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

The timing depends on where you live and whether you start seeds indoors or plant them straight in the ground. Indoor seed starting gives you a head start on the growing season—seedlings need 6 to 8 weeks under lights before they are ready to move outside. Direct sowing (planting seeds straight in the soil) works only after the soil has warmed to at least 60°F, which happens later in spring.

To find your last spring frost date, search online for "[your city] last frost date" or check with your local cooperative extension office. Once you know that date, count backward 6 to 8 weeks to find when to start seeds indoors. For example, if your last frost is May 15, start seeds indoors around March 20 to April 1. If you prefer direct sowing, wait until soil temperature reaches 60°F—usually 2 to 3 weeks after your last frost date.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date so seedlings are ready to transplant when soil warms.
  • Direct-sow seeds outdoors only after soil temperature reaches 60°F, which typically occurs 2 to 3 weeks after your last frost date.
  • Indoor-started seedlings need 12 to 16 hours of light per day under grow lights or on a sunny windowsill to grow stocky and strong.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting them in the garden.
  • Tomato seeds germinate best in soil kept between 70°F and 80°F, so a heat mat speeds germination from 5 to 10 days down to 3 to 5 days.

Starting Seeds Indoors Under Lights

Fill seed trays or small pots with seed-starting mix—a light, sterile blend of peat moss, coconut coir, or perlite that drains well and does not compact. Moisten the mix before filling containers so it is evenly damp but not waterlogged. Plant one seed per cell, pressing it lightly into the soil about 1/4 inch deep. Tomato seeds do not need light to germinate, so cover the tray with a humidity dome or plastic wrap to keep moisture in.

Place the tray in a warm spot—ideally 70°F to 80°F. A heat mat under the tray speeds germination to 3 to 5 days instead of 5 to 10. Once seedlings emerge and push through the soil, remove the cover when ready and move the tray under grow lights or to a bright south-facing window. Seedlings need 12 to 16 hours of light per day to stay compact and stocky. Without enough light, they become leggy (tall and thin with weak stems).

Water seedlings from below by setting the tray in a shallow pan of water for 10 to 15 minutes, then removing it to drain. This prevents damping-off disease, a fungal problem that kills seedlings at soil level. Keep soil moist but never soggy. When seedlings develop their first true leaves (the second set of leaves that appear), thin them to the strongest one per cell, or transplant each into its own small pot.

Hardening Off Seedlings Before Transplanting

Indoor seedlings are tender and will wilt or die if moved directly into the garden. Hardening off means gradually exposing them to outdoor sun, wind, and temperature swings over 7 to 10 days. Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase their time outside and move them into brighter light.

By day 7 or 8, seedlings should spend the full day outside in dappled shade or morning sun. On the final days, leave them out in full sun for a few hours if temperatures stay above 50°F. If frost threatens overnight, bring them back inside. Once seedlings are 6 to 8 inches tall with sturdy stems and at least two sets of true leaves, they are ready to plant in the garden.

Direct Sowing Seeds Outdoors

Direct sowing works best in warm climates or when you start late in spring. Wait until soil temperature reaches 60°F—use a soil thermometer to check, or wait until 2 to 3 weeks after your last frost date when soil has warmed. Plant seeds 1/4 inch deep in rows or hills, spacing them 2 to 3 inches apart. Water gently so seeds stay in place.

Seeds germinate in 5 to 10 days in warm soil. Once seedlings have two true leaves, thin them to 12 to 24 inches apart depending on the variety—check the seed packet for spacing. Direct-sown plants often catch up to indoor-started ones by mid-summer, but you lose 4 to 6 weeks of growing time, which matters in short-season areas.

Soil Temperature and Frost Protection

Tomatoes are frost-tender—even a light frost kills seedlings and young plants. Never plant outdoors until after your last spring frost date, and soil should be at least 60°F. Cold soil slows germination and can cause seeds to rot. If you transplant too early and frost threatens, cover plants with row covers, cloches, or blankets on the night of the frost.

In cool climates, warm the soil before planting by laying black plastic mulch over the bed for 1 to 2 weeks. This traps heat and raises soil temperature 5 to 10 degrees. Remove the plastic once plants are in the ground, or cut holes in it and plant through.

Timing for Different Tomato Types

Determinate varieties (bush types that ripen all at once) and early-maturing varieties can be started indoors 5 to 6 weeks before the last frost instead of 8 weeks. Indeterminate varieties (vining types that produce all season) benefit from the full 7 to 8 weeks because they need a longer head start to produce fruit before fall frost arrives. Check your seed packet for days to maturity—this tells you how long from transplant to first ripe fruit.

In very short-season areas (last frost in June or later), start seeds indoors as early as February to give plants enough time to set fruit before fall frost. In warm climates where frost rarely occurs, you can direct-sow in late winter or early spring and again in mid-summer for a fall crop.

Common Mistakes to Avoid

Starting seeds too early leads to overgrown, root-bound seedlings by transplant time. If seedlings are already flowering when you move them outside, they often drop flowers and produce less fruit. Starting too late means seedlings are too small when frost ends, delaying your harvest. Planting outdoors before soil warms causes seeds to rot or sit dormant for weeks.

Overwatering seedlings is the most common mistake—it causes damping-off disease and root rot. Water only when the top of the soil feels dry to the touch. Underwatering is less common but causes wilting and stunted growth. Leggy seedlings result from too little light, not from watering or temperature. If seedlings are already leggy, bury them deeper when transplanting—they will develop roots along the buried stem.

Frequently Asked Questions

Can I start tomato seeds in a regular potting mix instead of seed-starting mix?

Regular potting mix works but is heavier and holds more moisture, which increases damping-off risk. Seed-starting mix is lighter and drains faster, making it safer for seedlings. If you only have potting mix, mix it half-and-half with perlite or coconut coir to improve drainage.

What if I missed the indoor seed-starting window?

Direct-sow seeds outdoors once soil reaches 60°F. You will lose 4 to 6 weeks of growing time, but plants often catch up by mid-summer. In short-season areas, buy transplants from a nursery instead—they are already hardened off and ready to plant.

Do I need grow lights, or can I use a sunny windowsill?

A bright south-facing window works if you have 12+ hours of direct sun daily. Most indoor windows do not provide enough light, causing leggy seedlings. Inexpensive LED grow lights (set on a timer for 14 to 16 hours per day) produce much sturdier plants and cost little to run.

How do I know when seedlings are ready to transplant outside?

Seedlings are ready when they are 6 to 8 inches tall, have sturdy stems, at least two sets of true leaves, and have been hardened off for 7 to 10 days. Soil should be at least 60°F and all frost danger should have passed.

Can I save tomato seeds from my plants to start next year?

Yes, but only from heirloom or open-pollinated varieties. Hybrid varieties do not produce seeds that grow true to type. Ferment seeds in water for 2 to 3 days to remove the gel coating, rinse well, dry completely on a paper towel, and store in a cool, dry place. They stay viable for 4 to 6 years.