GrowingHQ
DiseaseSeedlingsStems

Damping off

Seedlings collapsing at soil level, healthy one morning and flat the next, often in a spreading patch across a tray. The stem is pinched and darkened where it meets the compost.

Caused by: Pythium, Rhizoctonia and Phytophthora species

What brings it on

Cold, wet, airless conditions and overcrowded trays. Early indoor sowings are most at risk because the compost stays wet for days at low temperatures and light levels.

source

Sourced guidancehigh confidence

  • Royal Horticultural Society: Damping off
  • Penn State Extension — Pennsylvania State University: Damping off of seedlings

Checked 2026-08-13

Usually seen: February, March, April, May

Preventing it

Ordered by how much difference each measure actually makes, not by how often it is repeated.

  • Sow thinly and give trays light and moving air. Crowding is the single biggest contributor.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Damping off — prevention

    Checked 2026-08-13

  • Use fresh compost and clean containers, and water with mains water rather than from a butt for early indoor sowings.
    Sourced guidanceDepends on conditionshigh confidence

    Applies when: Stored rainwater is fine for established plants; the concern is specifically seedlings under glass, where the organisms involved can be present in stored water.

    • Royal Horticultural Society: Damping off — hygiene

    Checked 2026-08-13

  • Do not sow earlier than the light and warmth available can support. A seedling growing slowly in the cold is exposed for far longer than one growing away quickly.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Sowing indoors

    Checked 2026-08-13

Dealing with it once it is there

  • Remove collapsed seedlings and the compost around them at once, improve ventilation, and let the surface dry between waterings. Affected trays rarely recover fully and re-sowing is usually quicker.
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Damping off

    Checked 2026-08-13

What it affects

Signs that lead here

Each of these lists the other things that produce the same sign, and how to tell them apart.

On sprays and treatments

GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.