Damping off: identify, prevent, control

Updated June 2026

Worst in cold, wet conditions in early spring and autumn sowings, March to May and August to October.

Damping off is a soil fungus that rots seeds and seedlings at or below the soil line, toppling them just after they emerge. It thrives in cold, wet, overcrowded seed trays, so it is the classic killer of early indoor sowings.

How to identify it

• Seedlings collapsing with a thin, pinched, water-soaked stem at soil level
• Patches of seedlings falling over within a day or two
• Seeds that rot before they emerge
• A white mould sometimes visible on the soil surface

How to prevent it

• Sow into fresh, sterile seed-raising mix rather than garden soil
• Avoid overwatering and let the surface dry slightly between waterings
• Sow thinly and provide good airflow and light to seedlings
• Use clean trays and tools, and warm the mix for warm-season seeds

Organic control, step by step

• Remove collapsed seedlings and any visible mould immediately
• Cut back watering and improve airflow around the tray
• Move trays somewhere warmer and brighter to speed growth past the vulnerable stage
• Water from below and avoid leaving trays sitting in water
• Water with a weak chamomile tea or seaweed solution, which can help suppress the fungus
• Start again with fresh sterile mix and clean trays if a batch is lost

Plants it attacks