Starting seeds indoors: a complete guide

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.

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.

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.