Start seeds indoors 6 to 8 weeks before your last spring frost

Tomato seedlings need to be started indoors because tomatoes require a long growing season — typically 70 to 85 days from transplant to first fruit. If you wait to plant seeds directly in the garden after your last frost date, you will not harvest until late fall or not at all. Starting seeds indoors gives you a head start and lets you move sturdy seedlings into the soil when conditions are right.

The timing matters. Count backward from your area's last spring frost date (your local cooperative extension office has this for your zip code) and start seeds 6 to 8 weeks earlier. If your last frost is May 15, start seeds around March 20. This gives seedlings time to develop true leaves and a small root system without becoming leggy or root-bound before transplant day.

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.
  • Use a sterile seed-starting mix (not garden soil), keep it moist but not waterlogged, and provide 14 to 16 hours of direct light daily from a grow light or sunny window.
  • Seedlings are ready to transplant outdoors when they have at least two sets of true leaves and are 3 to 4 inches tall, usually 6 to 8 weeks after planting.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before moving them to the garden permanently.
  • Bury the stem deeper than it was growing in the pot when you transplant — tomatoes will root along the buried stem and develop a stronger plant.

Gather containers, soil, and light before you plant

You need a container with drainage holes, a sterile seed-starting mix, and a reliable light source. Containers can be seed trays, small pots, yogurt cups with holes drilled in the bottom, or newspaper pots — anything that holds soil and drains water. Avoid reusing old garden soil or potting soil from last year; use fresh seed-starting mix, which is lighter and sterile, reducing the risk of damping-off disease (a fungal infection that kills seedlings at soil level).

Light is critical. Tomato seedlings need 14 to 16 hours of light daily. A south-facing window may work in spring, but most indoor growers use a grow light — a fluorescent or LED fixture positioned 2 to 3 inches above the seedlings. Adjust the light height as seedlings grow so they stay close to the source. Without enough light, seedlings become tall, thin, and weak (called "leggy"), and they will struggle after transplant.

Fill containers, moisten the mix, and plant seeds at the right depth

Fill your containers with seed-starting mix and water it thoroughly until it drains from the bottom. Let it sit for a few minutes so the mix absorbs water evenly. Tomato seeds are small; plant them about 1/4 inch deep — roughly the thickness of a pencil lead. You can plant 2 to 3 seeds per cell or pot; you will thin to the strongest seedling later.

Cover the container with a humidity dome or plastic wrap to trap moisture and speed germination. Place it in a warm spot — tomato seeds germinate best at 70 to 80°F. A heat mat under the container helps if your home is cool. Check daily and remove the cover as soon as you see green shoots, usually within 5 to 10 days.

Water carefully and keep the soil consistently moist

Once seedlings emerge, remove the humidity cover and move the container under your grow light. Water from below by setting the pot in a tray of water and letting it soak up moisture through the drainage holes, or water gently from above with a spray bottle. The goal is soil that stays moist like a wrung-out sponge — never soggy, never bone-dry.

Overwatering is the most common mistake. Wet soil invites damping-off and root rot. Check the soil surface daily; if the top feels dry to your finger, water. If it still feels moist, wait another day. As seedlings grow and the container fills with roots, they will dry out faster and need more frequent watering.

Thin seedlings to one per container when they have true leaves

Within 1 to 2 weeks, seedlings will sprout their first leaves — these are cotyledons (seed leaves) and look straightforward and oval. A week or two later, the next set of leaves will appear; these are true leaves and look like actual tomato leaves with serrated edges. Once you see true leaves, thin the container to the strongest seedling by snipping the others at soil level with scissors. Do not pull them out, because pulling disturbs the roots of the keeper seedling.

If you planted one seed per cell, no thinning is needed. Either way, keep the light close and the soil moist. Seedlings will grow steadily over the next 4 to 6 weeks.

Fertilize weekly once seedlings develop true leaves

Seed-starting mix contains little to no nutrients. Once true leaves appear, begin feeding seedlings weekly with a diluted liquid fertilizer — use half the strength recommended on the bottle. A balanced fertilizer (equal parts nitrogen, phosphorus, and potassium) works well, or use a fertilizer labeled for seedlings. Mix it into your watering can and explore it when you water.

Watch for signs of nutrient deficiency: pale or yellowing leaves, slow growth, or purple-tinged stems. If you see these, increase feeding to twice weekly. Stop fertilizing a week or two before transplanting outdoors, so seedlings are not too soft and tender.

Harden off seedlings 7 to 10 days before transplanting

Hardening off means gradually exposing seedlings to outdoor conditions — wind, direct sun, and temperature swings — so they toughen up before permanent transplant. Start 7 to 10 days before your last frost date. On day one, place seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase time outside and gradually move them into more sun and wind.

By day 7 or 8, seedlings should spend the full day and night outdoors in a spot that gets at least 6 hours of direct sun. Bring them in if temperatures drop below 50°F. After 7 to 10 days of this gradual transition, they are ready to plant in the garden.

Transplant seedlings into the garden when they are 3 to 4 inches tall

Seedlings are ready to move outdoors when they have at least two sets of true leaves and stand 3 to 4 inches tall — usually 6 to 8 weeks after planting. Wait until after your last spring frost date and soil temperature is at least 60°F (ideally 65°F or warmer). 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, creating a stronger root system.

Space seedlings 24 to 36 inches apart, depending on the variety. Water well after planting and keep soil moist for the first week. Mulch around the base with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature.

Frequently Asked Questions

Can I use regular potting soil instead of seed-starting mix?

Regular potting soil is heavier and can compact around seedlings, restricting root growth and increasing damping-off risk. Seed-starting mix is lighter and sterile. If you must use potting soil, mix it half-and-half with perlite or coconut coir to lighten it. Fresh seed-starting mix is inexpensive and worth the investment.

What if my seedlings are getting leggy?

Legginess means insufficient light or too much heat. Move the grow light closer (2 to 3 inches above the tops) and run it 14 to 16 hours daily. Lower the temperature to 65 to 70°F if possible. Leggy seedlings can still survive transplant, but they will be weaker. Once outdoors in full sun, they often recover and grow normally.

Do I need a heat mat?

A heat mat speeds germination and is helpful if your home is below 70°F, but it is not required. Seeds will germinate in a cool room; it just takes longer — 2 to 3 weeks instead of 5 to 10 days. Once seedlings emerge, remove the heat mat and keep the room at 65 to 75°F for healthy growth.

How do I know if seedlings are ready to transplant?

Seedlings are ready when they are 3 to 4 inches tall with at least two sets of true leaves and a visible root system (roots may show at drainage holes). If roots are circling the pot densely, transplant sooner rather than later to prevent root-bound stress. Timing is usually 6 to 8 weeks after planting.

Can I save seeds from my seedlings to grow next year?

Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes (often labeled F1) will not produce seeds that grow true to type. Let a ripe tomato stay on the plant until it is very soft, scoop out the seeds and gel into a jar, ferment for 2 to 3 days, rinse, and dry completely on a paper towel before storing in a cool, dry place.