Starting Seeds Indoors: A Complete Beginner's Guide
Updated June 2026 · SowWhen
Starting seeds indoors gives frost-tender, slow-growing crops the weeks of head start the outdoor season won't provide. Done well, it means earlier harvests, far more variety than any garden center sells as transplants, and a fraction of the cost. Done carelessly, it means a windowsill of pale, floppy seedlings that never recover. The difference comes down to four things: timing, light, warmth, and water.
Which crops to start indoors — and which not to
Start indoors: tomatoes, peppers, eggplant, and basil (all tender and slow), plus brassicas like broccoli, cabbage, and cauliflower, and often onions and leeks. These reward the head start. Do not bother starting indoors: root crops (carrots, beets, radishes, turnips) resent transplanting; and fast, frost-hardy direct-sowers like beans, peas, corn, squash, and cucumbers usually do better sown straight into warm soil. SowWhen labels each crop's preferred method, so let the calendar tell you which camp a plant is in.
Timing: count backward from your last frost
The whole schedule pivots on your average last spring frost. Most seed packets say something like "start indoors 6–8 weeks before last frost." Start too early and your seedlings outgrow their pots and get root-bound and leggy before it's safe to plant out; start too late and you give up the head start entirely. SowWhen does this counting for you — it prints the actual start-indoors weeks for your zone — but the underlying logic is always "weeks before last frost."
Light is the thing people get wrong
A bright windowsill is almost never enough. Seedlings reaching and leaning toward a window, with long thin stems and widely spaced leaves, are starved for light — the classic "leggy" seedling. The fix is a shop light or LED grow light kept just an inch or two above the tops of the plants and run 14–16 hours a day on a timer, raised as they grow. Under strong, close light, seedlings stay short and stocky, which is exactly what you want when it's time to transplant.
Warmth to germinate, cooler to grow
Most seeds germinate fastest in warm soil — around 70–75°F for tomatoes and peppers — so a seed-heat mat under the tray speeds things up dramatically. Once seedlings emerge, though, remove the heat and grow them a touch cooler; cooler air plus strong light produces sturdier plants than a hot, dim windowsill. A small fan for a few hours a day, or a gentle daily brush of your hand across the tops, thickens stems and hardens the plants off before they ever go outside.
Water, feed, and pot up
Water from the bottom when possible and let the surface dry slightly between waterings — constantly soggy soil invites "damping off," a fungal collapse that flattens seedlings overnight. Seed-starting mix has little nutrition, so begin a weak, diluted liquid feed once true leaves appear. If a plant fills its cell before it's safe to go out, pot it up into something larger rather than letting it stall. Finish with a week of hardening off — gradually introducing the seedlings to outdoor sun and wind — and they'll transition into the garden without the shock that sets unhardened plants back for weeks.
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