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. Once a seedling collapses it cannot be saved, but you can stop it spreading to the rest and, more importantly, prevent it next time. Here is how.
- Collapsed toppled at the base
- Pinched stem thin and dark at soil line
- White mould fuzzy film on compost
- Sudden wilt healthy then floored overnight
What causes damping off
- Too much water. Wet, airless compost is the perfect home for the fungi involved.
- Poor airflow. Still, humid air under a permanently closed lid lets mould take hold.
- 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 if it strikes
You cannot revive a collapsed seedling, but you can protect the survivors:
- Remove affected seedlings and the compost around them straight away.
- Take any covers off and improve airflow with 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 damping off
- Clean pots wash before use
- Fresh compost seed-grade
- Sow thinly space to breathe
- Water from below keep tops dry
- Good airflow vent the lid
- Warm, bright steady growth
Prevention is all about clean, warm, airy and not-too-wet. Use clean pots and fresh seed compost, sow thinly, water from below to keep the surface drier, and give seedlings light and a little moving air. Take covers off as soon as 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 kitchen hazard, but improve airflow and ease off the water to clear it.
Can I use tap water?
Yes — the bigger factors are how much you water and airflow, not the water itself.
Does it affect grown plants?
Damping off mainly strikes young seedlings; established plants are far more resistant.
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