The white mushroom, scientifically known as Agaricus bisporus, is the most widely recognized edible fungus globally, commonly sold as the button, crimini, or portobello mushroom. Cultivating this species at home is an accessible and rewarding project that involves managing distinct environmental phases to mimic its natural life cycle. This process requires attention to specific substrate composition, temperature control, and air quality. Success relies on providing the precise conditions this fungus needs to transition from vegetative growth to reproductive fruiting.
Preparing the Growth Medium and Environment
Agaricus bisporus differs from many other cultivated fungi because it requires a composted medium rather than simple wood or straw. The ideal substrate is a nitrogen-rich, selective compost, most often prepared from horse manure, straw, and gypsum, which selectively favors the mushroom’s mycelium over competitor molds. This specialized compost must be pasteurized, a process that eliminates harmful molds, insects, and pathogens while leaving beneficial, heat-tolerant microbes that aid growth. Pasteurization involves heating the substrate to an internal temperature of 140–160°F (60–71°C) for several hours.
Maintaining this heat range is important; temperatures too low will not kill contaminants, and those too high can destroy the beneficial microbes. After pasteurization, the compost must be cooled completely to below 77°F (25°C) before the next step, as higher temperatures will damage the fungal network. The containers used for cultivation should be placed in a dark area, as the mycelium does not require light during the initial growth phase.
Spawning and Colonization
Spawning is the process of introducing the mushroom spawn, which is grain inoculated with A. bisporus mycelium, into the prepared composted substrate. The spawn is thoroughly mixed into the cooled compost at a rate of about 0.5% to 2% of the substrate’s weight to ensure rapid and uniform growth. A higher spawning rate encourages quicker colonization, which helps the mushroom mycelium outgrow any remaining contaminants.
During this colonization period, often called the “spawn run,” the mycelium spreads as a white, thread-like network throughout the entire compost block. The environment must be kept warm and humid, with the substrate temperature ideally maintained between 75°F and 77°F (24°C and 25°C). High relative humidity is necessary to minimize the drying of the substrate surface. This phase typically lasts between 14 and 21 days, concluding when the compost is fully permeated with the white mycelial growth.
Inducing Fruiting
The transition from vegetative growth to reproductive growth is triggered by a specialized layer called the casing, which is applied over the fully colonized compost. The casing layer is a non-nutritive material, often a mixture of peat moss and calcium carbonate (lime), designed to hold moisture and create a microclimate that encourages mushroom formation. This layer is applied about 1 to 1.5 inches thick over the colonized substrate surface.
After the casing is applied and the mycelium begins to grow into it, the environment must be manipulated to initiate pinning, the formation of tiny mushroom primordia. The most significant trigger is a rapid drop in temperature, typically from the colonization temperature of 75°F down to a fruiting range of 55–65°F (13–18°C). This temperature change simulates the natural cooling of autumn, signaling the fungus to reproduce.
Simultaneously, the air exchange must be significantly increased to reduce the carbon dioxide concentration, which is high during the colonization phase. The combination of a temperature drop and fresh air exchange (FAE) is necessary to transition the mycelium to producing mushrooms. Humidity must be maintained at a high level, around 85–95%, to prevent the delicate pins from drying out as they form.
Harvesting and Subsequent Flushes
Mushroom growth, or “pinning,” occurs approximately 12 to 15 days after the casing layer is applied and fruiting conditions are established. The mushrooms should be harvested when the caps are firm and the veil (the membrane connecting the cap edge to the stem) is still intact or just beginning to stretch. Harvesting before the veil breaks ensures the best flavor, texture, and a longer shelf life.
The correct technique involves gently twisting the mushroom away from the casing layer rather than pulling it straight up, which can damage the mycelium and disturb the casing. Any remnants of the stem or aborted pins should be removed from the surface to maintain sanitation. After the initial harvest, known as the first “flush,” the casing layer must be re-misted to restore moisture. Subsequent flushes will appear every 7 to 10 days, though the yield typically decreases with each successive harvest as the compost nutrients deplete.