Damping-off is the sudden collapse and death of seeds or young seedlings caused by fungi and fungus-like organisms in the growing medium. It is the single most common reason a promising tray of seedlings keels over overnight, and it is almost always a propagation-conditions problem rather than bad seed. The good news: once you recognise it, it is far easier to prevent than to cure — and it is very preventable.
The short answer
If seedlings topple with a thin, dark, water-soaked pinch right at the soil line — often several at once, spreading in a patch across the tray — that is damping-off. Affected seedlings cannot be saved, but you can stop it spreading by improving airflow, easing off water, letting the surface dry, and bottom-watering. Prevent it with clean trays, fresh sterile medium, thin sowing and warmth that speeds germination.
What damping-off looks like
It comes in two forms. Pre-emergence damping-off rots the seed or sprout before it ever breaks the surface, so germination looks poor and patchy for no obvious reason. Post-emergence damping-off is the classic one: a seedling that had emerged fine suddenly develops a constricted, dark, water-soaked stem at the soil line, then topples over while the top may still look green. The collapse typically appears in patches and spreads outward across the tray, and you may see fuzzy white or grey mould on the medium surface or on fallen seedlings.
What causes it
Damping-off is caused by soil-borne pathogens — commonly Pythium, Rhizoctonia, Phytophthora and Fusarium — that thrive in the exact conditions many people accidentally create when sowing: cold, wet, airless medium. The classic risk factors are overwatering and medium that stays saturated, poor air circulation (especially under a humidity dome left on too long), sowing too thickly so seedlings crowd, cool temperatures that leave seedlings sitting slowly in damp medium, and — crucially — contaminated trays, tools or reused, unsterile potting mix and garden soil that carry the pathogens in.
How to tell damping-off from other problems
This is where most people go wrong, because several unrelated problems make seedlings flop. Use these distinctions:
- vs a dry seedling: a thirsty seedling wilts but the stem is intact and the medium is dry, and it revives within hours of watering. Damping-off leaves a rotted, pinched stem at the soil line and never recovers — and the medium is usually wet, not dry.
- vs ordinary overwatering: plain overwatering yellows and wilts seedlings gradually without the tell-tale soil-line constriction, and does not spread as an advancing patch of collapsed stems. Overwatering is, however, the main condition that invites damping-off — see overwatering vs underwatering.
- vs root rot in an established plant: root rot affects older, established plants at the root system. Damping-off strikes only seeds and very young seedlings, at the stem base, during and just after germination.
- vs transplant shock: transplant shock follows pricking out or potting on — a physical setback in handled seedlings — not a rotting stem in an undisturbed seed tray.
- vs heat or light collapse: seedlings grown too dark stretch tall, pale and floppy (leggy etiolation) but the stem is not rotted; seedlings in excess heat wilt and recover as it cools. Neither shows the dark, water-soaked pinch of damping-off.
What to do now
You cannot cure a seedling whose stem has already collapsed, and fungicides rarely rescue an active outbreak, so focus on halting its spread. Remove collapsed seedlings and the medium immediately around them. Take off any humidity dome, increase air movement (a gentle fan nearby helps enormously), move the tray somewhere warmer and brighter, and stop overhead watering — let the surface dry between waterings and water from below instead. In a bad outbreak it is often faster and safer to re-sow into fresh medium in a clean tray than to nurse survivors.
How to prevent it
- Start clean. Use fresh, sterile seed or propagation medium — not reused compost or garden soil — and wash trays and pots, ideally with a dilute bleach solution, before sowing.
- Do not overwater. Keep the medium moist, never saturated; water from below and let the surface dry slightly between waterings.
- Give it air. Remove humidity covers as soon as seeds germinate, space trays, and provide gentle air movement to keep the surface dry and the fungi in check.
- Sow thinly. Crowded seedlings hold moisture and let the disease jump plant to plant; thin sowing and thinning early both help.
- Provide warmth. Bottom heat (a seedling heat mat) speeds germination so seedlings grow up out of the vulnerable stage quickly rather than sitting cold and damp.
- Do not sow too deep, which keeps the sprout in wet medium longer than necessary.
When it is likely — and when it is not
Damping-off is a risk whenever seeds are started in cool, wet, still, crowded conditions or in reused medium; it is uncommon when propagation is clean, warm, airy and not overwatered. If mature plants are collapsing, or the problem is on established seedlings you have already potted on, look instead at root rot, botrytis, or a watering fault rather than damping-off. For the wider sowing method, see seed germination and starting seeds and plant propagation methods.
Related guides
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