Damping Off
Disease · Commonly affects: tomatoes, lettuce, cabbages, courgettes, basil, coriander, sweet peas, French beans
Damping off is a term for several fungal diseases that kill germinating seeds and young seedlings. It is most common when compost is cold, wet and poorly ventilated, often in early UK spring. Because seedlings can collapse overnight, it is a frustrating cause of gaps in trays, but clean, careful sowing greatly reduces the risk.
How to identify it
Seeds may fail to emerge, or seedlings suddenly topple at soil level. Their stems develop a narrow, water-soaked, brown or dark patch, often only 1-3 mm long, then become thin and thread-like. Roots may be brown and rotten rather than pale and firm. Affected seedlings collapse while leaves are still green, and neighbouring seedlings can follow quickly. The compost may feel persistently wet, compacted or smell stale.
Damage
Damping off can destroy an entire sowing before seedlings develop true leaves. It creates uneven trays, delays planting dates and wastes seed. Surviving seedlings may be weak, with damaged roots that struggle after pricking out. The disease is most serious in densely sown vegetables, herbs and flowers kept in cool, damp conditions.
Prevention
Use clean trays, pots and labels, fresh peat-free seed compost and clean mains water. Sow thinly so air can circulate, cover seed only to the recommended depth, and avoid compressing compost heavily. Keep seedlings at a suitable steady temperature and provide light and gentle ventilation as soon as they emerge. Water sparingly from below where practical, allowing the compost surface to dry slightly between watering. Avoid reusing diseased compost.
How to manage it
Remove collapsed seedlings and a small area of surrounding compost immediately, then improve airflow and reduce watering. Move trays to a brighter, suitably warm position, avoiding cold windowsills and stagnant corners. Prick out healthy seedlings promptly if they are crowded, using clean pots and fresh compost. Do not compost affected material. If losses are widespread, start again with washed equipment, new compost and a thinner sowing.
Damping Off: frequently asked questions
How do I identify damping off?
Seeds may fail to emerge, or seedlings suddenly topple at soil level.
How do I prevent damping off?
Use clean trays, pots and labels, fresh peat-free seed compost and clean mains water.
How do I get rid of damping off?
Remove collapsed seedlings and a small area of surrounding compost immediately, then improve airflow and reduce watering.
General guidance for UK home gardens and allotments, focused on prevention and non-chemical control. For a confirmed diagnosis, the RHS or a local garden centre can help. Page updated 19 Sep 2026.