Tomato seeds need warmth, moisture, and light to sprout — start them indoors 6 to 8 weeks before your last frost date
Tomato seeds germinate fastest in soil between 70 and 80 degrees Fahrenheit. You can start them indoors in seed trays, small pots, or recycled containers with drainage holes, filled with seed-starting mix (not garden soil, which compacts and holds too much water). Plant seeds about a quarter-inch deep, water gently so the soil stays moist but not waterlogged, and cover the tray with a humidity dome or plastic wrap until sprouts appear — usually 5 to 10 days.
Once seedlings break through, remove the cover when ready and move them to a bright location. A south-facing window works if it gets at least 12 to 14 hours of direct sun daily; a grow light positioned 2 to 3 inches above the seedlings is more reliable and prevents them from stretching toward weak light. Keep the soil consistently moist and the air temperature between 65 and 75 degrees during the day.
Seedlings are ready to transplant into larger pots when they develop their first true leaves — the second set of leaves that look like actual tomato leaves, not the smooth seed leaves. At that point, bury the stem deeper in fresh potting soil; tomato roots will form all along the buried stem, creating a stronger plant.
Key Takeaways
- Tomato seeds sprout fastest in soil between 70 and 80 degrees Fahrenheit and need consistent moisture but not waterlogging.
- Seedlings need 12 to 14 hours of bright light daily once they sprout, or they will stretch and weaken.
- Transplant seedlings into larger pots when they develop their first true leaves, burying the stem deeper to encourage root growth.
- Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting them in the garden.
Heat mats and grow lights make germination and growth faster and more reliable
A seed heat mat — a thin electric pad that sits under your seed tray — keeps soil at the ideal temperature even if your house is cool. They cost between $20 and $50 and cut germination time noticeably. If your home stays above 70 degrees, you do not need one, but if you start seeds in a basement, garage, or cool room, a mat prevents the slow, spotty germination that happens in cold soil.
A grow light is more important than a heat mat. Seedlings started on a windowsill almost always stretch — they grow tall and spindly as they reach toward the light source. A straightforward LED shop light or clip lamp with a full-spectrum bulb, positioned 2 to 3 inches above the seedlings and raised as they grow, produces compact, sturdy plants. Grow lights cost $15 to $60 depending on size and quality. Run them on a timer for 14 to 16 hours daily.
Water and fertilizer: the two things that kill seedlings most often
Overwatering is the most common mistake. Seed-starting mix drains quickly and dries out fast, but seedlings rot if the soil stays soggy. Water when the surface feels dry to your touch, and water from below by setting the tray in a shallow pan of water for 10 minutes, then removing it. This lets the soil absorb moisture from the bottom without waterlogging the surface where seedling stems sit.
Seedlings do not need fertilizer until they develop their first true leaves. Seed-starting mix contains no nutrients, but seedlings have enough stored energy in the seed itself for the first few weeks. Once true leaves appear, feed them every 7 to 10 days with a diluted liquid fertilizer — use half the strength recommended on the bottle. Too much fertilizer burns roots and stunts growth.
Hardening off: the step most people skip and regret
Seedlings grown indoors are soft. Moving them directly from a warm, still, protected environment to the garden kills them or sets them back weeks. Hardening off means gradually exposing them to outdoor conditions — wind, direct sun, temperature swings, and lower humidity — over 7 to 10 days.
Start by placing seedlings outside in a sheltered, shaded spot for 2 to 3 hours on the first day. Each day, increase the time outdoors and move them into more direct sun. By day 7 or 8, they should spend the full day outside. Bring them in at night until nighttime temperatures stay above 50 degrees. This toughens the stems and leaves and prevents transplant shock.
Timing: when to start seeds so they are ready when the soil warms
Tomatoes need soil at least 60 degrees Fahrenheit to grow well in the garden, but they thrive when soil reaches 65 to 70 degrees. Count backward from your region's average last frost date — the date when nighttime temperatures reliably stay above 50 degrees. Start seeds indoors 6 to 8 weeks before that date.
If you start too early, seedlings become root-bound and leggy while waiting for warm soil outdoors. If you start too late, you miss the growing season or plant weak, small seedlings that take months to produce fruit. Your local cooperative extension office publishes the frost date for your area, or you can search online for your ZIP code.
Common problems: leggy seedlings, damping off, and slow growth
Leggy seedlings — tall, thin, weak stems — happen when light is too far away or too dim. Move the light closer (2 to 3 inches above the tops of the plants) or run it longer (14 to 16 hours daily). A small fan running on low for a few hours daily also strengthens stems by creating gentle air movement.
Damping off is a fungal disease that causes seedlings to collapse at the soil line. It thrives in cold, wet conditions. Prevent it by using fresh seed-starting mix, watering from below rather than overhead, ensuring good air circulation with a small fan, and keeping the soil moist but not waterlogged. If seedlings start to collapse, remove affected plants when ready and let the soil dry out slightly.
Slow growth usually means light is insufficient, temperature is too cold, or seedlings are root-bound. Check that seedlings get 12 to 14 hours of bright light daily, that the room stays between 65 and 75 degrees, and that you transplant them into larger pots as soon as true leaves appear.
Varieties that are easiest to grow from seed
Determinate varieties — bush types that grow to a fixed size and produce most fruit at once — tend to be more forgiving from seed than indeterminate types. 'Roma', 'San Marzano', and 'Plum Regal' are reliable. Cherry tomatoes like 'Sungold' and 'Black Cherry' also germinate and grow quickly.
Heirloom varieties like 'Brandywine' and 'Cherokee Purple' are slower to germinate and grow, and seedlings are more fragile, but they are worth the extra care if you have the space and patience. Hybrid varieties bred for disease resistance, like 'Early Girl' and 'Mountain Magic', are also straightforward from seed and often produce faster than heirlooms.
Frequently Asked Questions
Can I save seeds from store-bought tomatoes?
Yes, but only from heirloom or open-pollinated varieties. Hybrid tomatoes produce seeds that grow into plants different from the parent — often smaller, weaker, or lower-yielding. Heirloom seeds saved from ripe fruit and dried thoroughly will grow true to type. Rinse the seeds, ferment them in water for 2 to 3 days to remove the gel coating, rinse again, and dry completely before storing.
How long do tomato seeds stay viable?
Tomato seeds stay viable for 4 to 6 years if stored in a cool, dry place — ideally below 50 degrees and below 50 percent humidity. A sealed envelope in the refrigerator works well. Older seeds germinate more slowly and less reliably, so start a few extra seeds if you are using stock more than 3 years old.
What if I do not have a grow light?
A bright south-facing window with at least 12 to 14 hours of direct sun daily works, but seedlings will likely stretch toward the light. Rotate the tray daily to keep growth even. A reflective surface like white cardboard or aluminum foil behind the seedlings bounces light back and helps. Grow lights are inexpensive and prevent this problem entirely.
Can I start seeds directly in the garden instead of indoors?
You can, but you will lose 4 to 6 weeks of growing time because seeds need warm soil and will not germinate until late spring. In short growing seasons, this means fewer or no ripe tomatoes by fall. Starting indoors lets you harvest fruit weeks earlier and is worth the small effort.
Why are my seedlings turning purple?
Purple or reddish leaves usually mean the soil is too cold or the seedlings are stressed. Move them to a warmer location, check that the soil drains well, and may support they are getting enough light. A heat mat under the tray helps if the room temperature is below 65 degrees.