Why Did My Broccoli Seedlings Collapse at Soil Level?

Seedlings that look healthy and upright one day, then are found lying flat with a dark, pinched or rotted section of stem right at soil level the next morning, have succumbed to damping off. This is caused by a group of soil-borne fungi and water moulds — most commonly Pythium, Rhizoctonia and Fusarium species — that attack the stem of young seedlings at or just below the soil surface. Once a seedling collapses from damping off, it cannot be saved.

Conditions that cause damping off

Damping off pathogens thrive in wet, poorly-ventilated conditions. The key triggers are overwatering (the most common cause), using old or contaminated compost, sowing too densely so seedlings cannot dry between waterings, and keeping trays in poor air circulation. Tap water applied overhead in the evening creates ideal conditions. The problem is worst in cool, damp weather when evaporation is slow.

Stopping it from spreading

Once damping off appears in a seed tray, remove affected seedlings immediately. Reduce watering sharply and allow the surface of the compost to dry between waterings. Move trays to a better-ventilated location. A light surface dusting of sharp sand over the compost around the stem base improves surface drainage and makes it harder for the fungi to establish. Healthy-looking remaining seedlings in the tray can often be saved if the conditions are corrected quickly.

Prevention next time

Use fresh, sterilised seed compost rather than garden soil or old reused compost. Sow thinly so seedlings have airflow around them. Water from below by standing trays in water briefly, rather than overhead. Do not allow compost to stay saturated. Clean seed trays with a dilute bleach solution between uses. These four habits eliminate damping off in almost every case.