Start with the right pot and soil mix

Use a pot that is at least 2 to 3 inches wide and has drainage holes in the bottom. A small yogurt container, seed tray, or nursery pot works well for starting seeds. Fill it with seed-starting mix — a light, fluffy blend of peat moss, coconut coir, or perlite that drains quickly and does not compact. Do not use garden soil or potting soil meant for mature plants; these are too heavy and hold too much moisture, which causes seeds to rot.

Dampen the seed-starting mix before you plant. Pour water into the pot until the soil is moist all the way through, like a wrung-out sponge. Let it sit for a few minutes so the water spreads evenly. This prevents you from washing seeds away when you water after planting.

Key Takeaways

  • Plant tomato seeds about one-quarter inch deep in moist seed-starting mix, not garden soil, in a pot with drainage holes.
  • Keep the soil temperature between 70 and 80 degrees Fahrenheit and the surface moist but not waterlogged for germination in 5 to 10 days.
  • Place the pot in a warm spot with indirect light until seedlings sprout, then move them under grow lights or a sunny window when ready.
  • Water from below by setting the pot in a tray of water for 10 minutes rather than pouring from above, which prevents damping-off disease.
  • Thin seedlings to one plant per pot once they have two true leaves, or transplant the strongest one to its own container.

Plant seeds at the correct depth

Make a small hole in the soil about one-quarter inch deep — roughly the thickness of a pencil lead. Drop one or two tomato seeds into the hole and cover them gently with soil. Pat the soil down lightly so the seeds make contact with the moisture below, but do not press hard. Planting too deep (more than half an inch) means the seed uses up its energy before reaching the surface and may not sprout.

If you plant two seeds per pot, you will thin them later. Planting two increases your chances that at least one will germinate, especially if your seeds are old. This is a common strategy when seeds are several years old or when germination rates are uncertain.

Provide warmth and moisture for germination

Tomato seeds germinate fastest between 70 and 80 degrees Fahrenheit. Place the pot on a heat mat, on top of the refrigerator, or in a warm corner of your kitchen — anywhere that stays consistently warm. A cold windowsill or basement will slow germination or prevent it altogether. Check the soil every day and mist the surface with water from a spray bottle to keep it moist but not soggy. Soggy soil invites mold and rot.

Germination usually takes 5 to 10 days. You do not need light yet — seeds sprout in darkness. Once you see the first green shoot breaking through the soil, move the pot when ready to bright light. Do not wait; seedlings that sit in darkness after sprouting will stretch and weaken before they ever see sun.

Move seedlings to light as soon as they sprout

The moment the seedling emerges, place the pot under a grow light, in a south-facing window, or under a bright LED lamp. Seedlings that do not get enough light become leggy — tall, thin, and pale — and will not grow into strong plants. If you use a grow light, keep it 2 to 3 inches above the seedlings and leave it on for 14 to 16 hours per day. A sunny windowsill works if you rotate the pot every day so the seedling grows straight instead of leaning toward the light.

Once seedlings are under lights, they no longer need the heat mat. Room temperature (65 to 75 degrees) is fine now. The combination of bright light and cooler air actually strengthens seedlings and prevents them from growing too fast and becoming weak.

Water from below to prevent disease

Watering from above — pouring water onto the soil surface — spreads a fungal disease called damping-off, which kills seedlings at the soil line. Instead, set the pot in a shallow tray of water for 10 minutes. The soil will absorb water from the drainage holes. Remove the pot and let it drain completely. Water this way whenever the top of the soil feels dry to the touch, usually every 2 to 3 days depending on how warm and dry your home is.

Never let the soil dry out completely, and never let it sit in standing water. Both extremes kill seedlings. The goal is consistently moist soil, not wet and not dry. If you forget to water and the soil dries out, soak the pot in a tray for 15 to 20 minutes to rehydrate it fully.

Thin or transplant when seedlings have true leaves

After 2 to 3 weeks, seedlings will have two sets of leaves: the first pair (called cotyledons) are smooth and straightforward, and the second pair (called true leaves) look like tiny tomato leaves with jagged edges. Once true leaves appear, thin the pot to one seedling if you planted two seeds.

To thin, snip the weaker seedling at soil level with clean scissors rather than pulling it out, which disturbs the roots of the one you want to keep. If both seedlings look equally strong, transplant the extra one into its own pot filled with the same seed-starting mix. Handle seedlings by their leaves, not their stems, which bruise easily and can rot if damaged.

Harden off seedlings before moving them outdoors

About 2 weeks before your last spring frost, seedlings are ready to move outside. But they cannot go straight from your warm kitchen to the garden — the sun, wind, and temperature swings will shock them. Harden off seedlings by placing them outdoors in a sheltered, shaded spot for 1 to 2 hours on the first day. Each day, add an hour of time outside and gradually move them into more sunlight. By day 7 to 10, they can stay outside all day and night and are ready to transplant into the garden or larger pots.

Hardening off takes patience but prevents wilting, sunburn, and stunted growth. Seedlings that skip this step often struggle for weeks after transplanting. If a cold snap is forecast during hardening off, bring pots back indoors until the danger passes.

Frequently Asked Questions

Why are my seedlings tall and spindly?

They are not getting enough light. Move them closer to a grow light (within 2 to 3 inches) or to a brighter window. Leggy seedlings can be saved by burying them deeper when you transplant — tomatoes will grow roots all along a buried stem.

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

Potting soil is too dense and holds too much water for seeds. It compacts easily and causes rot. Seed-starting mix is worth buying because it drains fast and stays light. You can reuse it for several seasons if you store it in a sealed container.

What temperature should the soil be?

Tomato seeds germinate best between 70 and 80 degrees Fahrenheit. Below 60 degrees, germination is slow or fails. A heat mat keeps soil warm and speeds sprouting, but once seedlings emerge, room temperature is fine.

How often should I water seedlings?

Water whenever the top of the soil feels dry to the touch, usually every 2 to 3 days. Always water from below by setting the pot in a tray of water for 10 minutes. This prevents damping-off disease and keeps stems dry.

When can I move seedlings to bigger pots?

Transplant seedlings to 4-inch pots once they have 4 to 6 true leaves and the roots start poking out the drainage holes. Use regular potting soil for larger pots, not seed-starting mix. Bury the stem up to the first true leaves — tomatoes develop roots along buried stems and grow stronger.