Quick answer
Microgreens usually mold because of too much moisture combined with poor airflow and warm, humid conditions, often made worse by sowing too densely or reusing dirty trays. Water from below, let the surface dry slightly, run a small fan, keep the room around 65–75°F (18–24°C) and clean trays between crops.
Mold is the problem beginners worry about most. The good news: it is predictable. Once you understand the handful of conditions it needs, you can prevent most of it with small changes to your routine.
Is it mold or root hairs?
| Root hairs (normal) | Mold (a problem) | |
|---|---|---|
| Where | On the roots, near the base of each seedling | Spreads across the surface, seeds and stems |
| Look | Fine, fuzzy and evenly spread | Cobweb-like patches, sometimes grey or colored |
| When misted | Collapse and almost disappear | Stay put |
| Smell | Fresh and earthy | Musty or sour |
The main causes of mold
- Overwatering and water sitting on the surface or in the bottom tray.
- Still air. Without airflow, the humid layer around the seedlings never dries.
- Warm, humid rooms, which speed up mold growth.
- Sowing too densely, which traps moisture between seeds.
- Dirty trays and tools carrying spores from the last crop.
How to prevent mold
- Bottom-water after blackout and pour off any water that is not absorbed.
- Run a small fan on low near the shelf for gentle, constant airflow.
- Aim for a room temperature around 65–75°F (18–24°C).
- Sow evenly and a little less densely if mold keeps returning.
- Wash trays, weights and spray bottles between every crop.
Watch out
If mold has spread across much of a tray, don’t try to rescue it. Discard the batch, clean everything thoroughly and start again. Don’t eat greens with visible mold.
FAQ
Is white fuzz on microgreens always mold?
No. Fine white root hairs are normal and collapse when misted. Cobweb-like fuzz that spreads over seeds and stems and smells musty is mold.
Does a fan really help with mold?
Yes. Gentle airflow dries the surface between waterings and is one of the most effective ways to prevent mold indoors.
