Starting Seeds Indoors: Timing, Light, and the Mistakes That Kill Seedlings

Starting seeds indoors buys a head start on the growing season, but it only pays off if the timing is right and the seedlings get enough light once they germinate. Get either wrong and you end up with leggy, pale plants that transplant poorly, which is worse than direct-sowing later would have been.

Count backward from your last frost date

Seed packets give a 'weeks before last frost' figure for a reason: start too early and plants outgrow their pots before it is safe to move them outside; start too late and you lose the head start entirely. Find your average last spring frost date, then count backward using the packet's window.

Light is the limiting factor, not warmth

A sunny windowsill supplies a fraction of the light intensity seedlings actually need, which is why windowsill seedlings almost always stretch toward the glass and topple. A basic shop light or dedicated grow light held 2-4 inches above the leaves and run 14-16 hours a day produces short, sturdy stems instead.

• Raise the light as seedlings grow so it stays 2-4 inches above the topmost leaves.
• Run a small fan nearby for an hour or two a day to strengthen stems before transplanting.
• Bottom water or water gently at the soil line to avoid fungal issues on wet foliage.
• Use a seed-starting mix, not garden soil, which compacts and can carry disease.

Damping off is the most common early failure

Damping off is a fungal disease that collapses seedlings at the soil line within days of emergence. It thrives in cool, wet, poorly ventilated conditions. A fan for airflow, bottom watering instead of overhead misting, and not overwatering are the three most effective preventions.

Hardening off before transplanting

Seedlings raised indoors have never felt direct sun, wind, or temperature swings, and moving them straight to the garden shocks or scorches them. Over 7-10 days, set them outside in a sheltered spot for a couple of hours the first day, gradually increasing sun and time until they are outside full days before the final transplant.

1. Days 1-2: a few hours in dappled shade, sheltered from wind.
2. Days 3-5: half a day, including some direct morning sun.
3. Days 6-8: full days outside, brought in only if a cold night threatens.
4. Days 9-10: left out overnight if temperatures are safely above the crop's minimum, then transplanted.