After spawning, the compost is maintained at approx. 24 o C, and relative humidity and CO 2 levels are kept high to minimize drying of the compost. The spawn will begin to grow and produce a thread-like network of mycelium throughout the compost. Complete colonization of the compost usually requires 12-20 days, depending on the spawning rate and environmental conditions. The important transition from the vegetative to the reproductive stage of A. bisporus takes place in the casing layer, which is usually a mixture of peat and limestone.
Mushrooms form only after the compost is covered with a layer of casing material.
In addition to stimulating fruit body formation, the casing layer provides moisture essential for high yields and anchorage for the developing mushrooms. Casing materials do not provide any nutrients to the mushroom mycelium. Environmental conditions after casing are the same as during spawn growth. The compost temperature is kept around 24 o C for up to 5 days after casing to allow for the spawn to grow through the casing layer. Most Agaricus is grown in a place with high relative humidity and not much light.