Why is there white mould, moss or green slime on my compost surface?

Anything growing on the top of a pot looks like a disease and almost never is. Compost is organic matter, and organic matter kept damp in still air grows fungi and algae the way a log in the woods does. The useful information is not the mould itself but what it tells you about the watering.

Step by step

1. Identify white fluffy threads first.A cobweb-like white or grey growth spreading over the compost surface, sometimes up the stem base, is a saprophytic mould living on bark and wood fibre in the compost. It does not attack living roots, and it is extremely common in peat-free composts, which contain a lot of woody material. Scrape it off, let the surface dry between waterings, and improve air movement.
2. Identify green slime and algae.A slick green film means the surface is permanently wet and there is enough light. Common on seed trays, on greenhouse benches and on pots that are overwatered or standing in saucers. It seals the surface, which makes water penetration worse. Scrape it off and water less often but more thoroughly.
3. Identify moss and liverwort.A flat green scaly crust with tiny umbrella-like structures is liverwort, and it forms a genuinely waterproof mat that stops water reaching the roots. It arrives on the surface of bought plants and thrives in constantly wet, cool, poorly drained compost. Scrape it off completely and replace the top layer.
4. Identify a white or yellow crust.Not alive at all. A powdery or salty crust on the compost surface and around the pot rim, often with a similar deposit on terracotta, is accumulated fertiliser salts and hard-water minerals. It can burn roots. Scrape off and replace the top few centimetres, and flush the pot with plenty of plain water.
5. Identify the yellow one that alarms everybody.A bright yellow, lumpy, custard-like growth appearing overnight on mulch or the top of a large pot is a slime mould. It is spectacular, entirely harmless, and disappears in a few days. Break it up with a stick if you dislike it.
6. Look for the small black flies while you are there.A permanently damp compost surface is also the breeding ground for fungus gnats. Mould and gnats together are the same message: the surface never dries.
7. Change the watering rather than treating the mould.Water thoroughly until it runs from the base, then leave the top two or three centimetres to dry before watering again. Empty saucers rather than leaving pots standing in water. That change ends nearly all of these.
8. Improve the physical conditions.Move the pot somewhere with more air movement and light. Top-dress with a centimetre of grit or fine gravel, which dries fast, discourages algae and moss, and blocks fungus gnats from laying.

Tips

• Peat-free composts encourage surface mould more than the old peat-based ones did, and it is not a fault in the compost. It comes with the wood fibre.
• A pot that grows moss on top but is dry underneath is the classic peat-free trap — the surface tells you nothing about the rootball. Lift the pot to judge weight, or push a finger in properly.
• Do not use fungicide on compost mould. It is not a plant pathogen and nothing sold for plants is intended for it.
• Mushrooms and toadstools in a pot or in a mulched bed are feeding on the organic matter, not on the plant. Knock them off if children or pets are around, since some are toxic if eaten.

Common mistakes

• Repotting the whole plant to get rid of surface mould — It grows back within weeks in the same conditions. The watering and the air movement are what changed nothing.
• Leaving liverwort in place because it looks like moss and moss looks fine — Liverwort is genuinely waterproof and progressively starves the rootball of water. It has to come off.
• Assuming a white crust is mould and ignoring it — Salt build-up burns roots and browns leaf tips. Scraping and flushing is a different job from scraping alone.

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