Seeds & growth
Damping off
A fungal disease that makes young seedlings collapse and rot at the base, encouraged by cold, wet, overcrowded conditions.
Damping off is a disease that kills seedlings just as they are getting going. It is caused by several soil-borne fungi and fungus-like organisms that thrive in cold, wet, airless compost. They attack the seedling at its weakest point, the thin stem at soil level, causing it to shrivel, darken and topple over. Once a seedling has damped off it cannot be saved, and the rot spreads quickly through a crowded tray, so prevention is everything.
What it looks like
The classic sign is a seedling that has collapsed at the base, with the stem pinched thin and brown or water-soaked right where it meets the compost. The top of the plant may still look green for a day or so, but it is already doomed. You might also see fuzzy white or grey mould on the compost surface, or seeds that rot before they ever break through. It tends to spread in patches, taking out a cluster of seedlings rather than the odd one here and there.
What causes it
The fungi are almost always present; they only cause trouble when conditions favour them over the seedlings. The main triggers are:
- Cold, wet compost — damp, chilly conditions slow the seedlings down while the fungi flourish.
- Overcrowding — seedlings sown too thickly trap moisture and stop air moving between them.
- Poor airflow — still, humid air on a windowsill or in a covered propagator keeps everything damp.
- Dirty trays or reused compost — old pots and garden soil can carry the fungi from one batch to the next.
How to prevent it
You cannot cure damping off, but it is largely avoidable with clean, careful sowing:
- Start clean. Wash and dry seed trays and pots before use, and always sow into fresh, sterile seed compost rather than reused or garden soil.
- Sow thinly. Space seeds out so seedlings are not packed shoulder to shoulder, which also helps avoid weak, leggy seedlings.
- Water from below. Stand trays in a shallow tray of water and let the compost draw moisture up, then drain — this keeps the surface and the vulnerable stems drier.
- Give them air. Take lids off propagators once seeds are up, leave space between trays and run a gentle breeze or a cracked window to keep air moving.
- Use clean water and don't overwater. Mains water is safest, and letting the surface dry slightly between waterings makes life much harder for the fungi.
In a UK garden
In the UK it strikes most often in late winter and early spring, when seedlings are raised on cool, gloomy windowsills and the compost stays damp for too long between waterings.
Example
A tray of tomato seedlings that looked healthy one evening keels over by morning, each stem pinched thin and brown right at soil level — that's damping off.