Production Cycle

Compost preparation usually happens outdoors on a concrete slab known as a wharf, though it can be done under a roof. A compost turner aerates and waters the ingredients, and a tractor-loader feeds the turner. Phase I begins by mixing and wetting the ingredients as they are stacked.

Gypsum is added at the outset to cut the greasiness compost tends to have. It causes certain chemicals to bind to the straw rather than clog the pores between the straws — which lets air permeate the pile. Air matters: without it the pile turns anaerobic and forms compounds that make the compost far less selective for mushrooms.

Phase II has two purposes. The first is pasteurisation, to kill any insects, nematodes, pest fungi or other pests present in the compost. The second is to drive off the ammonia formed during Phase I — above 0.07% it is often lethal to spawn growth. Most people can smell ammonia above 0.10%.

Compost is filled to a uniform depth and density so gas can exchange freely, letting ammonia and carbon dioxide be replaced by outside air. The whole phase is a controlled, temperature-dependent process that keeps the heap in the range where de-ammonifying organisms thrive.

Compost must be inoculated with mushroom spawn before anything will grow. A mushroom is the fruit of the organism much as a tomato is of the plant — but where a tomato carries handleable seeds, a mushroom cap forms microscopic spores far too small to sow.

So the mushroom is instead raised from mycelium: fine, thread-like cells, the same white webbing you see on rotting wood or damp straw. Spawn carries that mycelium into the compost, where it spreads throughout the bed.

Casing is a top-dressing laid over the spawn-run compost, and it is where the mushrooms eventually form. Clay-loam field soil, a mix of peat moss with ground limestone, or reclaimed weathered compost can all be used.

It carries no nutrients — its job is to hold water and to give rhizomorphs somewhere to form. Rhizomorphs look like thick strings and appear when very fine mycelium fuses together. Mushroom initials form on them, so no rhizomorphs means no mushrooms. Casing is pasteurised to remove insects and pathogens.

Once rhizomorphs have formed in the casing, mushroom initials develop as tiny outgrowths along them. When an initial quadruples in size it becomes a pin; pins keep expanding through the button stage until a button becomes a mushroom.

Harvestable mushrooms appear 18 to 21 days after casing. Pinning is triggered by lowering the carbon dioxide in the room to around 0.08% or less — depending on the cultivar — by introducing fresh air.

Flush, break and bloom are all names for the repeating 3–5 day harvest periods of the cropping cycle, each followed by a few days with nothing ready to pick. The rhythm repeats for as long as mushrooms keep maturing.

Most growers harvest for 35 to 42 days; some run to 60, and a crop can be carried as far as 150. Air temperature during cropping is held between 57–62 °F (about 14–17 °C), which suits the mushroom and keeps the crop cool.