CVG

Coir, vermiculite and gypsum are a lean substrate that some litter fungi accept, and that shiitake does not.

Three materials, three jobs

CVG is a bench abbreviation, not a species. It stands for coir, vermiculite, gypsum. None of the three is a fertiliser in the garden sense, and none of the three alone is a decent fruiting substrate. Together they make a mattress that holds water, leaves a little air, does not pack into a clay, and offers little sugar. That “little” is the point. The fungi that do well on it are not the wood eaters. They are litter and compost fungi, used to a food that is already degraded, lean, moist. Button mushroom, almond agaricus, many Coprinus. The oyster prefers straw. Shiitake prefers beech. Putting them on CVG because “it is a substrate” is mistaking the container for the diet.

Coir is the pith of the husk, washed and compressed into the garden forms, then broken up. It holds a lot of water and little nutrition compared with grain or a composted manure. That poverty is a defence: moulds and bacteria looking for free sugar run less hard on it than on an enriched sawdust. It is also a limit. It is not the compost of an industrial farm, the one that has been through hot fermentation phases and a bulk pasteurisation, with a flora selected by the heat. Home CVG is a simpler stand-in. It colonises. It fruits, if the species is the right one and if you case afterwards. It does not reproduce the output of a house that built the compost for weeks. Anyone who promises you that is describing another craft.

Vermiculite is an expanded mica. It does not feed. It holds water like a mineral sponge and holds voids open, so the mattress does not become an anaerobic slab. Without it, wet coir closes up. With too much, the block dries in patches and the spawn seeds stay suspended in a material that is not food. Gypsum, calcium sulfate, is not lime. Lime shifts pH upward, sometimes harshly. Gypsum adds calcium and sulfur and helps the particles stay discrete, with a much shyer effect on pH. It goes in as a small part. Increasing it does not increase the mushrooms. It leaves them without a useful change and, if you overdo it, it leaves you a powdery, unbalanced substrate.

Pasteurised CVG is this mix already hydrated and already through the heat, in one or two kilos. Pasteurised, not sterilised. The same logic as straw applies: the heat has lowered the competitors, it has not made a void. You spawn it with colonised grain, not with a few millilitres of syringe hoping a slow fungus will win a race in an open volume.

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Why it is not a wood, and why it is not sterile

A lean pasteurised substrate is a compromise suited to a mycelium that knows how to live on little, provided it starts at many points. Grain spawn is those points. One part spawn to five or ten of CVG is a sensible order of magnitude: more spawn shortens the time the coir is undefended, less spawn lengthens a species that was already slow. Button mushroom is not an oyster. It does not close a bag in ten days for sport. If you spawn it thinly, the bacteria left from pasteurisation have an extra week to make a film, and you call “weak strain” a problem of ratio.

You mix evenly, minimum opening, clean hands. CVG should be moist like a squeezed sponge, not a soup. If the bag is dripping, let it drain first. A sodden core does not colonise: it stays dark, smells shut in, and the white stops at the border. If it is dry and crumbles to powder, the mycelium does not have the film of water it needs to jump from one fibre to the next. You do not water it by the glass after you have closed it. You recognise the bag.

The colonisation temperature of these fungi is not the pink oyster’s. Button mushroom is often happy around 20–24 °C while it feeds, and it wants cooler conditions when it fruits. Almond agaricus wants more warmth in both phases. Keeping them in the same corner and expecting the same calendar is how you condemn one of them. Neither belongs on a radiator. Neither gets opened halfway to see whether “it is white underneath”.

Casing comes afterwards

A casing is a lean layer, leaner still than the CVG, set on the surface when the substrate is already colonised. It is not a top dressing of fertiliser and it is not garden soil. Mycelium reaches it, finds little food, plenty of water, and a gas exchange different from the core of the block. That step is part of the signal that makes primordia form, especially in Agaricus. Without a casing, a colonised CVG can stay white and still for weeks, not because it is dead, but because it has not had the surface on which to fruit. With a casing made on spawning day, you cover food that is still bare. The mycelium has to win underneath first. Then you cover it.

The layer is a few centimetres, even, moist without being mud. A waterlogged casing closes the air off and rots. A dry casing becomes a crust the mycelium does not cross, or crosses in ropes that then dry out. The simple recipe is peat or coir with vermiculite, pasteurised, almost without nutrition. If you use the same CVG as the substrate for a “casing”, you feed it again: the mycelium eats it and forms a thick skin, overlay, instead of knotting primordia. Overlay gets torn with a clean fork only once you have understood that the casing was too rich or the air too still. Better a leaner casing from the start.

You do not use soil from the plant pot. It is a reservoir of Trichoderma. You do not use soil from the bed. You pasteurise the casing material separately, you let it cool, you lay it on the colonised surface with clean hands, you keep it moist with water misted in the air above, not with a jet that digs craters. When mycelium starts to show in the casing as sparse threads, not as a sheet, you increase air exchange and, for button mushroom, you lower the temperature a little. Primordia come from that microclimate. If they do not come, the usual causes are a casing too wet, too dry, too rich, or air still saturated with CO2. It is not a deficiency to correct with more gypsum.

