Why Did My Kale Seedlings Collapse at Soil Level?

You check your seedling tray or bed and find previously upright kale or spinach seedlings lying flat, still green above but with a pinched, dark, thread-thin stem right at soil level where the seedling meets the compost. This is damping off — a collective name for seedling death caused by several soilborne fungi including Pythium, Rhizoctonia and Fusarium species. It is one of the most common seedling problems and can collapse an entire tray of seedlings in a day or two once it starts.

Why damping off happens

The fungi that cause damping off are common in soil and old, reused compost — they cannot be eliminated, but they only become problematic when conditions favour them: high humidity, poor air circulation, cold compost, and overwatering. Overcrowded seedlings in a closed, airless environment create perfect conditions. The fungi attack the stem at or just below soil level, constricting and destroying it before the seedling has developed enough resistance. The seedling collapses and usually dies within hours of the stem being girdled.

Can you save affected seedlings?

Once a seedling has damped off and collapsed, it cannot be saved — the stem is destroyed. Remove affected seedlings immediately to prevent spread to neighbours. If only a few seedlings are affected and the rest look healthy, improve conditions immediately: remove any propagator lid, increase ventilation, move to a brighter spot, and stop watering until the compost surface just begins to dry. Avoid misting the seedlings from above. In some cases, removing the worst affected seedlings and improving conditions saves the rest of the batch.

Prevention: the full checklist

Use fresh, sterile seed compost — never old, reused compost for seedlings. Ensure seed trays and pots are clean. Sow at the recommended density — overcrowding is a major contributor. Water from below (standing the tray in water briefly) rather than from above to avoid wetting the stem and compost surface. Provide good air circulation — do not keep seedlings in a sealed propagator after germination. Thin seedlings early to prevent overcrowding. Avoid cold, wet compost: germinate in a warm spot and do not water cold, wet compost further.

Outdoors versus indoors

Damping off is more common with indoor seed-starting than direct outdoor sowing. Outdoors, the soil is better ventilated and natural microbial competition is more balanced. Kale sown directly into the ground in warm conditions (not cold, wet spring soil) damping off is much less likely. If you consistently struggle with damping off indoors, consider direct sowing outdoors or using a cold frame where air circulates more freely than a windowsill propagator.