The Portobello mushroom, the fully matured form of the common button mushroom (Agaricus bisporus), is a popular culinary choice. Cultivating this fungus at home requires precise environmental and substrate preparation. The method begins with tissue cloning, moving through specialized substrate preparation and strict environmental controls before mushrooms can emerge.
Creating the Initial Spawn Culture
The first step in using a store-bought mushroom is to clone its tissue to create a culture, which will then become the spawn. Select a fresh, healthy Portobello, preferably one that has not yet opened fully, and work in a clean environment to minimize airborne contaminants. This involves taking a small piece of the mushroom’s sterile inner flesh and transferring it to a nutrient-rich medium, such as agar.
To obtain the sterile tissue, snap the mushroom stem in half rather than cutting it with a knife, exposing the clean interior. Using a sterilized scalpel or needle, immediately extract a piece of this inner stem tissue, about the size of a grain of rice. This sample is placed onto a sterilized agar plate containing nutrients like malt extract. The mycelium will appear as white, cottony growth radiating from the tissue over seven to ten days if kept near 75°F (24°C).
Once the mycelium has fully colonized the agar plate, this culture is used to inoculate a larger medium, such as sterilized grain, creating the mushroom spawn. This grain spawn serves as the dense, clean starting material that will introduce the desired genetics into the bulk substrate. While using a spore print from the cap is possible, cloning the tissue provides a more direct and genetically reliable way to propagate the mushroom.
Preparing the Compost Substrate and Casing Layer
Portobello mushrooms require a nutrient-rich compost substrate, often based on pasteurized straw and manure mixes. The bulk substrate must undergo pasteurization, heating the material to 140°F to 160°F (60°C to 71°C) for several hours. This process eliminates most competing organisms and molds while preserving beneficial, heat-tolerant microbes necessary for the growth cycle.
After the bulk substrate has cooled, the prepared grain spawn is mixed in thoroughly, and the entire block is placed in an incubation area for the mycelium to fully colonize the compost. Once the mycelium has permeated the substrate, a casing layer must be applied to the surface to initiate fruiting. This layer is typically a blend of peat moss and calcium carbonate (lime) to buffer the naturally acidic peat and adjust the pH to a slightly alkaline range, ideally between 7.5 and 8.0.
The casing layer’s primary function is to retain high moisture levels and create a microclimate that encourages the formation of primordia, or “pins.” Unlike the bulk substrate, the casing layer is often only lightly pasteurized or left unsterilized, as the presence of certain bacteria helps trigger the mushroom’s reproductive phase. A thickness of one to two inches is recommended for the casing layer to effectively manage the surface environment.
Controlling Environmental Conditions for Fruiting
The cultivation of Agaricus bisporus is separated into two distinct environmental phases: the vegetative incubation phase and the reproductive fruiting phase. During the initial incubation phase, the colonized substrate requires warm temperatures, ideally between 75°F and 77°F (24°C and 25°C), to encourage rapid and complete mycelial growth. The carbon dioxide (CO2) concentration should remain high during this time, often reaching 10,000 to 15,000 parts per million (ppm), and fresh air exchange is minimal.
The shift to the fruiting phase requires a controlled “shock” to signal the mycelium to begin forming mushrooms. This change is induced by simultaneously lowering the temperature and increasing the fresh air exchange. The air temperature is dropped to between 60°F and 68°F (16°C and 20°C), and the CO2 levels must be reduced significantly, ideally to 1,000 to 2,000 ppm, to promote the formation of pins.
This reduction in CO2, combined with the temperature drop, triggers the mushroom’s reproductive cycle. High humidity, maintained near 95% during mycelial growth, is then lowered to about 85% to 88% during the pinheading stage. Precise control over these factors is necessary because if CO2 levels remain too high, the resulting mushrooms will develop elongated stalks and open prematurely.
Harvesting and Maintaining Subsequent Flushes
The timing of the harvest depends on whether the goal is to produce Crimini or full Portobello mushrooms. Crimini, or baby bellas, are harvested when the veil beneath the cap is still intact, before the cap has fully opened. To produce a full Portobello, the mushroom is allowed to mature until the cap is completely open, exposing the dark gills beneath.
When harvesting, it is best to gently twist and pull the mushroom from the casing layer rather than cutting it off with a knife. Twisting minimizes the amount of residual mushroom tissue left behind, which can otherwise decay and introduce contamination to the growing bed. Any small, undeveloped pins or aborts should also be carefully removed to maintain the health of the substrate for future growth.
Following the initial harvest, the mushroom bed will enter a resting period before producing flushes. The casing layer may need to be rehydrated between flushes by misting the surface to replace the moisture lost during the growth and harvest. While later flushes yield fewer mushrooms than the first, maintaining high humidity and removing spent tissue encourages the mycelium to conserve energy and produce additional harvests.