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

Container tomato seeds need to be started indoors in most climates because they need warmth and light to sprout, and outdoor soil is usually too cold in early spring. Count backward from your last frost date — the day when nighttime temperatures stay above 50°F in your area — and plant seeds 6 to 8 weeks before that date. This timing lets seedlings grow strong enough to transplant outdoors when the weather is warm enough.

You can find your last frost date by searching your zip code on the National Weather Service website or asking at a local garden center. Starting too early means seedlings get leggy and weak while waiting for warm weather; starting too late means you miss the peak growing season.

Key Takeaways

  • Fill small containers with seed-starting mix (not garden soil), moisten it, and plant one seed per pot at a depth equal to twice the seed's width.
  • Keep soil temperature between 70°F and 80°F using a heat mat, and provide 14 to 16 hours of direct light daily with a grow light or sunny window.
  • Water from below by setting pots in a tray of water rather than pouring from above, which prevents damping-off disease that kills seedlings.
  • Thin seedlings to the strongest one per pot once they develop their first true leaves, and move them to larger containers when roots fill the original pot.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting into garden containers.

Choose containers and soil that drain well

Use small containers — 2 to 3 inches wide — filled with seed-starting mix, not garden soil or potting soil. Seed-starting mix is lighter and fluffier, which lets tiny roots push through easily and reduces the risk of damping-off, a fungal disease that kills seedlings at the soil line. Garden soil is too dense and often carries disease spores; regular potting mix holds too much moisture.

Containers can be plastic cell packs, small pots, or even paper cups with drainage holes drilled in the bottom. Wet the seed-starting mix thoroughly before planting so it absorbs water evenly. Dry mix repels water and creates air pockets that leave seeds sitting in dust rather than moist soil.

Plant one seed per pot at the correct depth

Tomato seeds are tiny — about the size of a grain of sand — and should be planted at a depth of roughly twice their width, which is about 1/4 inch deep. Drop one seed into the center of each pot and cover it lightly with seed-starting mix. Press down gently so the seed makes contact with the soil, but do not pack it hard.

Planting one seed per pot prevents overcrowding and means you will not have to thin seedlings later, which damages roots. If you plant multiple seeds per pot to hedge your bets, you will need to remove the weaker seedlings once they sprout, leaving only the strongest one.

Provide warmth and light for germination and growth

Tomato seeds germinate fastest at soil temperatures between 70°F and 80°F. A heat mat — an electric pad that sits under the seed trays — keeps soil warm even if your house is cool. Without heat, seeds germinate slowly or not at all in cold basements or unheated rooms. Once seedlings emerge, remove the heat mat and keep the room at 65°F to 70°F to prevent them from stretching.

Light is equally critical. Seedlings need 14 to 16 hours of direct light daily, which a sunny south-facing window can provide if it gets at least 6 hours of unobstructed sun. Most indoor spaces need a grow light — a fluorescent or LED fixture positioned 2 to 3 inches above the seedlings. Without enough light, seedlings become tall and spindly, reaching toward the light source instead of growing compact and sturdy.

Water from below to prevent disease

Water seedlings by setting their pots in a tray of water and letting the soil absorb moisture from the bottom up, rather than pouring water over the top. This method, called bottom watering, keeps the leaves and soil surface dry, which prevents damping-off and other fungal diseases. Check the soil daily — it should feel moist but not waterlogged, and the surface can dry out slightly between waterings.

Overwatering is the most common mistake. Soggy soil suffocates roots and invites disease. If water sits in the tray after 30 minutes, drain it. Once seedlings are a few inches tall and have developed their first true leaves (the second set of leaves that appear), you can switch to careful overhead watering, pouring slowly at the base of each plant and avoiding the leaves.

Thin seedlings and move them to larger pots as they grow

Once seedlings develop their first true leaves — the second set of leaves that look like actual tomato leaves, not the smooth rounded cotyledons that emerged first — thin them to one per pot if you planted multiple seeds. Pinch off the weaker seedlings at soil level rather than pulling them out, which disturbs the roots of the one you are keeping.

When seedlings are 2 to 3 inches tall and their roots begin to fill the original pot, move them to larger containers, usually 4 to 6 inches wide. Bury the stem deeper in the new pot — tomatoes can develop roots along their buried stem, which creates a stronger root system. Handle seedlings by their leaves, not their stems, because stems bruise easily.

Harden off seedlings before moving them outdoors

Seedlings grown indoors are tender and will wilt or die if moved directly into outdoor sun and wind. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing pots in a sheltered, shaded spot outdoors for 1 to 2 hours, then bring them back inside. Each day, increase the time outdoors and gradually move them into more direct sun.

By the end of the hardening-off period, seedlings should spend a full day and night outdoors before transplanting into their final containers. This process toughens the leaves and stems so they can handle wind and direct sun without shock. If frost threatens during hardening off, bring pots back inside — even a light frost kills tender seedlings.

Frequently Asked Questions

What is the difference between seed-starting mix and potting soil?

Seed-starting mix is lighter and contains no nutrients, which forces young roots to grow outward searching for food and creates stronger seedlings. Potting soil is heavier, holds more moisture, and contains fertilizer meant for established plants. For seeds, the lighter texture and lower moisture of seed-starting mix prevent damping-off disease and allow roots to push through easily.

Can I use a sunny windowsill instead of a grow light?

A south-facing window with at least 6 hours of direct sun can work, but most indoor windows do not provide enough light. Seedlings grown in windows often become tall and spindly because they stretch toward the light. A grow light positioned 2 to 3 inches above seedlings and left on for 14 to 16 hours daily produces stockier, stronger plants.

Why are my seedlings falling over at the soil line?

This is damping-off disease, caused by fungi in wet soil. It happens when seedlings are overwatered, the air is too humid, or soil is not sterile. Prevent it by using fresh seed-starting mix, watering from below, providing good air circulation with a small fan, and keeping the soil moist but not waterlogged. Once damping-off appears, the seedling cannot be saved.

How do I know when seedlings are ready to transplant into containers?

Seedlings are ready when they are 6 to 8 inches tall, have developed their first true leaves, and roots are visible at the drainage holes of their current pot. They should also have been hardened off and the outdoor soil temperature should be at least 60°F at night. Transplanting too early into cold soil slows growth; waiting too long means roots become cramped.

Can I plant seeds directly into large containers instead of starting small?

You can, but it wastes space and soil. Seeds need consistent moisture and warmth to germinate, which is easier to manage in small pots. Starting in small containers also lets you move seedlings to a grow light and control conditions precisely. Once they are established, moving them to larger containers is quick and the roots recover fast.