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

Tomato seeds germinate best in warm soil—between 70 and 80 degrees Fahrenheit—and take 5 to 10 days to sprout. If you live where frost arrives late (May or June), starting seeds indoors gives you a head start so plants are already 6 to 8 inches tall and ready to harden off by the time soil is warm enough to plant outside. If your last frost date is early (March or April) or you have a short growing season, indoor starting is almost necessary. In warm climates where frost never arrives, you can direct sow seeds straight into garden soil once it reaches 60 degrees, though indoor-started plants still mature faster.

The choice between indoor and outdoor sowing depends on your climate and how much time you have. Indoor sowing gives you control over temperature and light, and lets you start earlier. Direct sowing is simpler if you have the time and warmth. Most home gardeners in cooler regions use the indoor method.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date in a warm spot with grow lights or a sunny window.
  • Use seed-starting mix (not garden soil), keep it moist but not waterlogged, and maintain soil temperature between 70 and 80 degrees for fastest germination.
  • Once seedlings have their first true leaves, thin them to one plant per cell or pot so they don't compete for light and nutrients.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before transplanting into the garden.
  • Direct sow seeds outdoors only after soil temperature reaches 60 degrees and all frost danger has passed, planting them half an inch deep.

What you need to start seeds indoors

Seed-starting mix is lighter and fluffier than garden soil, which lets roots push through easily and drains well without staying soggy. You can buy it bagged at any garden center. Do not use potting soil or garden soil from your yard—both are too dense and can introduce soil-borne diseases that kill seedlings.

You will need containers with drainage holes—seed trays, cell packs, or small pots all work. Recycled yogurt cups or egg cartons work too, as long as you poke holes in the bottom. Fill them with moist seed-starting mix before planting.

Tomato seeds need warmth and light. A heat mat under your seed tray keeps soil at 70 to 80 degrees and speeds germination. Once seedlings sprout, they need 14 to 16 hours of bright light per day—a south-facing window works if it gets direct sun most of the day, but grow lights (LED or fluorescent) are more reliable and keep seedlings from getting leggy and stretched. Position lights 2 to 3 inches above the seedlings and raise them as plants grow.

Planting seeds indoors step by step

Fill your containers with moist seed-starting mix. Press the mix gently so it is firm but not compacted. Plant one tomato seed per cell, pushing it about a quarter inch deep—tomato seeds are small, so do not bury them. Cover the tray with a humidity dome or plastic wrap to keep moisture in while seeds germinate.

Place the tray on a heat mat set to 75 degrees. Check the soil daily and mist it if the surface begins to dry out. Seeds should sprout in 5 to 10 days. Once you see green shoots, remove the humidity dome and move the tray under grow lights or into a bright window. Keep the soil moist but not waterlogged—soggy soil causes damping off, a fungal disease that kills seedlings at the soil line.

When seedlings have their first true leaves (the second set of leaves that appear, which look like actual tomato leaves), thin them to one plant per cell. Pinch off the weaker seedlings at soil level rather than pulling them out, which can disturb the roots of the one you are keeping. If you planted multiple seeds per cell, you now have to choose.

Caring for seedlings until transplant time

Water seedlings from below by setting the tray in a shallow pan of water for 10 to 15 minutes, then removing it. This keeps water off the leaves and reduces disease. The soil should feel moist like a wrung-out sponge, not soggy.

Keep lights on for 14 to 16 hours per day. You can use a straightforward timer so you do not have to turn them on and off by hand. Keep the room temperature between 65 and 75 degrees during the day. Seedlings grow slower in cold, and too much heat makes them weak.

When seedlings are 2 to 3 inches tall and have several sets of true leaves, you can move them to slightly larger pots if they are getting crowded. Tomatoes can be planted deeper than they were growing—bury the stem up to the first true leaves, and roots will form all along the buried stem, making a stronger plant.

Hardening off before moving outdoors

Hardening off means gradually exposing seedlings to outdoor sun, wind, and temperature swings so they toughen up and do not go into shock when planted in the garden. Start 7 to 10 days before your planned transplant date.

