Why seedlings collapse and die

First check: look at the stem where it meets the compost. A rotting or constricted stem suggests a different problem from an intact seedling leaning towards the light.

Could it be damping off?

Damping off is caused by several fungi and fungus-like organisms. Seedlings may decay before emerging or collapse afterwards, sometimes with whitish mould. Crowding, persistently wet compost, humid air and poor ventilation favour problems, especially when seedlings grow slowly.

A failed sowing alone does not confirm disease. Before starting again, check seed age, sowing depth and the temperature needed by that crop.

What to do now

Remove collapsed, decaying seedlings and keep the affected tray apart from healthy batches. A seedling whose stem has rotted through will not recover. If losses continue, a fresh sowing in a clean container and fresh seed compost is usually a more useful investment than repeated rescue treatments.

Improve airflow without exposing tender plants to cold draughts. Check compost before watering, let excess water drain, and avoid keeping trays constantly saturated. Remove propagator covers after germination as appropriate for the crop.

Check the alternatives

Long, pale, leaning seedlings: investigate inadequate light in relation to warmth. Move them into better light promptly; a warm windowsill may be too dim for strong growth.

Dry compost and intact stems: investigate drying out. Small cells can lose moisture quickly, especially near a heater or in sun. Water gently and reassess the surviving plants.

Missing or chewed seedlings: inspect for pests, including after dark. A severed stem or missing leaves needs a different response from decay.

Collapse after planting out: consider cold, drying wind, root damage and insufficient hardening off. Compare the weather with the crop's requirements.

Make the next batch easier to manage

Use clean containers, fresh suitable seed compost, clear labels and enough space between seedlings. Sow a manageable quantity so you can check each tray regularly. Use clean mains water where available for vulnerable seedlings, and keep stored water clean and covered.

Record the crop, sowing date, compost, germination and first losses. Change the suspected cause in the next batch rather than adding several unproven remedies at once. Avoid household chemical drenches and improvised fungicides.

Back to Solve a Growing Problem · Browse seed-starting guides

Further reading: RHS guidance on damping off, sowing vegetable seeds and transplanting.

By Simon Adams · Published 27 October 2026. Adapt advice to your crop and conditions.