Starting seeds indoors: a complete guide
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
Starting seeds indoors buys you weeks of season for warm crops and gives you varieties no nursery carries. It needs light and attention more than equipment.
Get the timing right
Work back from your last frost date by the crop head-start on the seed packet. Too early is a real mistake: a root-bound, leggy six-week-old tomato stuck waiting three extra weeks for warm soil does worse than one sown on time.
The seed-starting calculator lists the sow-indoors date for every crop once you enter your frost date or state.
How many weeks before the last frost to sow indoors
What to start indoors - and what to sow direct
Start indoors: crops that need a long season or transplant well - tomatoes, peppers, eggplant, onions, leeks, celery, broccoli, cabbage, kale and many herbs.
Sow directly outdoors: root crops (carrots, beets, radishes, parsnips), beans, peas and corn. Their roots resent being disturbed, and they grow quickly from seed once the soil is warm.
Cucumbers, squash and melons can be started indoors three to four weeks ahead in individual pots to beat a short season or early pests - but no earlier, because older transplants stall.
Containers and mix
Any clean container with drainage works: cell trays, soil blocks, yogurt pots. Use a light, sterile seed-starting mix, not garden soil or heavy potting soil, which stay too wet and carry disease.
Sow at the depth on the packet — roughly twice the seed diameter. Tiny seeds (lettuce, basil) need light to germinate and are barely covered.
Warmth to germinate, light to grow
Most seeds germinate fastest in soil around 70–80°F; a seedling heat mat helps for peppers and eggplant. Remove the mat and any humidity dome once seedlings emerge.
From the moment they sprout, seedlings need bright light within a couple of inches of a shop-style LED or fluorescent fixture for 14–16 hours a day. A windowsill alone almost always produces stretched, weak plants.
A simple light setup
Two shop-style LED fixtures hung on chains over a shelf light a standard 10×20-inch tray well. Plug them into an outlet timer set for 14-16 hours, and keep them 2-3 inches above the leaves, raising them as the plants grow.
A small fan on low for a few hours a day strengthens stems and dries the surface of the mix, which helps prevent damping-off. Brushing a hand lightly across the tops of the seedlings each day has a similar effect.
Water, feed, and pot up
Keep the mix evenly moist, not soggy; bottom-watering avoids knocking seedlings over and damping-off. Once true leaves appear, feed weekly with a half-strength liquid fertilizer.
Move seedlings to larger pots when roots fill the cell or when tomatoes reach the first true leaves — burying the stem deeper each time for tomatoes.
Harden off before transplanting
Over 7–10 days, set plants outside in a sheltered spot for a gradually longer time each day, starting with an hour in shade and building to full sun and a night out. Skipping this step shocks or kills plants that were otherwise ready — see the hardening-off guide for the full schedule, and transplanting without shock for the move itself.
Troubleshooting
Tall, pale, floppy seedlings are short of light - lower the lights. See leggy seedlings explained.
Seedlings that pinch and collapse at the soil line have damping-off, a fungal disease of cool, wet mix. Sow in fresh sterile mix, water from below and keep air moving.
Nothing came up? Cold mix, old seed or sowing too deep are the usual causes - see why seeds are not germinating.
A purple tinge under tomato leaves usually means the mix is too cool for the roots to take up phosphorus. It normally fades once the seedlings are warmer and fed.
Common questions
How early should I start seeds indoors?
Count back from your average last frost date: about 8-10 weeks for onions, leeks and peppers, 6-8 for tomatoes and eggplant, 4-6 for brassicas and lettuce, and just 3-4 for squash, cucumbers and melons. The seed-starting calculator gives each date.
Can I start seeds on a windowsill?
You can, but most windowsills give far less light than seedlings need, so they stretch toward the glass. A bright south-facing window works for sturdy crops if you turn the tray daily; for tomatoes and peppers, a simple LED shop light gives much better plants.
Do seeds need light to germinate?
Most don't - they need warmth and moisture. Lettuce and a few herbs do need light, which is why their tiny seeds are barely covered. Once any seedling emerges, though, it needs strong light straight away.
When can seedlings go outside?
After 7-10 days of hardening off, and when the weather suits the crop: hardy brassicas and onions around the last frost date, tender tomatoes, peppers and squash one to two weeks after it, once nights stay above about 50°F.