Fuzzy grey growth: cobweb mold and how to stop it

Grey, fluffy, and it wasn’t there yesterday. Cobweb mold is the fastest thing you’ll see in a tub and it looks far worse than it is. Of the common contaminants it’s the one that’s genuinely worth treating rather than binning.

The speed is the giveaway. Cobweb can cover a surface overnight. Almost nothing else moves like that.

What you’re looking at

It grows as a loose, grey, three-dimensional fuzz that sits on top of the substrate rather than binding into it. Think dust bunny, not mat. Early on it can be pale enough to read as white, but it greys within a day and stays grey — it never turns green.

Left alone it will grow over your pins and fruits, and where it touches a mushroom it causes a soft wet rot at the contact point.

Telling it apart from healthy mycelium

This is the distinction that matters, because the treatment for cobweb is actively harmful to a healthy tub.

Density. Your mycelium is dense, bright white, and hugs the substrate. Cobweb is sparse, grey-tinged and stands up off the surface. If you can see through it, it’s cobweb.

Speed. Healthy mycelium colonises the surface over days. Cobweb doubles overnight. If the difference between last night and this morning is dramatic, that’s your answer.

Edge. Cobweb spreads as a diffuse cloud with no defined border. Trichoderma in its white phase spreads as a circular patch with a sharp edge. That difference is the one worth learning — it decides whether you treat or bin.

Colour on ageing. Cobweb stays grey. Trichoderma goes green within a few days. If you’re unsure and there are no fruits at risk yet, waiting 24 hours costs you very little and resolves the question.

Can it be saved?

Usually, yes — if it’s actually cobweb and you catch it before it’s spread across the whole surface.

Cobweb is a surface organism. It hasn’t penetrated the substrate the way Trichoderma does, which is why removing what you can see actually accomplishes something.

No if you’re not confident it’s cobweb. Treating Trichoderma with peroxide misting wastes days while it seeds your room.

Treating it

Work gently. The whole risk here is disturbing the fuzz and aerosolising it.

1. Mist the patch directly with 3% hydrogen peroxide, straight from the bottle, no dilution. Get it wet — you want the patch saturated, not dusted.
2. Wait a few minutes. It will collapse and darken.
3. Remove the dead material with a spoon or gloved hand, taking a small margin of substrate with it. Move slowly; don’t fan it.
4. Mist the area again lightly.
5. Check every twelve hours for three days. A second patch is common and means the underlying condition hasn’t changed.

Hydrogen peroxide at 3% breaks down into water and oxygen within minutes and does not meaningfully harm established mycelium at the surface. It will damage young pins it lands on, so accept losing the pins in that patch.

What caused it

Cobweb is a stagnant, over-humid air problem almost every time. Two knobs fix it:

Not enough fresh air exchange. Still, saturated air sitting on the surface is exactly the condition cobweb wants. If your tub has visible condensation running down the walls and the surface never dries between mistings, you’re running too wet and too still.

Surface staying wet. Misting directly onto the substrate rather than the walls keeps a film of water on the surface. Mist the sides of the chamber and let humidity come off that.

Secondary causes worth checking:

• Casing layer or substrate contaminated at mixing. Cobweb spores are common in soil and in coir that’s been stored damp.
• Temperatures at the high end. It moves faster above 75°F, as most things do.
• Dead organic matter on the surface — an aborted pin left in place, a harvested stem butt still sitting there. These are the usual starting points, which is why harvesting cleanly matters.

Preventing the next one

Increase fresh air exchange before you increase anything else. Most cobweb in monotubs traces back to a tub that’s holding humidity beautifully and exchanging air badly.

Pull stem butts and aborts out at every harvest. Those are the beachhead.

If you’ve had cobweb once in a space, expect it again — the spores are in the room now. Wipe surfaces down with a peroxide or dilute bleach solution between grows, and keep a spray bottle of 3% peroxide next to the tub so treatment happens the hour you notice it rather than the following weekend.

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Grey and fast is cobweb. Green and edged is Trichoderma. If you remember one thing from this page, remember which one you’re allowed to treat.