Starting Seeds Indoors, Step by Step — Timing, Light, Heat, and the Mistakes That Kill Seedlings

Starting seeds indoors is the highest-leverage skill in gardening: a $4 packet becomes forty plants, in varieties no garden center stocks. It fails for the same three reasons every time — timing, light, and water. Get those right and the rest is patience.

Count backward, not forward

Every packet says something like “start 6–8 weeks before last frost.” Find your area’s average last-frost date, then count back. Tomatoes and peppers: 6–8 weeks. Brassicas: 4–6. Squash and cucumbers: 2–3, if at all — they sprint. Onions from seed: 10–12. Starting too early is the beginner’s signature move; a leggy, root-bound March tomato is outrun by a calm April one within a month of transplant.

Light is the whole game

A sunny windowsill is not enough — it delivers a fraction of outdoor light, from one side. Seedlings raised on sills grow tall, pale, and bent. Any cheap full-spectrum LED shop light works: hang it 2–4 inches above the leaves, raise it as they grow, and run it 14–16 hours on a timer. Closer than looks reasonable is correct.

Heat for sprouting, cool for growing

Most seeds germinate fastest around 70–80°F soil temperature — a heat mat under the tray cuts tomato germination from two weeks to five days. But the mat’s job ends at sprouting: move seedlings off heat and a few degrees cooler afterward, or they stretch.

Water from below, and let it breathe

Damping off — the fungus that fells a whole tray of seedlings at the soil line overnight — is a moisture-management disease. Use sterile seed-starting mix, not garden soil. Bottom-water by filling the tray and letting cells drink for twenty minutes. Run a small fan nearby: moving air firms stems and dries the surface fungus needs.

Harden off like you mean it

Indoor seedlings have never met wind or true sun. A week before transplant, give them an hour of dappled shade outdoors, then ramp up daily. Skip this and a sunny afternoon will bleach every leaf white — survivable, but it costs two weeks.

Start small: two trays, three varieties, notes on dates. The packet is the cheapest part of the operation; the light and the timing are the crop.