Damping-off: why seedlings collapse, and how to prevent it

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What is it?

Damping-off is a soil fungus that rots seeds and young seedlings right at the base, so they keel over and die within days of coming up. It's heartbreaking with a fresh tray — but it's very preventable.

How do I spot it?

• Seedlings that flop over with a thin, water-soaked, pinched stem at soil level.
• Seeds that never come up at all.
• A white fuzzy mould on the surface of the mix, and whole patches collapsing at once.

Which plants does it affect?

Growing any of these? See our guides to lettuce, tomatoes, broccoli, coriander and parsley.

Any seedling is vulnerable when young — tomatoes, lettuce, brassicas and the rest.

What causes it?

Overwatering, poor drainage, crowded seedlings, dirty trays and cold, wet mix all invite it in. Damp mix also draws fungus gnats, whose larvae add to the damage.

How do I prevent it?

• Use fresh seed-raising mix and clean containers.
• Don't overwater — let the surface dry a little between waterings.
• Sow thinly for airflow and give the seedlings good light.
• A raised bed's free-draining soil helps once they're planted out.

How do I manage it?

There's no cure once a seedling has collapsed. Remove the affected ones, ease right off the water, improve airflow, and re-sow with fresh, clean mix.

For the bigger picture, see our guide to managing pests and problems safely.

Plan your patch: our free garden calendar shows what to plant now where you live. Ready to grow? Browse our raised garden beds or build your own with the garden bed builder.

Image: University of Georgia Plant Pathology, Bugwood.org, CC BY 3.0, via Wikimedia Commons.