Swiss Chard Seedlings Collapsing — Is It Damping Off?

Damping off is one of the most disheartening problems for gardeners starting swiss chard from seed. Seedlings emerge looking healthy, then suddenly topple over at the soil line, the stem pinching and turning dark at the base. In severe outbreaks, entire trays of seedlings can be lost within a couple of days. The condition is caused by a group of fungal and oomycete pathogens—primarily Pythium, Rhizoctonia, and Fusarium species—that attack the stems of very young plants before their tissues toughen.

Recognising Damping Off

The classic symptom is a seedling that appears healthy above the soil but has a constricted, water-soaked or dark stem right at the soil surface. The stem collapses under the weight of the leaves and the seedling falls over. Sometimes seeds fail to emerge at all (pre-emergence damping off). In a tray situation, the collapse often spreads outward from a central point as the water-borne pathogens move through saturated compost. A musty or sour smell from the tray is another indicator.

The Three Conditions That Cause Damping Off

Three factors almost always combine to cause damping off: overwatering (saturated, poorly drained compost), poor air circulation (which keeps the surface wet and humid), and sowing too densely (seedlings touching create a humid microclimate at soil level). Low light contributes too—seedlings in poor light grow weak and spindly, making them more susceptible. Understanding these causes makes prevention straightforward.

Prevention: Start with the Right Compost and Containers

Use fresh, sterile seed-sowing compost—not garden soil or reused compost, which harbour the pathogens. Ensure all containers have adequate drainage holes and are clean (wash with a weak bleach solution if reusing trays). Sow seeds thinly so seedlings have space and airflow from the start. Water from the base (stand the tray in shallow water for 20 minutes, then drain) rather than from above, which keeps the surface drier and less hospitable to fungal growth.

Airflow and Light Are Critical

Place trays in your brightest, most ventilated location. A gentle fan running nearby for a few hours each day strengthens stems and dries the surface quickly after watering. If growing under lights, ensure they are close enough to prevent leggy growth. Open ventilation covers on propagators during the warmest part of each day to prevent humid air from building up around seedlings. Once seeds have germinated and the first true leaves appear, remove covers entirely.

What to Do When Damping Off Strikes

Act immediately. Remove and discard all collapsed seedlings. Do not compost them. Allow the compost surface to dry out before watering again. Increase airflow around surviving seedlings. In a tray situation, quickly remove any surviving seedlings that look healthy and pot them on individually into fresh compost. The surviving plants may recover if removed from the contaminated environment promptly. Do not try to save the original tray—the pathogens persist.