Button, almond, Coprinus

The button mushroom, Agaricus bisporus, is the white mushroom of the market. In a growing house it grows on compost prepared in two phases, then a peat casing. At home CVG is the pasteurised stand-in, and the casing imitates the surface. The mycelium is white, sometimes with a smell slightly different from oyster, more of damp woodland than of anise. Fruiting wants cool conditions, often below 18 °C, high humidity, exchange without a drying draught. The pins are grey and round before they are caps. Harvesting late, with the veil broken and the gills black, is a kitchen choice, not a failure: it is the fungus sporulating. It is not more nourishing or more “real”. It is older, and it dirties the casing with dark spores that then confuse the reading of the next flush.

Button mushroom liquid culture is the inoculum of this path. Ten millilitres go into grain, millet or rye, not into the CVG bag. Button mushroom is slow. A liquid front in a pasteurised substrate loses to the residues of pasteurisation more often than an oyster does. When the grain is evenly white, that grain goes into the CVG. Then you wait for colonisation. Then the casing. Then the cool. Skipping a link does not turn it into an oyster.

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Almond agaricus, Agaricus subrufescens, is a relative and it is not the same calendar. You also meet it under the name blazei. It wants more warmth than button mushroom, both while it colonises and while it fruits. A cool corner that makes the market white happy leaves the almond standing still. The casing is still required. Grain spawn is still required. The name on the syringe is not a kit that fruits in a drawer. It is a mycelium to run across a bridge and then across a CVG, with the patience of a species that does not close a block in a weekend. It is not health advice. It is a fungus to grow, with a smell some people find almond-like and others find merely fungal. The smell measures nothing except the species.

Coprinus, in the broad sense of the inkcaps, sit on similar substrates: moist, lean litter, not hardwood. Some, such as the shaggy mane, digest their own cap within hours once they mature. They turn to ink. That is not rot arriving from outside. It is the way that fungus releases spores, an autolysis. You harvest earlier, while the cap is still a pale cylinder. This does not make CVG an “ink substrate”. It makes the moment of harvest part of the biology, not an oversight. Not every inkcap behaves this way. You learn the species you have, and you do not leave a mature cap on the casing to dirty the rest.

None of these species owes you a basket. First attempts with casing get the water wrong. You correct the water of the casing, you do not change substrate halfway, and you do not promise the block a second life by dosing it with feed. The CVG has already given the little food it was supposed to give.

Gypsum powder is not a seasoning for the bag

If the bag is already pasteurised CVG, the gypsum is in it. Adding a spoonful “to strengthen it” does not strengthen the mycelium. It dirties a surface that was supposed to stay even, and it can dry into a crust. Powdered gypsum is for when you build the mix yourself, from raw coir and vermiculite, before the heat. There it is the structural ingredient, a small percentage of the dry weight, mixed in before the water so it does not stay a white lump.

Gypsum powder is calcium sulfate. You do not substitute hydrated lime, or carbonate, or builder’s plaster full of additives if you do not know what is in it. Lime is a different tool, the one in alkaline baths for straw, and a dosing error burns the mycelium. Gypsum does not. It is shy, and it should be kept shy. In a casing you can put a little, for structure, not to “feed”. If you are not formulating, the jar stays closed. The finished bag does not need it.

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A home-made CVG has to be pasteurised, not treated as ready because the coir “is dry and therefore clean”. Dry is not sterile. The water you reconstitute it with wakes whatever was there. The heat, in the pasteurisation window, is the step. Sterilising a CVG in an autoclave is possible, and then it has to be treated as a sterile substrate: minimum opening, plentiful spawn, because you have removed even the small residual competition and any mistake shows. For these species pasteurisation is the usual compromise. It is not a lesser rite. It is what leaves a lean mattress still habitable by a slow fungus.

What you see when it goes wrong

Powdery green: Trichoderma, often from a casing made with dirty soil or from a long opening. You close it, bag it, discard it. You do not “take the spot off” and cover it again. A slimy film and a sour smell: bacteria, almost always too much water. You do not dry it by leaving it open. A white that covers the casing in a skin: overlay, still air or a casing too rich. You increase exchange, you do not water more. A block that never turns white: too little spawn, the wrong temperature for that species, or a sodden core. You wait a time proportionate to button mushroom, not to oyster, and then you judge. Throwing an Agaricus out on day six is throwing the fungus out while it was still walking.

Shiitake on this mattress does not go “a bit more slowly”. It is in the wrong place. The lignin it is looking for is not here. The brown crust has no dense block to form on. You move it to sterilised beech and you leave the CVG to whoever knows how to use it.

What this guide is not

It is not a farm compost and it is not dietary advice. It is a lean mattress, pasteurised, spawned with grain, covered only afterwards, with a gypsum that holds structure and does not stand in for food. If the pins do not come, look at the casing water and the air. Do not add nutrition out of tiredness.