Green mold on your substrate: identifying and handling Trichoderma

If you’re seeing green on your substrate, it’s Trichoderma. There are other green molds, but in a mushroom grow this is the one you’ll meet, and the answer to the question you’re about to ask is: no, don’t try to save it.

The rest of this page is about being sure that’s what you have, getting it out of your space without spreading it, and understanding what went wrong so the next tub doesn’t do the same thing.

What you’re looking at

Trichoderma starts white. That’s the part that catches people out — the early stage looks like healthy mycelium, and by the time it turns green it has already been growing for several days.

It goes white → pale yellow-green → forest green, and the green is spores. Once you can see colour, it’s sporulating, and spores travel.

Telling it apart from healthy mycelium

In its white phase, three things distinguish it:

Growth pattern. Your mushroom mycelium advances from spawn points as a broad front. Trichoderma grows as a circular patch with a defined edge, usually starting somewhere on the surface.

Speed. It’s faster than almost any cultivated species. A patch that’s visibly bigger than it was yesterday, while the surrounding mycelium has barely moved, is a bad sign.

Texture. Mushroom mycelium is cottony and often rhizomorphic — you can see strands. Trichoderma is smoother and flatter, more like a stain spreading across the surface than a mat growing over it.

If you’re unsure, mark the edge of the patch with a pen on the outside of the tub and check in twelve hours. Healthy mycelium won’t have moved much. Trich will have.

Not to be confused with

Cobweb mold is grey, wispy, and even faster — it looks like actual cobweb and it stays grey rather than going green. It’s also the one contaminant that’s often treatable with a light hydrogen peroxide misting. If it’s grey, don’t throw the tub out yet.

Metabolites — yellow or amber droplets on healthy mycelium — are normal, not contamination.

Blue bruising on some species is a physical reaction to handling, not mold. It appears where you touched, not as a spreading patch.

Can it be saved?

Sporulating (green): no. Removing the visible patch doesn’t remove the spores already through the substrate, and the tub will re-contaminate. Every hour it stays in your grow space it’s seeding the next one.

White phase, single small patch, bulk substrate: sometimes, briefly. Some growers cut out the patch plus a generous margin and continue. Success is maybe one in four, and the failure mode is contaminating your other tubs while you find out. If you have other grows in the same room, the answer is no.

Grain jars: no, ever. Grain is too nutritious and too enclosed. A contaminated jar is a spore bomb waiting for you to open it.

Getting it out safely

Do not open a sporulating tub indoors. A disturbed Trichoderma colony releases a visible cloud, and those spores will find your next grow.

1. Seal the tub or bag before moving it — lid on, or into a contractor bag tied shut
2. Carry it outside, still sealed
3. Open and dispose of it well away from your grow space, downwind
4. Bag the spent substrate for trash — don’t compost it near where you grow
5. Wipe the tub with 70% isopropyl, then wash with soap and water
6. Change clothes before going back to your grow area

Trichoderma spores are a respiratory irritant, and heavy exposure is worth avoiding. Wear an N95 for the handling and disposal.

What caused it

In rough order of likelihood:

Under-sterilized grain. Grain needs 15 psi for 90–120 minutes depending on jar size, with full pressure reached before you start timing. A pressure cooker that hasn’t come up to pressure properly is the most common single cause.

Sterile technique during inoculation. Trichoderma spores are already in your room. Still air box or flow hood, alcohol on everything, minimal movement, don’t talk over open jars.

Substrate too wet. Water pooling at the bottom of a tub creates anaerobic conditions that favour Trich over your mycelium. Field capacity means a firm squeeze produces a few drops, not a stream.

Spawn rate too low. Under-spawned bulk substrate colonizes slowly, and slow colonization gives contaminants time to establish. Aim for 1:2 to 1:4 spawn to bulk.

Temperature. Trichoderma prefers it warmer than most cultivated species. Colonizing above 80°F tilts the race in its favour.

pH. Trich likes acidic conditions. Gypsum at around 5% of substrate dry weight buffers pH and is cheap insurance.

Preventing the next one

The pattern in that list is that Trichoderma doesn’t beat healthy mycelium in a fair race — it wins when something has slowed your colonization down. Sterilize properly, spawn heavily, hydrate correctly, and keep temperatures moderate, and your mycelium takes the substrate before anything else can get established.

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Photograph your contaminated tubs before you dispose of them. Trichoderma at each stage looks quite different, and having your own reference makes the next one easier to catch early.