What's wrong with my seedlings?

Pick what you can see. You get the usual causes and what to do.

Tall, pale and floppy

Long thin stems leaning toward the window.

Not enough light. Window light is usually not enough indoors (Clemson Extension). Move to a brighter place or add a grow light 2 to 4 inches above the seedlings and raise it as they grow.
Too warm after germination. Once seedlings are up they do not need the germination heat; move them to a cooler, brighter spot.
Crowding. Thin or prick out into individual pots.

A seedling falls over; the stem is thin, dark or water-soaked at the base.

Damping off, a fungal problem favoured by wet mix, cool conditions, crowding and dirty containers (RHS, Penn State Extension). Remove affected seedlings and the mix around them. Thin the rest, let the surface dry a little and ventilate. Next time, use clean pots and fresh seed mix, and sow thinly. No fungicide is available to home gardeners (RHS).

White, grey or green growth on the surface.

Too wet and too little air. Take the lid off, let the surface dry, scrape the fuzz away and improve airflow. Water from below less often.
Contaminated or old mix. If seedlings start to collapse as well, treat it as damping off and start again with fresh mix.

Leaves turning pale yellow.

Overwatering: roots cannot breathe in soggy mix. Let the mix dry slightly between waterings and make sure there are drainage holes.
Hunger: seed mix holds little food. From the first true leaves, feed every other watering with diluted liquid feed (Clemson Extension), or pot on into fresh compost.
Cold. Move somewhere warmer, out of draughts.
Normal ageing of the seed leaves. The first pair (cotyledons) yellow and drop when true leaves take over; nothing to do.

A purple tint on leaves or undersides.

Cold roots. Some varieties also colour naturally. Warm the mix and avoid cold draughts; check the packet for the variety.
Hunger. If growth has also stalled, start feeding with a diluted liquid feed.

Papery brown patches, often after being moved outdoors.

Sun scorch or cold from skipping hardening off. Shade and shelter the plants and carry on hardening off gradually. The RHS says to cut out cold-damaged growth and continue.
Grow light too close, or fertiliser too strong. Keep tubes at the recommended distance and dilute feed.

Drooping leaves in the daytime.

Dry mix. Water from below until the mix is damp, then drain.
Waterlogged mix, rotting roots. If the mix is wet and the plant is still drooping, let it dry out; check for drainage and for damping off.
Transplant shock. Shade, water and give them a week.

Seedlings sit at the same size for weeks.

Too cold. Raise the temperature of the mix, and move away from cold windows.
No food or no space. Feed from the first true leaves and pot on.
Not enough light. More light, closer lights.
Some crops are simply slow (celery, lavender, parsley). Check the crop page and give it time.

The empty seed coat is still wrapped around the leaves.

The mix dried out as the seedling emerged, or humidity is low. Mist gently and let the seedling free itself; a stuck coat can be moistened and eased off, but often it is best left alone.

Holes, nibbled stems or whole seedlings gone.

Slugs and snails. Remove by hand, use biological controls (RHS), and guard young plants.
Birds. Taut netting (RHS).
Mice. The RHS suggests raising seedlings indoors where mice are a problem.

Small dark flies hovering over the mix.

Fungus gnats, which breed in constantly damp mix. Let the surface dry between waterings and keep the mix clean; they are usually a symptom of overwatering.

Leaves rolled, cupped or distorted.

Gaps in the tray or row.

Seed viability: old or poorly stored seed. Test a sample on damp kitchen paper next time.
Uneven moisture, depth or temperature across the tray. Sow evenly, water from below, and rotate the tray.
Slow seed still to come. Wait; see the troubleshooter.

Seedlings all bend in the same direction.

Light from one side. Turn the tray every day, or put reflective card behind the seedlings.
No air movement, so stems stay weak. Gentle fan, or brush the tops with your hand daily.