Mushroom initials develop after rhizomorphs have formed in the casing. The initials are extremely small but can be seen as clumps on a rhizomorph. As these structures grow and expand, they are called primordia or pins (Figure 16). Mushroom pins continue to grow larger through a prebutton stage and ultimately enlarge to mature mushrooms. Mushroom harvesting begins 15–21 days after casing, which is normally 10–12 days after flushing and 7–8 weeks after composting started. The cultural practices used during pin development and cropping include the management of air and compost temperatures and CO2 content of room air, and is often dependent on the strain and number of pins the grower wishes to form and develop.

Figure 16. The developmental stages of the fruiting process.




Pea-Sized Pin


Air-handling systems regulate the amount of fresh air entering the room and temperatures within the room. Ventilation requirements depend on the amount of mushrooms to be grown on the beds, heat, and CO2 production, which increases with temperature. Uniform air movement and circulation is important to prevent stale air with high CO2 levels from building up around the mushrooms, which lowers fresh quality. Air temperature is maintained in a range of 60–66ºF (15–17ºC); CO2 levels range from 1,000 to 2,500 ppm (1–2.5 percent) during the pinning and cropping stages. The most critical stage of the mushroom’s development for fresh quality and yield improvement is during the Rapidly Expanding Stage (RES), when the mushroom doubles in size every 24 hours (Figure 17). This expansion stage depends on temperature, moisture of the compost, and casing. The environment inside a production room determines the rate of transpiration, which aids in the flow of nutrients and moisture into the mushrooms.

Mushroom size is dependent on the number of pins that develop for a break or flush and by how the crop is prepared and managed. Portabella mushroom growers have learned to manage the pin set to achieve enough pins for good yield, yet, more important, to attain the right amount of pins to produce the large mature mushrooms for the Portabella market.

Figure 17. Mature mushrooms ready for harvesting.

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