Mould vs Mycelium: How to Tell What You’re Seeing

A closed tropical jar is a damp, still, organic box. Fungi will show up. That is not automatically failure. Small amounts often feed on dead bits, as they would on a woodland floor. A carpet of fluff is a management signal, not a reason to throw the Kilner in the bin.

Beginners see white and reach for bleach. Don’t. First decide what the white is, then change weather and debris — not the whole ecosystem.

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A first-jar Kilner with a small cottony patch on cork or mix, plants otherwise fine.

Fuzzy vs threads (rules of thumb, not a lab)

Treat as mould (act): cottony or cobweb patches on mix, moss, wood, or glass; may grey, green, or blacken as it ages; may smell musty; often in isolated puffs that thicken.

Treat as mycelium (usually leave): finer, hair-like white threads through substrate, leaf litter, or wood; more even; usually a neutral or mushroomy smell, not sour. This network is how fungi digest litter. In a bioactive mix it is often a sign the woodland is switching on, not that the jar is dying.

Also not mould:

• Hard mineral crust on glass (hard water) — wipe; change water. See watering & condensation.
• Tiny still clusters if springtails are already in — often eggs.

Visual ID is imperfect. When unsure, treat spreading surface fluff as mould. Do not claim species names from a photo.

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Set of four, same lighting: (1) cottony fluff on cork, (2) fine threads in leaf litter, (3) hard mineral crust on glass with a fingernail, (4) tiny still egg clusters. One caption strip: Fluff / threads / crust / eggs. Macro if you can; a 50mm phone close-up is enough.

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A beginner’s finger next to fluff vs threads for scale — fluff is a puff, threads look like cobweb hair in the mix.

Why it appears in this jar

Mould wants moisture, still air, and something to eat (dead leaves, new wood sugars, nursery compost left on roots). A gloomy shelf and a tight lid stack the odds. Closed Tropical Mix holds water on purpose.

Typical UK beginner triggers:

• Over-misting after setup (see watering).
• Leaving yellow Fittonia or Pilea depressa leaves in the jar.
• Fresh cork or hardwood — a white bloom in the first fortnight is common even on boiled wood.
• Not enough light (bright shade, not a dark sideboard).
• Skipping quarantine on garden-centre plants. See quarantine & settling.

A little fluff while the jar cycles is expected. Panic-rebuilds teach less than picking off debris and reading the glass.

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New cork with a typical week-two white bloom, plants still upright.

Response ladder (vent / crew, not nuke)

Work down this list. Stop when the fluff recedes.

1. Mechanical. Tweezers out dead leaves, rotting Pilea depressa bits, and heavily fuzzed wood crumbs. Get clippings out of the jar — left-behind trimmings are a mould meal.
2. Weather. Vent. If the glass never clears, the jar is too wet — lid off hours, blot, siphon the false bottom. Move out of gloom into bright shade. Do not use direct sun as a steriliser.
3. Wait. Small saprotrophic films often shrink once the easy food is gone and air moves.
4. Crew, after cycling. Springtails graze hyphae and spores. They are the long-term grazers, not a magic powder you dump into a flood. Add them when the first fortnight has settled and the mix is damp, not on day one into a swamp. See adding springtails and cycling.
5. Optional spot measures (hobby, not medicine). UK pages mention cooled chamomile mist on mild patches, or a cotton bud of 3% hydrogen peroxide on hardscape after testing. Peroxide is indiscriminate: it can bleach moss and knock back the very microbes you want. Never soak the jar. Never use barbecue charcoal, bleach, or cinnamon as a soil blanket (cinnamon can waterproof the surface).

Do not sterilise the mix into a dead box if you want a living jar. Clean inputs (rinse plants, skip rotting wood) are not the same as a lifeless oven-baked desert.

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15–25s: tweezers lifting a fuzzed leaf out of the jar (show it landing on kitchen paper, not back in), then lid left ajar. No bleach bottle in frame.

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Springtails on damp mix or glass after a mist — tiny pale specks, not a product pack shot.

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What not to do — a bleach bottle or cinnamon tin beside a jar, crossed in the caption, not as a joke meme. Keep it calm.

When to nuke (escalate)

Rebuild or strip when:

• Foul, anaerobic smell (swamp, not woodland).
• Plants going mushy, not just a few yellow leaves.
• Fluff returns within days of every clean despite venting.
• Black growth on living tissue, or household mould-allergy symptoms from opening the jar.

Then: take plants out, bin the worst mix and wood, wash the glass, start again with damp (not wet) Closed Tropical Mix, and cycle before fauna. That is the troubleshooting pillar, not a different mould article.

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A jar that does need a reset — blackened mix, collapsed plants — shot from the side, lid off by a window. Don’t linger; it’s a warning frame.

This jar, specifically

First closed jar: Closed Tropical Mix, Fittonia, cushion moss, Pilea depressa. Springtails after cycling.

• Fittonia: grey mould can sit on damaged leaves in still humidity — pick off affected material. Do not leave fainted-then-rotting leaves “to see.”
• Moss: slime and green films like constant wet plus light on glass. Vent and wipe; do not drown.
• Pilea depressa: grey mould is common on pileas in still humidity — pick off affected leaves and take them out. Trailing stems that stay wet against glass rot, then fur. Trim so leaves are not plastered to the wall.
• Springtails: after cycling. They will not out-eat a flood.
• Later warning — Soleirolia: not in this trio. If someone adds it later, dense wet carpets against glass rot and then mould. Do not confuse shop “baby tears” labels: some mean Pilea depressa.

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Pilea depressa stem against wet glass with a small furred patch, tweezers in frame.

Safety

People with mould allergies or asthma should limit time over an open, badly furred jar. Open by a window. Do not recommend household fungicides inside a planted jar.

Science note

Fungi arrive as spores on plants, wood, mix, and room air. In still air a wet film sits on every surface; a short vent breaks that film so spores dry instead of germinating. Mycelium is the feeding body of a fungus; the fluff you hate is often the same kingdom in a messier mood. Springtails graze; they do not erase fungi, and you do not want a jar with none.