Indoor Seed Starting 101

By GardenPlanner Team · July 17, 2026

Starting seeds indoors buys a plant like a tomato or pepper extra weeks of growing time before your last frost, so it can start setting fruit as soon as the weather allows outside. Not everything needs this head start — root crops like carrots hate being transplanted and should be sown directly in the ground — but for slow-to-mature, frost-tender crops, it’s the difference between a short harvest window and a long one.

The four things that matter

Warmth. Most seeds germinate fastest around 70–85°F soil temperature, which a windowsill often can’t provide in late winter. A seedling heat mat closes that gap.

Light. As soon as a seed sprouts, it needs strong, close light or it’ll stretch into a thin, weak stem reaching for a window. A dedicated grow light isn’t required — Wirecutter’s own testing found basic LED shop lights work just as well for a fraction of the cost.

Moisture. Consistently damp, never soggy. A humidity dome over the tray until seeds sprout, removed once they’re up, prevents both drying out and damping-off (a fungal disease that kills seedlings at the soil line).

Timing. This is the one people get wrong most often — starting too early leads to leggy, rootbound plants sitting in trays for weeks past when they should’ve gone outside. Our planting calendar gives you an exact indoor-start date for your zone, worked backward from your last frost date (or, in zones with a distinct seasonal calendar like the low desert Southwest, from the actual research-based planting windows for your area).

What to start indoors vs. direct-sow

As a rule: slow-maturing, frost-tender crops (tomatoes, peppers, broccoli) benefit from an indoor head start. Fast-growing or transplant-shy crops (carrots, beans, corn, most root vegetables) do better direct-sown outside. Every plant page on this site — like growing tomatoes — states which method it needs and why.