Microgreens Troubleshooting: Mold, Damping Off and Leggy Stems (Diagnosis Table)
The diagnosis table
| Symptom | Likely cause | Fix / prevention |
|---|---|---|
| White fuzz on roots/stems, vanishes when misted | Root hairs (normal) | Do nothing — it's not mold. |
| Webby growth across the medium, musty smell, brown leaf tips | Mold (powdery/cottony fungus) | Increase airflow: fan across trays 2–4 h/day; spray medium only, not canopy; harvest affected tray, sanitize it. |
| Pale, spindly stems; tips flopping | Not enough light (distance or hours) | Move light closer (15–30 cm), extend to 14–16 h/day; see LED guide. |
| Stems thick but very pale after a dark period | Blackout too long (5–7+ days) | Keep blackout to 3–4 days max; resume light promptly. |
| Seedlings topple at soil line, sunken dark stems | Damping off (Pythium) | Fresh clean medium every batch; better airflow; don't overwater. Affected tray → sanitize or discard. |
| Seeds not germinating after 5–7 days | Stale seed, too cold/wet, sown too deep | Sow on the surface (barely covered), keep medium moist at 20–24 °C; test a fresh packet in parallel. |
| Bitter taste, flowers starting | Past harvest window (bolting) | Harvest earlier next batch — see days-to-harvest chart. |
| Medium smells rotten/soapy, stays wet for days | Overwatering / reused wet medium | Drain fully after bottom-water; fresh medium each batch; don't seal lids airtight. |
The 3-knob diagnostic (use before anything else)
- Water: is the medium dripping? If yes — reduce frequency or drain time. Is it bone dry on top? If yes — you're underwatering the surface layer.
- Light: measure distance (fingertip to canopy ≈ 20–30 cm) and hours (12+). Check for stretching over the last 3 days, not just today's look.
- Airflow/heat: is there a fan moving air across the trays? Is the zone above ~18 °C? Stagnant warm humidity = mold factory.
One-tray rule
If a single tray fails badly (mold/damping), isolate it: move it away from the stack, fix or discard, and sanitize. Never let one bad tray contaminate four good ones — that's how beginners lose a whole batch.
Sources
- Cornell Small Farms Program — microgreen best practices (disease section).
- Crockett & Briggs, Utah State Extension — microgreens guide.
- Extension guidance on Pythium/damping off in seedlings (general).