Day 1 and 2: Place seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours. Bring them back inside. Day 3 and 4: Move them to dappled shade for 4 to 5 hours. Day 5 and 6: Give them 2 to 3 hours of morning sun and afternoon shade. Day 7 and 8: Increase sun to 4 to 6 hours. Day 9 and 10: Leave them out all day in sun, but bring them in at night. On the final day or two, leave them out overnight if nighttime temperatures stay above 50 degrees. By the end, they are ready to stay outside.

Water seedlings well during hardening off—they dry out faster in sun and wind. If a late frost is forecast after you have started hardening off, move plants back indoors or cover them with a blanket overnight.

Direct sowing seeds outdoors

If you are direct sowing, wait until soil temperature reaches at least 60 degrees and all danger of frost has passed. You can check soil temperature with a soil thermometer or wait until nighttime lows stay above 50 degrees for several days in a row.

Prepare the planting spot by mixing compost or aged manure into the top 6 to 8 inches of soil. Plant seeds half an inch deep, spacing them 2 to 3 inches apart. Water gently so the soil stays moist but not waterlogged. Seedlings should emerge in 5 to 10 days. Once they have their first true leaves, thin them to one plant every 24 to 36 inches, depending on the variety—check the seed packet for spacing.

Direct-sown plants mature later than indoor-started ones because they start from seed in warm soil rather than from a larger transplant. In short-season climates, this can mean the difference between ripe tomatoes in August and green fruit in October.

Transplanting seedlings into the garden

Harden-off seedlings are ready to plant when they are 6 to 8 inches tall and have several sets of true leaves. Choose a spot that gets at least 6 to 8 hours of direct sun per day—tomatoes need sun to produce fruit and to resist disease.

Dig a hole slightly deeper than the root ball. Tomatoes can be planted deeper than they grew in their pot—bury the stem up to the lowest leaves, and new roots will form along the buried stem. This creates a stronger root system. Space plants 24 to 36 inches apart depending on variety. Water well after planting and keep soil consistently moist for the first 2 to 3 weeks while roots establish.

If you are planting in spring and a late frost is forecast, cover plants with a frost cloth or blanket overnight. Once nighttime temperatures stay above 50 degrees consistently, plants are past frost danger.

Frequently Asked Questions

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

Potting soil is heavier and holds more moisture than seed-starting mix, which can cause seedling roots to rot. Seed-starting mix is designed to be light and drain quickly. It costs a few dollars more but prevents damping off and gives seedlings a much better start. If you are out of seed-starting mix, you can make your own by mixing equal parts peat moss or coco coir, perlite, and compost.

What if my seedlings are tall and spindly?

Leggy seedlings mean they are not getting enough light. Move grow lights closer (within 2 to 3 inches of the tops) or place the tray in a brighter window. Increase light duration to 16 hours per day if you are not already. Once seedlings are leggy, you cannot fix it, but you can plant them deeper when transplanting—bury the stem up to the first true leaves, and the buried part will root.

How do I know when soil is warm enough to direct sow?

Use a soil thermometer pushed 2 inches into the ground in the morning. Tomato seeds germinate best at 70 to 80 degrees, though they will sprout slowly at 60 degrees. If you do not have a thermometer, wait until nighttime lows stay above 50 degrees for a week and daytime highs are consistently in the 70s.

Do I need a heat mat if I have a warm house?

A heat mat keeps soil warm even if your house is cool, and it speeds germination from 10 days to 5 to 7 days. If your home stays at 75 degrees or warmer, you may not need one, but a mat is inexpensive and gives more reliable results. Without one, germination takes longer and is less even.

Can I reuse seed-starting mix from last year?

Old seed-starting mix can harbor fungal spores that cause damping off. Fresh mix is safer. If you do reuse it, sterilize it first by baking it in a 180-degree oven for 30 minutes in a covered pan, then let it cool completely before using.