. When transplanting seedlings, you can bury the stem deeper—up to the first true leaves—to encourage a stronger root system.