I want to get a head start on my spring garden but I've never done indoor seed starting before. Should I use regular potting soil or something special, and how much light do seedlings actually need? I have a sunny windowsill but I'm not sure if that's enough.
Start with a seed-starting mix rather than regular potting soil - it's lighter and drains better, which keeps seedlings from rotting. A sunny windowsill can work, but seedlings typically need 14-16 hours of consistent, fairly intense light, and most windows don't quite deliver that, so a cheap LED grow light a few inches above the trays makes a huge difference and is worth the investment. Keep the soil consistently moist but not waterlogged, and once they sprout, make sure there's decent air circulation with a small fan to prevent mold and strengthen the stems.
I've killed plenty of seedlings by babying them too much indoors, so here's what actually matters: seed-starting mix is definitely the way to go over regular potting soil, and yeah, a windowsill probably won't cut it unless you're getting like 16+ hours of direct sun daily. The thing people miss is that even with decent light, seedlings get leggy and weak if they're competing for rays, so most folks end up needing a basic grow light anyway - doesn't have to be fancy, just something close enough to keep them stocky and compact. Start with that assumption going in and you'll save yourself the frustration of transplanting these sad, stretched-out plants later.
When I first tried indoor seed starting, my biggest mistake was assuming my south-facing windowsill would be enough - it wasn't. The seedlings got leggy and weak because they were stretching toward the light source. If you want decent results, you really do need supplemental lighting. A basic LED grow light or even shop lights with cool fluorescence will make a massive difference. Keep them about 2-3 inches above the seedlings and run them for 14-16 hours daily. Your windowsill can be part of the setup, but it shouldn't be your only light source unless you're in an extremely bright location and can rotate the trays constantly.
On the soil front, the earlier answers nailed it - seed-starting mix is worth the investment. Regular potting soil compacts too easily and holds moisture in ways that invite damping-off disease, which will kill your seedlings fast. The lighter texture also lets roots establish better early on. I'd add that you don't need anything fancy; a basic seed-starting mix from any garden center works fine.
Beyond those two things, stay consistent with watering. Keep the soil moist but not waterlogged - the seedlings hate sitting in standing water. Use a spray bottle or water from below if you can, rather than pouring from above and disturbing the tender plants. Temperature matters too; most vegetable seeds germinate best around 65-75°F. Once they've sprouted, you can drop it a few degrees if needed, which also helps prevent those stretchy, weak seedlings.
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