Damping off and seedling mould
You check on your seedlings and find them toppled over at soil level, their stems pinched and thin — or a film of white, fuzzy mould on the compost. This is damping off: a fungal problem that thrives in cold, wet, still air. A collapsed seedling cannot be saved, but you can stop it spreading.
How to recognise it
Sign | What it looks like |
|---|---|
Collapsed | toppled at the base |
Pinched stem | thin and dark at the soil line |
White mould | fuzzy film on the compost |
Sudden wilt | healthy in the evening, floored by morning |
What causes it
- Too much water. Wet, airless compost is the perfect home for the fungi involved.
- Poor airflow. Still, humid air under a lid that never comes off.
- Overcrowding. Densely sown seedlings trap moisture and shade each other.
- Cold compost. Slow-growing seedlings in cold, damp mix are far more vulnerable.
- Dirty pots or reused compost. Old trays and used compost can harbour the fungi.
What to do now
- Remove affected seedlings and the compost around them straight away.
- Take any covers off and add a gentle fan.
- Let the surface dry out a little: stop misting and water sparingly, from below.
- Move the tray somewhere brighter and a touch warmer.
- If a whole tray is lost, start again with clean pots and fresh compost. With fast crops you lose little time.
How to prevent it
What | How |
|---|---|
Clean pots | wash before use |
Fresh compost | seed-grade |
Sow thinly | space to breathe |
Water from below | keeps the surface drier |
Good airflow | vent the lid |
Warm and bright | steady growth |
It comes down to clean, warm, airy and not too wet. Take covers off as soon as the seeds germinate. Strong, fast-growing seedlings rarely succumb.
Frequently asked questions
Can I save a seedling that has collapsed? No. Once the stem has gone, that seedling is lost. Focus on protecting the rest.
Is the white mould harmful to me? It is a seedling problem, not a hazard to you. More airflow and less water clear it.
Can I use tap water? Yes. How much you water and how much air there is matter far more than the water itself.
Does it affect grown plants? Damping off mainly strikes young seedlings. Established plants are far more resistant.
The full version of this guide is on seedschoice.com.
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Updated on: 05/10/2026
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