Seedlings raised in cool, wet, or reused growing medium can collapse from damping-off
Seedlings raised in cool, wet, or reused growing medium can collapse from damping-off, where soil-borne fungi rot the seed or girdle the young stem at the soil line before the plant establishes.
Promising A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Damping-off is caused by soil-borne fungi (chiefly Rhizoctonia and Fusarium species and the water mold Pythium) that infect the germinating seed or the young stem at the soil surface, rotting it so the seedling fails to emerge or topples at the base. The pathogens thrive in cool, wet medium with poor drainage, and anything that slows the seedling's own growth (low light, overwatering, cool soil, high fertilizer salts) widens the window in which they attack.
What to do about it
Start seed in fresh sterile or soilless mix (never reused mix or garden soil) in clean containers with drainage holes; keep the medium moist but never soggy; give the tray warmth (about 70 °F) and air movement; and sow thinly, at the recommended density. Damping-off spreads once it appears in a tray and there is no cure for an infected seedling, so the whole defense is preventive.
How much it matters: No incidence figure is stated or estimated. The risk is highest for seed started in cool, overwatered, or reused medium and lowest in fresh sterile mix kept just moist and warm; because the outcome depends on the medium and watering the gardener controls, the rule only advises.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
The fungi, Rhizoctonia spp. and Fusarium spp., along with the water mold Pythium spp. are the most common pathogens responsible for damping off
The fungi include species of Rhizoctonia, Pythium, Fusarium, Phytophthora, Sclerotinia, Sclerotium, Botrytis, and others
Folklore it answers
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Wording made clearer
- A person read the sources and confirmed it
- Added to the rules
Cite this page
Milpa Gardens. "Seedlings raised in cool, wet, or reused growing medium can collapse from damping-off, where soil-borne fungi rot the seed or girdle the young stem at the soil line before the plant establishes." Evidence grade: Promising. https://milpa.garden/rules/seedlings-raised-cool-wet-reused-growing-medium/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
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- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
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- The claim did not survive its sources. It is kept so the folklore can be answered.