Portobello mushrooms are a recognizable grocery staple, but they are not a distinct species. They are the fully mature stage of Agaricus bisporus, the same fungus that produces the smaller white button and cremini varieties. The extended time allowed for the cap to expand and the gills to darken gives them their rich, meaty texture and flavor profile. From the start of cultivation, the entire process to harvest your first fully mature Portobello typically requires 7 to 12 weeks. This timeline is influenced by strict environmental controls and the natural biological pace of the fungus.
Substrate Preparation and Colonization Phase
The journey begins with preparing the substrate, a nutrient-rich compost mixture that provides the food source for the fungus. This preparation phase, often involving controlled composting and pasteurization, takes approximately 14 days before the material is ready for inoculation. Pasteurization eliminates harmful contaminants while preserving beneficial microorganisms needed for the Agaricus bisporus to thrive.
Once the substrate is prepared, it is inoculated with grain spawn, which is the mycelium of the fungus grown on sterilized grains. This starts the colonization phase, where the white, thread-like mycelium begins to grow and spread throughout the compost. This growth period, known as the mycelial run, typically lasts 10 to 14 days until the substrate is completely colonized.
Environmental conditions, particularly temperature, are managed during this phase to encourage rapid mycelial growth and prevent competing molds from taking hold. The compost temperature is maintained around 75 to 78 degrees Fahrenheit (24–26°C) for optimal colonization. The first four to six weeks of the cultivation cycle are dedicated to this subterranean growth as the fungus builds the biological network necessary to produce mushrooms.
The Timeline from Pinning to First Harvest
After the substrate is fully colonized, a moisture-retentive casing layer, typically peat moss and lime, is applied to the surface. This casing does not provide nutrition but signals the fungus to begin the next stage of its life cycle. Environmental conditions are then adjusted by lowering the air temperature and increasing fresh air exchange to reduce carbon dioxide levels.
These changes trigger the formation of primordia, or “pins,” which are the tiny mushrooms that first appear on the casing layer. Pinning initiation occurs 5 to 10 days after the casing is applied and the environmental shock is introduced. These pins represent the first visible sign of the upcoming harvest.
The growth from pins to a harvestable button mushroom is quick, usually taking 7 to 10 days. If the crop were intended for white button mushrooms, harvest would occur when the cap is still tightly closed. Allowing the mushrooms an additional 3 to 7 days of growth past the button stage results in the cremini mushroom.
Achieving Portobello Size and Subsequent Harvest Cycles
To achieve the large Portobello size, the grower must allow the cremini to continue maturing for several more days. This final stage of expansion requires an additional 3 to 7 days beyond the cremini harvest window. During this time, the cap expands dramatically, flattening out to a diameter of 4 to 6 inches, and the veil underneath breaks, exposing the dark, mature gills.
The fully opened cap and exposed dark gills are the defining characteristics of a Portobello. The total time from the application of the casing layer to the first harvest of a full-sized Portobello is generally 21 to 28 days.
Mushroom production does not end with the first harvest, as the mycelial network is prepared to produce subsequent waves of mushrooms known as “flushes.” A new flush typically emerges 7 to 10 days after the previous crop has been harvested, as the fungus recovers its energy. A single growing bed can sustain multiple flushes, providing a continuous harvest for several weeks before the substrate is exhausted.