White or yellow mould on the compost
A saprophytic fungus living on organic matter at the surface. Harmless to the plant, and a reliable sign that the top of the pot never dries.
What it usually is
1 Compost that stays damp at the surface
| How to tell | A white, grey or yellow fuzzy growth or crust on the compost, sometimes with small mushrooms, in a pot that is watered little and often or that stands in a still, humid room. |
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| The fix | Scrape the visible mould off and throw it away, then let the top two or three centimetres dry between waterings. Water more thoroughly and less often. Improve the air movement in the room, and take any decorative cover pot off while the surface dries. |
2 Organic matter lying on the surface
| How to tell | Fallen leaves, dead flowers or a bark mulch on the compost, with the mould growing on and around them. |
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| The fix | Clear the surface every week and top the pot with a centimetre of horticultural grit. The fungus feeds on dead organic matter, and a mineral surface gives it nothing to eat. |
3 A yellow blob that moves
| How to tell | A bright yellow or orange growth that appears overnight and changes shape or position over a day. This is a slime mould, not a fungus, and it is entirely harmless. |
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| The fix | Scrape it off if you dislike the look of it, and let the surface dry. It eats bacteria in the compost and is a curiosity rather than a problem. |
Through the year. Almost entirely a winter complaint, when plants use less water, rooms are warm and closed, and the compost surface stays damp for weeks.
The mould on the surface of a pot is almost never attacking the plant. It is saprophytic, which means it lives on dead organic matter, and potting compost is largely dead organic matter. It is eating the compost, not the roots.
That said, it is telling you something worth hearing. These fungi need the surface to stay damp continuously. A pot that dries at the top between waterings does not grow them. So a persistent bloom of white fuzz is the same message as a persistent population of fungus gnats: the plant is being watered a little and often, when it would prefer to be watered thoroughly and then left.
The routine, in order. Scrape off what is visible and bin it, because it is easier to prevent regrowth on a clean surface. Clear away fallen leaves and spent flowers, which are what it is feeding on. Water properly, until it runs from the bottom, and then wait until the top two or three centimetres are dry. Take the plant out of its decorative cover pot while the surface dries, because a cover pot traps damp air around the compost. And if the room is closed and still, open a window for an hour a day.
A centimetre of horticultural grit on the surface is the neat permanent answer. It stays dry, it looks better than bare compost, and it denies the fungus and the gnats their damp organic layer at once.
Two things not to do. Do not repot a plant just because the surface has gone mouldy; the compost below is usually fine and the disturbance costs more than the mould does. And do not reach for a fungicide, which will not reach the mycelium through the compost and is not needed for an organism that is not attacking anything.
The one case to take seriously is a white growth that is on the stem rather than on the compost, or a compost surface that smells sour and stagnant. That is a different problem and belongs with the entries on rot.
Entries that name it
What the journal says about this symptom on each plant.
| Garlic | White rot, which persists in soil for years. Never plant garlic or onions in the same ground twice running, and destroy affected bulbs rather than composting them. |
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| Lavender | Compost kept damp under a plant that wants to dry out. Top the pot with grit, water far less, and check the drainage hole is not blocked. |
| Lucky bamboo | Algae in the water and on the pebbles, encouraged by light. Move it out of direct light, wash the pebbles and the vessel, and change the water every week. |
| Onion | Onion white rot or neck rot in wet ground. Do not grow onions in the same ground for several years, and dry the harvested bulbs thoroughly before storing. |
Questions from the margin
- Is the white mould on my plant's soil harmful?
- Not to the plant. It is a saprophytic fungus feeding on dead organic matter in the compost, not on the roots. What it tells you is that the surface never dries, which usually means the plant is watered a little and often when it would prefer a thorough soak and a wait.
- How do I stop mould growing on the compost?
- Scrape off what is visible, clear fallen leaves from the surface, water thoroughly and then let the top two or three centimetres dry, and take the pot out of any decorative cover while it does. A centimetre of horticultural grit on top is the permanent answer.
Checked against RHS, Moulds and fungi on compost; Oregon State University Extension, Saprophytic fungi in potting media.