Start seeds indoors 6 to 8 weeks before your last spring frost
Growing tomatoes from seed means starting them indoors in late winter or early spring, then moving them outside once the soil warms up. You do not need special equipment — a sunny windowsill, seed-starting mix, and small containers with drainage holes are enough to get seedlings going. The timing matters: count backward from your area's last frost date (your local cooperative extension office has this) and start seeds 6 to 8 weeks before that date arrives.
If you do not know your frost date, search "[your city] last frost date" or call your county's cooperative extension office. They can tell you the exact date for your area. Starting too early means seedlings get leggy and weak while waiting for warm soil; starting too late means you miss the growing season.
Key Takeaways
- Start seeds indoors 6 to 8 weeks before your last spring frost date, which varies by location and determines when you can move plants outside safely.
- Use seed-starting mix (not garden soil), keep it moist but not waterlogged, and provide 14 to 16 hours of light daily from a sunny window or grow light.
- Seedlings are ready to move outside when they have their first true leaves, the soil has warmed to at least 60°F, and nighttime temperatures stay above 50°F.
- Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the ground or containers.
- Bury the stem deeper than it grew indoors — tomatoes can root along the buried stem and develop a stronger root system.
Fill containers with seed-starting mix and plant seeds ¼ inch deep
Use small containers — seed trays, yogurt cups with drainage holes drilled in the bottom, or recycled plastic takeout containers all work. Fill them with seed-starting mix, which is lighter and fluffier than garden soil and holds moisture evenly without compacting. Garden soil is too dense for seeds and can introduce disease.
Moisten the mix before planting. Plant one or two tomato seeds per container, pressing them ¼ inch into the soil. Cover lightly with mix. Label each container with the variety and date — you will have many seedlings and will forget which is which. Tomato seeds germinate best at 70°F to 80°F, so place containers on a warm shelf or heating mat if your house is cool.
Seeds usually sprout in 5 to 10 days. Once they do, remove the heating mat and move them to bright light when ready. Seedlings that do not get enough light stretch toward the window and become thin and weak.
Provide 14 to 16 hours of light daily from a window or grow light
A south-facing windowsill works if it gets direct sun most of the day, but most indoor windows do not provide enough light. Seedlings need 14 to 16 hours of bright light daily to stay compact and healthy. If your seedlings are stretching or leaning toward the window, they are not getting enough light.
A straightforward grow light — a fluorescent or LED fixture hung 2 to 3 inches above the seedlings — solves this problem. You can buy a basic two-bulb fixture for $20 to $40, or use a desk lamp with a daylight bulb. Set the light on a timer so it runs 14 to 16 hours per day. Raise the light as seedlings grow so it stays close to the tops of the plants.
Keep the seed-starting mix moist but not soggy. Water from below by setting containers in a tray of water and letting them soak up moisture, or water gently from above. Overwatered seedlings develop damping off, a fungal disease that kills them at the soil line. If you see this, improve air circulation with a small fan and let the soil dry slightly between waterings.
Thin seedlings and pot them up as they grow
If you planted two seeds per container and both sprouted, remove the weaker seedling by snipping it at soil level with scissors. Do not pull it out, because pulling disturbs the roots of the seedling you want to keep.
When seedlings have their first true leaves — the second set of leaves that look like actual tomato leaves, not the smooth oval seed leaves — they are ready to pot up into larger containers. Use 4-inch pots filled with potting mix. Bury the stem deeper than it was growing in the seed tray. Tomatoes can root along the buried stem, creating a stronger root system. Water gently and place the pots back under the light.
Continue watering when the top of the soil feels dry to your finger. Feed seedlings every two weeks with a diluted liquid fertilizer (follow the label rate for seedlings). This keeps them growing steadily without becoming too leggy.
Harden off seedlings before moving them outside
Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start this process 7 to 10 days before your last frost date. On the first day, place seedlings in a sheltered spot outdoors for 1 to 2 hours. Each day, increase the time and expose them to more direct sun and wind. By day 7 or 10, they should spend the full day and night outside.
Watch the weather forecast. If frost is predicted during hardening off, bring seedlings back inside. Tomatoes cannot survive frost, so do not rush this step. The goal is to have seedlings that are stocky, dark green, and ready to handle outdoor conditions without wilting or turning purple from cold stress.
Plant seedlings in the ground or containers when soil is warm
Wait until your soil temperature reaches at least 60°F and nighttime temperatures stay above 50°F before planting seedlings outside. Cold soil slows growth and can rot roots. A soil thermometer (available at garden centers for $10 to $15) takes the guesswork out of this.
Choose a spot with at least 6 to 8 hours of direct sun daily. Dig a hole slightly deeper than the seedling's root ball and bury the stem up to the first true leaves. Tomatoes develop roots along the buried stem, anchoring them better and helping them access water deeper in the soil. Space plants 24 to 36 inches apart depending on the variety — check the seed packet for the mature size.
Water gently after planting and keep the soil consistently moist (not waterlogged) for the first two weeks while roots establish. Once plants are growing steadily, water deeply but less frequently to encourage deep rooting. Mulch around the base with 2 to 3 inches of straw or shredded leaves to retain moisture and keep soil temperature steady.
Support plants and watch for common problems
As tomato plants grow, they need support to keep fruit off the ground and improve air circulation. Use stakes, cages, or string trellises. Determinate varieties (bush types) need less support; indeterminate varieties (vining types) grow tall and need sturdy support.
Watch for pests like hornworms and spider mites, and diseases like early blight (brown spots on lower leaves). Remove affected leaves promptly and improve air circulation by pruning lower branches. Water at the soil level, not on the leaves, to reduce fungal disease. Most problems are easier to prevent than to fix, so scout plants twice a week and act early if you spot damage.
Frequently Asked Questions
Can I plant tomato seeds directly in the garden instead of starting indoors?
You can, but you will harvest much later. Direct seeding works only in warm climates where the growing season is long. In most areas, starting indoors gives you a head start of 6 to 8 weeks, so you harvest ripe tomatoes before fall frost arrives. If you live in a warm zone (9 or warmer), direct seeding is an option.
Why are my seedlings tall and spindly?
Seedlings stretch when they do not get enough light. Move them closer to a window, add a grow light, or both. If they are already stretched, you can bury them deeper when you pot them up — the buried stem will root and create a stronger plant. Going forward, provide 14 to 16 hours of bright light daily.
What if I miss my frost date and my seedlings are still small?
Plant them outside anyway once nighttime temperatures stay above 50°F. Small seedlings catch up quickly once they are in warm soil and full sun. They may not be as large as seedlings planted earlier, but they will still produce fruit. If frost is forecast after planting, cover plants with cloth or row cover overnight.
Do I need to fertilize seedlings?
Seed-starting mix contains little nutrition, so seedlings benefit from feeding every two weeks once they have their first true leaves. Use a diluted liquid fertilizer made for seedlings or a balanced all-purpose fertilizer at half strength. Stop fertilizing a week or two before moving them outside so they are not too tender.
How long does it take from seed to first ripe tomato?
Most varieties produce ripe fruit 60 to 85 days after transplanting outdoors. Add 6 to 8 weeks for growing seedlings indoors, so the total from seed to harvest is roughly 4 to 5 months. Early varieties are faster; large slicing tomatoes take longer. Check the seed packet for the specific variety's days to maturity.