Mould on bonsai soil: harmless or dangerous?

Found mould on bonsai soil? First, take a breath: white fluff on the soil is usually harmless to the tree. It is almost always fungi that live on dead, organic material in the soil, such as bark, peat or humus. They don't attack healthy roots. Still, the mould is a warning sign: it tells you the pot is too wet.

Harmless or a real problem? How to tell

• White, cotton-like fluff or fine threads only on top of the soil, tree looks healthy → usually a harmless soil fungus. The threads are called mycelium, which is basically the fungus's root network.
• White threads among the roots inside the root ball, especially on pines and other conifers → often mycorrhiza. These are useful fungi that work together with the roots and help the tree take up water and nutrients. Don't remove them!
• A hard white or yellowish crust on the pot rim or the soil that doesn't wipe away like fluff → probably lime from hard tap water, not mould. Not pretty, but not an emergency.
• Mould plus yellow leaves, limp shoots and a rotten smell → now it's serious. This usually means root rot from waterlogging. See yellow leaves.

Why does the soil go mouldy?

1. Too much water. The soil never dries out properly. This is the main reason.
2. Lots of organic soil. Many bonsai from garden centres sit in peaty, fine potting compost. It holds water for a long time and gives fungi plenty of food. Real bonsai soil with coarse, mineral grains (such as akadama, a Japanese clay granule) dries much faster.
3. Little air. A spot with no air movement, such as a dark corner or crowded among other plants.
4. Little light and cool temperatures. Especially in winter, less water evaporates and the soil stays wet longer.
5. No drainage. A blocked drainage hole or water left standing in the saucer.

Step by step

1. Stay calm and take a look. Tree healthy, just fluff on the surface? Then these simple steps are enough.
2. Remove the top layer. Gently scrape off the mould with a spoon, about 1 cm deep. Take care not to damage the roots. Top up with a little fresh bonsai soil if needed.
3. Let the soil dry. Only water again when the soil feels dry about 1–2 cm down. Our guide to watering bonsai explains exactly how.
4. Empty the saucer and check the drainage hole is clear.
5. Give it air and light. Move the tree somewhere brighter and airier, but not into cold draughts. Airing the room regularly already helps a lot.
6. Long term: better soil. If the mould keeps coming back, your tree is probably in soil that is too fine and organic. Switch to airy bonsai soil at the next repotting in spring. Your roots will thank you too.
7. If you suspect rot, check the roots. Gently lift the tree out of its pot. Healthy roots are light and firm, rotten ones are dark and mushy.

What NOT to do

• No fungicide in a panic. A harmless soil fungus doesn't need treatment. With mycorrhiza you would even destroy the helpful partners.
• Don't wash out all the roots just because something white is growing on top.
• Don't water even more because the soil looks "sick". The opposite is right.
• Don't confuse lime crust with mould. You can simply wipe it off the pot rim.

When to see a bonsai nursery

Get help from a specialist nursery if the tree is also clearly struggling, the roots are mushy, the soil smells rotten, or the mould doesn't go away after a few weeks of drier care. Especially when repotting into new soil, it's worth having an expert look over your shoulder. More tips for trees indoors in our guide to indoor bonsai.