Why Are My Radish Seedlings Collapsing at Soil Level?

Radish seedlings that emerge and then suddenly topple over at soil level — the stem pinching to a thin point at the soil surface and the seedling collapsing — are suffering from damping off. This is a condition caused by several soil-borne fungal pathogens (Pythium, Rhizoctonia, Fusarium species) that attack the stem of newly emerged seedlings at or just below the soil surface. A ring of collapsed seedlings spreading across a bed from a central point is the classic pattern.

What causes damping off

Damping off pathogens are always present in garden soil. They become a problem only when conditions favour their rapid development: consistently wet or waterlogged soil, cool temperatures that slow seedling growth while keeping the pathogens active, poor surface drainage, and dense sowing with minimal airflow between seedlings. The pathogens colonise the stem at soil level and cause it to rot, cutting off the seedling from its roots.

Preventing damping off through drainage

The soil surface around newly sown radish should never be constantly waterlogged. Water after sowing but then allow the top surface to dry slightly before watering again. In raised beds, ensure drainage is adequate. In open ground, incorporate fine grit into the top few centimetres of the seedbed if the soil tends to compact and pool water. A seed covering of fine vermiculite or sharp sand rather than heavy topsoil provides better drainage and airflow around emerging stems.

Airflow and spacing

Overcrowded seedlings create humid microclimate conditions at soil level that favour pathogen development. Sow radish at the recommended rate rather than thickly, and thin promptly to the correct spacing. Row cover placed directly on the seedbed traps humidity; if damping off is a recurring problem, lift the cover edges periodically to allow air circulation.

Starting fresh after an outbreak

Once damping off has established in a bed, affected seedlings cannot be saved. Remove the entire affected patch, allow the area to dry, and resow with fresh seeds — ideally with a thin sand or grit covering rather than soil. Consider moving to a different area of the bed as the pathogen population in the affected spot will be elevated.