Start seeds indoors 6 to 8 weeks before your last spring frost, or direct sow them outside after soil warms

Tomato seeds germinate best in warm soil—between 70 and 80 degrees Fahrenheit—and they need light once they sprout. If you live where frost arrives late, starting seeds indoors under grow lights gives you a head start and stronger plants by transplant time. If your growing season is long and your soil warms early, you can plant seeds directly in the garden and skip the indoor step altogether. The choice depends on your climate and how much time you have before your first frost date.

Your local cooperative extension office has your frost dates. Search "[your county] cooperative extension frost date" to find the exact dates for your area. Count backward 6 to 8 weeks from that date to know when to start seeds indoors.

Key Takeaways

  • Indoor seed starting requires a seed tray, seed-starting mix, grow lights, and a heat mat or warm location; outdoor direct sowing needs only warm soil and seeds.
  • Tomato seeds need soil between 70 and 80 degrees to germinate, which usually takes 5 to 10 days.
  • Seedlings need 14 to 16 hours of light per day from a grow light positioned 2 to 3 inches above the leaves.
  • Harden off indoor seedlings by exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • Direct-sown seeds in the garden should go 1/4 inch deep in soil that has warmed to at least 60 degrees Fahrenheit.

Setting up for indoor seed starting

You will need a seed tray with drainage holes, seed-starting mix (a sterile, lightweight blend—not garden soil), a grow light, and a heat source. Seed-starting mix is sold at garden centers under names like Jiffy Seed Starting Mix or Pro-Mix. Fill your tray with moistened seed-starting mix, leaving 1/4 inch at the top.

Place one tomato seed per cell, pressing it gently into the mix. Cover it with a thin layer of seed-starting mix—tomato seeds need darkness to germinate, so cover them completely. Mist the surface with water until it is damp but not waterlogged. Tomato seeds do not need light until they sprout, but they do need warmth. A heat mat under your seed tray will keep soil at 75 to 80 degrees. If you do not have a heat mat, place the tray on top of a refrigerator, on a heating pad set to low, or in a warm corner of your home—anywhere the temperature stays consistently warm.

Check the soil daily. It should feel damp, not soggy. Soggy soil causes damping off, a fungal disease that kills seedlings at the soil line. If the surface dries out, mist it lightly with water.

Germination and the first light stage

Tomato seeds usually sprout in 5 to 10 days at 75 to 80 degrees. As soon as you see the first green shoot, remove the cover and move the tray under a grow light. Position the light 2 to 3 inches above the seedlings and leave it on for 14 to 16 hours per day. A straightforward fluorescent shop light works well and costs less than a specialized grow light. Set it on a timer so the seedlings get consistent light without you having to turn it on and off by hand.

Keep the soil damp but not wet. Water from below by setting the tray in a shallow pan of water for 10 minutes, then removing it and letting it drain. This prevents the soil from splashing onto the leaves and reduces disease risk.

Once seedlings have their first true leaves—the second set of leaves that look like actual tomato leaves, not the smooth seed leaves—you can remove the heat mat. The room temperature is warm enough now. Seedlings grow fastest at 65 to 70 degrees during the day and 55 to 60 degrees at night, so a cool basement or unheated room works well.

Thinning and transplanting into larger pots

If you planted multiple seeds per cell, thin them to one strong seedling per cell once they have true leaves. Snip the weaker ones at soil level with scissors rather than pulling them out, which disturbs the roots of the keeper plant.

When seedlings are 2 to 3 inches tall and have 2 to 3 sets of true leaves, they are ready to move into larger pots—4-inch pots work well. Tomato seedlings can be planted deeper than they were growing; bury them up to the first true leaves. Tomatoes will grow roots all along the buried stem, creating a stronger root system. Use the same seed-starting mix or a general-purpose potting mix. Keep the light 2 to 3 inches above the new growth as the seedlings get taller.

Continue watering from below and keep the soil consistently damp. Once seedlings are in larger pots, you can begin fertilizing with a diluted liquid fertilizer every 2 weeks—follow the label rate and cut it in half.

Hardening off before transplanting outdoors

Indoor seedlings are tender and will wilt or die if moved directly into the garden. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing the pots in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase the time outside and gradually move them into more sunlight. By day 7 or 8, they should be in full sun for most of the day. Bring them inside at night until nighttime temperatures stay above 50 degrees.

Harden off seedlings only after your last frost date has passed and nighttime temperatures stay above 50 degrees. If you harden off too early and a frost comes, the seedlings will die.

Direct sowing seeds outdoors

If you prefer to skip the indoor step, you can plant seeds directly in the garden after soil has warmed to at least 60 degrees Fahrenheit—ideally 65 to 70 degrees. Soil thermometers are inexpensive and tell you the exact temperature. Plant seeds 1/4 inch deep in a small mound or hill of soil. Space seeds 6 inches apart if you plan to thin them later, or plant them in clusters of 3 to 4 seeds per spot and thin to the strongest seedling once they have true leaves.

Keep the soil consistently moist until seedlings are 2 to 3 inches tall. Once they are established, water deeply but less often to encourage deep roots. Direct-sown seedlings will be smaller than transplants at first, but they often catch up by mid-summer and may have fewer transplant shock issues.

Transplanting seedlings into the garden

Harden-off seedlings are ready to plant in the garden after they have been outside for 7 to 10 days and nighttime temperatures stay above 50 degrees. Dig a hole slightly deeper than the root ball and plant the seedling up to the first true leaves, just as you did when potting up indoors. Bury the stem; tomatoes will root along it and develop a stronger base.

Space seedlings 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 2 weeks while roots establish. After that, water deeply but less often to encourage deep rooting.

If seedlings are tall and leggy from insufficient light indoors, you can plant them at an angle in a shallow trench and they will straighten up as they grow.

Frequently Asked Questions

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

No. Garden soil is too heavy and often carries fungal spores that cause damping off. Seed-starting mix is sterile and drains well. It costs a few dollars per bag and is worth the investment.

What if my seedlings are tall and spindly?

They are not getting enough light. Move the grow light closer—it should be 2 to 3 inches above the leaves—or leave it on longer. Tall, weak seedlings can still survive, but they will be slower to produce fruit. Thicker, stockier seedlings are always better.

Do I need a heat mat if my house is warm?

If your home stays at 75 to 80 degrees consistently, a heat mat is optional. Most homes are cooler, so a heat mat speeds up germination from 10 to 14 days down to 5 to 7 days. It is not required, but it helps.

How deep should I plant seeds outdoors?

Plant direct-sown seeds 1/4 inch deep. Tomato seeds are small and do not need to be buried deep. If soil is too cold or too wet, they will rot before they sprout.

What if I miss my hardening-off window and frost is coming?

Keep seedlings indoors under lights until nighttime temperatures stay above 50 degrees. Frost will kill tender seedlings. It is better to wait a week or two than to lose your entire crop to a late freeze.