Preventing damping off in greenhouse seedlings

Recognise the problem without guessing

The RHS damping-off guide explains that several fungi and fungus-like organisms can affect seedlings before or after they emerge. A tray may germinate patchily, or a group of young plants may suddenly fall over with darkened, narrowed stems at compost level. Whitish growth can appear on decaying seedlings. A seedling bent towards light but with a firm, intact stem may instead be leggy; a dry tray may simply have wilted. Inspect the stem base and root zone before deciding what happened.

The problem is particularly damaging under glass during low-light, cool periods, when seedlings grow slowly. Crowding, high humidity and poor air movement favour disease. It can spread through contaminated compost, trays and water. There is no useful reason to keep a collapsed plant among healthy seedlings while you investigate.

Prevent it at sowing

Use fresh commercial seed compost and clean containers with drainage holes. For reusable pots and trays, brush off all old compost, wash them and let them dry; follow the RHS advice on appropriate disinfection when needed. Do not reuse containers from a tray that already had damping off for another seedling batch. The greenhouse hygiene guide covers the wider workflow for separating dirty and clean equipment.

Sow at the recommended spacing and thin when appropriate. A crowded canopy stays wet and reduces circulation around stems. Moisten compost evenly, but do not leave trays standing in stagnant water. Check below the surface before watering again: a pale, dry-looking top does not prove the root zone is dry. Water gently so seed and compost are not splashed between cells. For seedlings, the RHS recommends mains water where possible; a covered, clean water butt is still no guarantee of pathogen-free water.

Balance warmth, light and air

A propagator cover can retain the humidity needed for germination, but it should not become a permanent sealed chamber after seedlings emerge. Open vents or remove the lid according to the crop and check condensation. Give plants enough light and space to grow promptly. Ventilate the greenhouse when conditions allow, without directing a cold draught at tender seedlings. Extra heat without adequate light produces weak growth rather than protection.

Check trays at least daily during germination. A warm, wet, crowded tray can change quickly. Put suspect trays away from clean sowings, avoid moving their drainage water between benches, and wash hands and tools after handling them. The seed-starting guide sets out the full sequence from sowing to growing on.

What to do after a loss

Remove affected seedlings and their immediate contaminated growing material promptly. If collapse is widespread, discard the whole sowing rather than transplanting apparently healthy plants out of a diseased tray into a new one. Clean the bench and tools, review water and airflow, then resow in fresh medium and new or properly cleaned containers. Consider whether the batch was sown too early for the available light.

The RHS states that there is no fungicide available to gardeners for damping-off control. Avoid marketing a general greenhouse cleaner or pot brush as a cure. The useful purchase decision is modest: a brush to remove old media, clean trays where suitable, and enough bench space to avoid crowding. The lasting improvement is a more consistent propagation routine, not a chemical treatment.