Start tomato seeds indoors 6 to 8 weeks before your last spring frost
Indoor tomato planting works best when you start seeds in late winter or early spring, timing them to mature into seedlings just as outdoor soil warms up. Count backward from your area's last frost date — the date when freezing temperatures typically stop — and start seeds 6 to 8 weeks before that date. This gives seedlings time to develop true leaves and a strong root system before transplanting outdoors.
You do not need a garden or outdoor space to begin. A sunny windowsill, a shelf under grow lights, or a seed-starting setup indoors all work. The goal is to give seeds warmth, light, and moisture until they are ready to harden off — the process of gradually exposing them to outdoor conditions before planting in the ground.
Key Takeaways
- Start seeds indoors 6 to 8 weeks before your last frost date, using seed-starting mix in small containers with drainage holes.
- Keep soil moist but not waterlogged, and provide 14 to 16 hours of light daily using a sunny window or grow lights positioned 2 to 3 inches above seedlings.
- Thin seedlings to one plant per cell once they develop their first true leaves, and transplant into larger pots if they outgrow their starter containers before outdoor planting time.
- Harden off seedlings by gradually exposing them to outdoor sun and wind over 7 to 10 days before moving them to the garden.
- Plant seedlings outdoors after the last frost date, burying the stem deeper than it was indoors to encourage a stronger root system.
Fill containers with seed-starting mix and sow seeds at the right depth
Use small containers — seed trays, cell packs, or even recycled yogurt cups — as long as they have drainage holes in the bottom. Fill them with seed-starting mix, a lightweight blend designed to hold moisture without compacting. Do not use garden soil or potting soil meant for mature plants; seed-starting mix drains better and reduces the risk of fungal disease in young seedlings.
Plant tomato seeds about one-quarter inch deep. Place one or two seeds per cell, press them gently into the mix, and water until the soil is moist throughout. Cover the containers with a clear plastic dome or plastic wrap to trap humidity, which speeds germination. Most tomato seeds sprout in 5 to 10 days at temperatures between 70 and 80 degrees Fahrenheit.
Once seedlings emerge, remove the plastic cover when ready and move the containers to a light source. Seedlings that stay in darkness become leggy — tall and thin with weak stems — even if you move them to light later.
Provide 14 to 16 hours of light daily using a window or grow lights
A south-facing windowsill works if it receives direct sun most of the day, but most indoor growers use grow lights because they deliver consistent light regardless of weather or season. Position lights 2 to 3 inches above the seedling tops and raise them as the plants grow. Seedlings need 14 to 16 hours of light daily; use a timer to turn lights on and off automatically.
Without enough light, seedlings stretch toward the light source and become weak. If you notice seedlings leaning or growing tall and spindly, move the light closer or add more hours of light per day. Keep the growing area at 65 to 75 degrees Fahrenheit during the day and slightly cooler at night.
Water consistently and thin seedlings when they develop true leaves
Keep the seed-starting mix moist but not soggy. Water from below by setting containers in a tray of water and letting them absorb moisture through the drainage holes, or water gently from above with a spray bottle. Overwatering causes damping off, a fungal disease that kills seedlings at the soil line.
Once seedlings develop their first true leaves — the second set of leaves that look like actual tomato leaves, not the smooth oval cotyledons that emerged first — thin them to one plant per cell. Snip the weaker seedling at soil level with scissors rather than pulling it out, which can disturb the roots of the one you want to keep.
If seedlings outgrow their starter containers before outdoor planting time, transplant them into 4-inch pots filled with potting soil. Bury the stem deeper than it was in the starter mix; tomatoes develop roots all along the buried stem, creating a stronger plant.
Harden off seedlings over 7 to 10 days before moving them outside
Indoor seedlings are tender and cannot survive the shock of outdoor sun, wind, and temperature swings. Hardening off means gradually exposing them to outdoor conditions over a week to ten days. Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase the time outdoors and gradually move them into more direct sun.
By day 7 or 8, seedlings should spend the full day outside in dappled sun or afternoon shade. By day 10, they can handle full sun and stay outside overnight if temperatures stay above 50 degrees Fahrenheit. If frost is forecast during hardening off, bring seedlings back indoors.
Hardening off takes time but prevents transplant shock and sunscald — white or pale patches on leaves and stems caused by sudden exposure to intense sun. Seedlings that are properly hardened off establish faster in the garden and produce earlier.
Plant seedlings in the garden after the last frost date
Once seedlings are hardened off and outdoor soil has warmed to at least 60 degrees Fahrenheit — ideally 65 to 70 degrees — they are ready for the garden. Plant them after your area's last frost date. 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 the plant and improving water and nutrient uptake. Space seedlings 24 to 36 inches apart depending on the variety — determinate (bush) types need less space than indeterminate (vining) types.
Water thoroughly after planting and keep soil consistently moist for the first two weeks while roots establish. Mulch around the base with 2 to 3 inches of straw or shredded leaves to retain moisture and regulate soil temperature.
Troubleshoot common indoor seedling problems
Leggy seedlings — tall with long spaces between leaves — signal insufficient light. Move lights closer or add more hours of light per day. Seedlings that wilt despite moist soil may have damping off; improve air circulation with a small fan and avoid overwatering.
Yellow lower leaves are normal as seedlings grow and shed their cotyledons, but yellowing throughout the plant suggests nitrogen deficiency. Feed seedlings with a diluted liquid fertilizer every 7 to 10 days once they develop true leaves. Purple or reddish leaves indicate cold stress; move seedlings to a warmer location.
If seedlings are ready to transplant outdoors but frost is still forecast, keep them indoors under lights and continue watering and fertilizing. They can stay in pots for several extra weeks if necessary, though they may become root-bound. Transplanting into larger pots buys more time.
Frequently Asked Questions
Can I start tomato seeds in a regular potting soil instead of seed-starting mix?
Regular potting soil is heavier and holds more moisture than seed-starting mix, which increases the risk of damping off and fungal disease in young seedlings. Seed-starting mix is inexpensive and designed specifically for germination. If you must use potting soil, mix it half-and-half with perlite or vermiculite to improve drainage.
What if I do not have grow lights — can I use a regular lamp?
Regular incandescent or LED household bulbs do not provide the right light spectrum for seedlings and can overheat plants if placed close enough to be effective. Fluorescent shop lights or LED grow lights designed for plants are inexpensive and widely available. A basic two-bulb fluorescent fixture costs $20 to $40 and works well for dozens of seedlings.
How do I know when seedlings are ready to transplant outdoors?
Seedlings are ready when they are 6 to 8 inches tall, have several sets of true leaves, and have been hardened off for 7 to 10 days. The most important factor is timing: wait until after your last frost date and soil has warmed. A seedling that is too small but planted at the right time will catch up faster than a large seedling planted too early and damaged by cold.
Can I save seeds from indoor-grown tomatoes to start next year?
Yes, if the tomatoes are from an open-pollinated or heirloom variety. Hybrid tomatoes produce seeds, but the plants grown from those seeds will not match the parent plant. Collect seeds from ripe tomatoes, ferment them in water for 2 to 3 days to remove the gel coating, rinse, dry completely on a paper towel, and store in a cool, dry place until next planting season.