A white or grey fuzz on top of potting soil is usually a harmless mould that grows on decaying matter in damp soil. It is a sign that the soil is staying wet and the air around it is still, not a sign that the plant is sick.
What it usually is
- Fluffy white or grey patches on the soil surface, sometimes with a musty smell. Typically a saprophytic mould feeding on organic matter.
- A white crust on the soil or pot rim that looks hard rather than fuzzy. This is usually mineral salts, not mould.
What to do
- Scrape off the top inch or so of soil and throw it away.
- Let the soil dry out more between waterings.
- Make sure the pot has drainage and empty the saucer.
- Move the plant to a spot with brighter light and some air movement.
- Top up with fresh potting mix if you removed a lot.
When to worry
If the plant also wilts, yellows, or the stem base is dark and soft, the real problem may be root rot. See our page on it. Also, if you have allergies or asthma, wear a mask when handling mouldy soil.
When to scan
If you can't tell whether it is mould, salts or something else, take a close-up photo of the soil in daylight and scan it.