Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow them outdoors after soil warms to 60°F

Tomato seeds germinate fastest in warm soil—between 70°F and 80°F—and sprout in 5 to 10 days under the right conditions. You have two routes: start seeds indoors under lights in late winter, then transplant seedlings outside after frost danger passes, or sow seeds directly into garden soil once the ground has warmed. Indoor starting gives you a head start on the growing season and works in any climate. Direct sowing is simpler but only works in regions with a long enough frost-free period.

The choice depends on where you live and how much time you have. If your last frost date is in May and you want mature plants by July, start indoors. If you live in a warm zone with frost-free weather by late April, direct sowing works fine. Either way, the seed itself needs warmth, moisture, and light to sprout—and light only matters after the seedling breaks through soil.

Key Takeaways

  • Indoor seeds need a seed-starting mix (not garden soil), a warm spot around 70°F, and consistent moisture—not waterlogged soil.
  • Seedlings need 14 to 16 hours of direct light daily from a grow light or sunny window, or they will stretch and weaken.
  • Harden off seedlings by exposing them to outdoor conditions for 7 to 10 days before transplanting, or they will wilt in the garden.
  • Direct-sown seeds go into warm soil (60°F minimum) about ¼ inch deep, spaced 2 inches apart, then thinned to final spacing once they have true leaves.
  • Tomato seeds stay viable for 3 to 4 years if stored in a cool, dry place, so old seed packets may still sprout.

Starting Seeds Indoors: Containers, Mix, and Setup

Use a seed-starting mix—a sterile blend of peat moss, coconut coir, and perlite—rather than garden soil or potting mix. Garden soil compacts and holds too much water. Seed-starting mix is light, drains well, and contains no soil-borne diseases that kill seedlings. Fill small containers (2-inch pots, cell packs, or even paper cups with drainage holes) to the rim, then water the mix until it is evenly moist but not soggy.

Place one or two seeds per cell, pressing them ¼ inch into the surface. Tomato seeds are small, so you can use a pencil tip to make a shallow hole. Cover the container with a humidity dome or plastic wrap to trap moisture and speed germination. Set the container on a heat mat or warm shelf—a seedling heat mat keeps soil at 75°F, which is ideal. Without warmth, germination takes twice as long or fails entirely.

Check the soil daily and mist it if the surface dries out. Once the first green shoot appears (usually 5 to 10 days), remove the cover when ready and move the container under lights. Leaving the cover on after sprouting traps too much moisture and causes damping-off disease, which kills seedlings at the soil line.

Lighting and Temperature for Strong Seedlings

Seedlings need 14 to 16 hours of direct light daily to grow stocky and strong. A south-facing window works if it gets unobstructed sun all day, but most indoor growers use a fluorescent or LED grow light positioned 2 to 3 inches above the seedlings. As the seedlings grow, raise the light to keep that distance constant. Without enough light, seedlings stretch toward the window, become thin and pale, and fall over when transplanted.

Keep the room temperature between 65°F and 75°F during the day. At night, a 10-degree drop is fine and actually strengthens seedlings. Avoid placing seedlings on a hot radiator or in a drafty window—temperature swings stress them. Once seedlings have their first true leaves (the second set of leaves that appear, which look like actual tomato leaves), you can reduce watering slightly. The soil should feel moist but not wet; overwatering is the most common mistake at this stage.

When seedlings are 2 to 3 inches tall and have at least two sets of true leaves, you can transplant them into larger pots (4-inch pots work well). Bury the stem up to the lowest leaves—tomatoes will root along the buried stem and develop a stronger root system. Water gently after transplanting and keep the light at the same distance.

Hardening Off Before Moving Seedlings Outdoors

Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start 7 to 10 days before your transplant date. On day one, place seedlings in a sheltered spot outdoors (a porch or against a wall) for 2 to 3 hours in dappled shade. Bring them back inside.

Each day, increase outdoor time by 1 to 2 hours and move them into slightly brighter light. By day 7, seedlings should spend the full day outside in partial sun. By day 10, they can handle full sun and stay out overnight if temperatures stay above 50°F. If a cold snap is forecast, bring them inside. Skipping this step causes seedlings to wilt, turn purple, or stop growing when planted in the garden.

