Leggy seedlings

also called Etiolation, Stretching

Seedlings stretching pale and thin toward the window, falling over under their own weight. Not enough light — and no amount of feeding will fix it.

Disorder Serious Doesn't spread

What you'll see

A long, thin, pale stem between the seed leaves and the soil, sometimes several centimeters of it.
Seedlings leaning hard toward the window and needing turning every day.
Leaves pale, small and widely spaced up the stem.
Plants toppling over or kinking at the base under their own weight.
A tray that grew fast, looked promising for a week, and then collapsed.

Why it happens

A seedling in low light gambles everything on height, because in nature the light it cannot reach is above it. It spends its stored reserves on stem rather than leaf, and the tissue it makes is soft and weak. A windowsill in February gives a fraction of the light of an overcast day outdoors, and warmth without light makes the stretching worse rather than better.

Sowing too early for the light available, a north-facing windowsill, seedlings too far under a grow light, crowded trays, and a propagator kept warm in a dim room. Warmth accelerates it; the fix is always light, not heat.

Treating it

Get the light right first: an inexpensive LED grow light 5–10 cm above the seedlings for 14–16 hours a day transforms them.
Drop the temperature. Cool and bright grows a stocky seedling; warm and dim grows a leggy one.
Repot tomatoes, brassicas and cucurbits deeply, burying the bare stem up to the seed leaves — tomatoes will root along the buried stem and be the better for it.
Brush the seedlings with your hand or run a fan on them for a few minutes a day; the movement thickens the stems.
Start again if they are badly gone. A fresh sowing under proper light will overtake a stretched tray within a fortnight.

Keeping it away

Sow later. Almost every leggy seedling problem starts with sowing weeks before the light can support it.
Use a grow light from germination, not from when the trouble shows.
Thin and space seedlings early so none is shaded by its neighbor.
Keep the propagator warm only until germination, then move the tray somewhere cooler and brighter.
Harden plants off properly, which continues the same job of thickening the tissue.

Who gets it

Any seedling raised indoors — tomatoes, peppers, brassicas and annual flowers show it worst.