How-to Guide
How to Start Seeds Indoors (Step by Step)
The short answer
Start seeds in a sterile, light seed-starting mix, sowing each seed about twice as deep as it is wide (some fine seeds need light and are barely covered — check the packet). Keep the mix warm and moist to germinate, then put seedlings under bright light held just 2-4 inches above them for 12-16 hours a day so they don't stretch and flop.
Timing is everything: count backward from your average last-frost date by the weeks the seed packet lists — starting too early gives you overgrown, weak transplants. Harden seedlings off outdoors over about a week before transplanting.
Step by step
- 1Use sterile seed-starting mix — Light, moisture-retaining, well-draining mix — not garden soil (which can carry damping-off fungi).
- 2Sow at the right depth — About 2x the seed's width; barely cover seeds that need light to germinate.
- 3Warm to germinate — Keep the mix warm and evenly moist; a heat mat speeds warm-season crops. Water from below to avoid dislodging seeds.
- 4Bright light, close and long — As soon as they sprout, give strong light 2-4 inches above the tops, 12-16 hours a day; raise lights as they grow.
- 5Time it and harden off — Start the number of weeks before last frost the packet says, then harden off outdoors over ~1 week before transplanting.
→ Leggy, stretched seedlings almost always mean the light is too weak or too far away — see the seedling-problems guide.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Minnesota Extension — Starting seeds indoors — Start seeds in a sterile mix at an appropriate depth, keep warm and moist to germinate, provide strong light close to seedlings, and count back from the frost-free date to time sowing.
- University of Maryland Extension — Grow Lights for Starting Seeds Indoors — Keep grow lights no more than about 4 inches above seedlings (2 inches ideal) for 12-16 hours a day; seedlings need a dark period too.