Direct Sowing Seeds Outdoors

Wait until soil temperature reaches at least 60°F—ideally 65°F or warmer. Use a soil thermometer to check; air temperature alone is not reliable. Plant seeds ¼ inch deep and 2 inches apart in rows or clusters. Water gently so the soil stays moist but not waterlogged. Germination takes 7 to 14 days depending on soil warmth.

Once seedlings have their first true leaves, thin them to 12 to 18 inches apart (depending on the variety—check the seed packet). Thinning is crucial: crowded seedlings compete for light and nutrients, and the result is weak, disease-prone plants. Pinch off extra seedlings at soil level rather than pulling them, which disturbs roots of the remaining plant.

Direct sowing works best in warm climates where frost danger has passed by late April or early May. In cooler regions, the soil may not warm up in time for seeds to germinate and grow into mature plants before fall frost arrives. If you are unsure whether you have enough time, start indoors instead.

Soil Preparation and Spacing in the Garden

Tomatoes need rich, well-draining soil with plenty of organic matter. Before planting, mix 2 to 3 inches of compost or aged manure into the top 8 to 10 inches of soil. Tomatoes are heavy feeders and will produce better fruit in amended soil. If your soil is clay-heavy and drains poorly, build a raised bed or mound to improve drainage.

Space transplants 24 to 36 inches apart (check your seed packet for the variety's mature size). Determinate varieties, which stay compact, can be closer; indeterminate varieties, which grow tall and sprawling, need more room. Plant seedlings deep—bury the stem up to the lowest leaves, just as you did when potting up indoors. This buried stem develops roots and creates a stronger plant.

Water seedlings well after transplanting and keep soil consistently moist for the first 2 to 3 weeks while roots establish. Once plants are growing actively, water deeply but less frequently—about 1 to 2 inches per week depending on rainfall and heat. Inconsistent watering causes problems like blossom-end rot and fruit cracking.

Common Mistakes and How to Avoid Them

Using garden soil instead of seed-starting mix: Garden soil compacts, drains poorly, and often carries diseases. Seed-starting mix is inexpensive and worth the investment.

Starting seeds too early: If you start 12 weeks before your last frost date instead of 6 to 8 weeks, seedlings become overgrown and root-bound before transplant time. They never recover fully. Count backward from your last frost date and start on schedule.

Skipping hardening off: Seedlings grown indoors are tender. Moving them straight to the garden shocks them. The 7 to 10 day hardening-off period is not optional.

Overwatering: Wet soil causes damping-off disease and root rot. Water when the top of the soil feels dry to your finger, not on a fixed schedule. Drainage holes are essential.

Insufficient light: Seedlings under a regular household bulb or in a dim window stretch and weaken. A grow light 2 to 3 inches above seedlings is the most reliable solution.

Frequently Asked Questions

Can I use old tomato seeds from last year?

Yes, tomato seeds remain viable for 3 to 4 years if stored in a cool, dry place. Germination rates may drop slightly with age, so you can sow a few extra seeds to account for lower sprouting. Store seeds in an airtight container in a cool closet or refrigerator, not on a sunny shelf.

What if my seedlings are tall and spindly?

Spindly seedlings mean insufficient light. Move the grow light closer (2 to 3 inches above the seedlings) and increase light duration to 16 hours daily. You can also bury the spindly stem deeper when transplanting—tomatoes will root along the buried portion and recover. In the future, use a grow light from day one.

How do I know when soil is warm enough for direct sowing?

Use a soil thermometer inserted 2 to 3 inches into the ground. Tomato seeds need soil at least 60°F to germinate reliably; 65°F to 70°F is better. Check soil temperature in the morning for the most accurate reading. If soil is still cold, wait a week and check again.

Do I need to fertilize seedlings indoors?

Seed-starting mix contains little nutrition, so once seedlings have their first true leaves, feed them weekly with a diluted liquid fertilizer (half strength). Stop fertilizing about a week before transplanting outdoors. Overfertilized seedlings become soft and weak.

Can I save seeds from my tomato plants?

Yes, but only from heirloom and open-pollinated varieties. Hybrid tomatoes (marked F1 on the packet) produce seeds that do not grow true to type. To save seeds, let a ripe tomato ferment in water for 2 to 3 days to remove the gel coating, rinse the seeds, dry them completely on a paper towel, and store them in an airtight container in a cool place.