Coco coir, vermiculite and gypsum (CVG)

CVG is the default bulk substrate for tub growing, and its main virtue is consistency. Every batch behaves the same way, it holds water beautifully, and it is nutritionally poor enough that contaminants get very little help from it.

That last property is deliberate. CVG is not trying to feed your mushrooms — the grain spawn does that. CVG provides water, structure and volume.

The recipe

For a standard monotub:

650 g brick of coco coir (one standard brick)
4 quarts / ~1.2 kg vermiculite
1 cup / ~200 g gypsum
4–5 litres boiling water

Proportionally: roughly 2 parts coir to 1 part vermiculite by volume, plus gypsum at about 5% of dry weight.

What each part does

Coco coir — the bulk. Coconut husk fibre, sold as compressed bricks. Holds several times its weight in water, has almost no available nutrition, and naturally resists some contaminants.

Vermiculite — water retention and structure. Expanded mica; it holds water in its layers and releases it slowly, and it keeps the coir from compacting into a dense mat.

Gypsum — calcium sulphate. Buffers pH against the acidification that contaminants prefer, supplies calcium, and reduces clumping.

Some growers drop the vermiculite and run coir plus gypsum. It works. The substrate dries faster and is less forgiving of a mistimed misting.

Pasteurising

This is CVG’s best feature: it pasteurises with boiling water and a bucket.

Put the dry coir brick, vermiculite and gypsum into a clean bucket or tub.
Pour in 4–5 litres of boiling water.
Seal it immediately — lid on, or wrap in a bin bag.
Leave overnight, or at least 6–8 hours.

The brick expands and breaks apart, the residual heat pasteurises, and the sealed container keeps it clean while it cools.

That is the entire process. No pressure cooker, no thermometer.

Getting the water right

This is the only part that goes wrong.

Target field capacity: squeeze a handful hard. A few drops between your fingers is correct. A stream is too wet. Nothing is too dry.

Coir bricks vary in how much water they take, so the 4–5 litre figure is a starting point rather than a rule. Start at the low end — you can add more, but removing water from a mixed batch means squeezing it out by hand.

Too wet is the more common error and the more damaging one. Standing water at the bottom of a tub causes exactly the anaerobic conditions that favour Rhizopus and bacterial contamination over your mycelium.

Spawning

1:2 to 1:4 spawn to CVG by volume. Higher rates are worth it — CVG offers your mycelium no nutritional advantage, so speed of colonisation is your only defence.

Mix thoroughly. Layer in two or three passes and mix again. See spawn to bulk.

Casing

A casing layer — half an inch of moist coir, or a 50/50 coir and peat mix, over the fully colonised surface — is where CVG earns its reputation.

It holds water exactly where pins need it, produces more even pinning, and largely prevents the surface crusting that causes aborted pins.

Apply after full colonisation, then wait 5–7 days for the mycelium to run through it before introducing fruiting conditions.

What grows well on it

CVG suits species that fruit from a cased surface: many of the common tub species, wine cap, and oysters at a pinch.

It does not suit wood decomposers. Lion’s mane, shiitake, king oyster, reishi and maitake all need hardwood. CVG does not contain what they eat.

Common problems

Too wet. Pooling water, slow colonisation, bacterial smell. The most common CVG failure. Mix in dry coir if you catch it before spawning.

Too dry. Colonisation stalls at the edges, surface crusts, pins abort.

Compacted. Packing the tub down hard excludes air. Firm, not compressed.

Contamination despite pasteurisation. Usually the container was not sealed fast enough while cooling, or the mixing bucket was not clean.

Cost

A brick of coir, a bag of vermiculite and a bag of gypsum will run several tubs and costs less than a single grow kit. This is the cheapest reliable bulk substrate there is, which is a large part of why it became standard.