Portobello mushrooms are the fully mature stage of the common button mushroom, Agaricus bisporus, which also includes the cremini variety. Cultivating this species at home provides large, fresh mushrooms and allows for complete control over the growing medium. While Agaricus bisporus has specific requirements, mastering the environmental controls makes home cultivation a rewarding project involving distinct phases.
Essential Supplies and Environmental Setup
The foundational material is viable mushroom spawn, which serves as the “seed” material to colonize the substrate. This spawn, often grain-based, should be sourced from a reputable supplier specializing in Agaricus bisporus. The mushroom requires a specific substrate, typically a composted, manure-based material enriched with straw or other organic components. Appropriate containers, such as plastic trays or specialized growing bags, are necessary to hold the substrate. Precise control over temperature and humidity is mandatory throughout the cultivation cycle. Incubation requires 70–75°F (21–24°C) for mycelial growth. Once colonized, the temperature must be lowered to 55–65°F (13–18°C) to trigger fruiting, maintaining high relative humidity (85–90%) with a hygrometer.
Substrate Preparation and Inoculation
The specialized composted substrate must first undergo pasteurization, which is distinct from sterilization. Pasteurization involves heating the substrate to 140°F to 160°F (60–71°C) for several hours. This heat treatment eliminates most harmful molds and competing organisms while preserving beneficial microorganisms necessary for Agaricus bisporus. Once the pasteurized substrate cools, inoculate it by thoroughly mixing in the mushroom spawn to ensure even colonization. Place this mixture into containers, cover loosely, and move it to the incubation area. The incubation period, or spawn run, is maintained at 70–75°F (21–24°C) for two to four weeks. During this time, the fungal network (mycelium) spreads as a dense, white, thread-like network. This phase is complete when the substrate is entirely bound together by this growth.
Casing and Inducing Mushroom Growth
After the substrate is fully colonized, apply the casing layer, which is a non-nutritive, moisture-retaining material, usually a mixture of peat moss and hydrated lime. This layer is necessary for Portobellos to successfully transition into the fruiting phase and form normal mushroom caps. The hydrated lime helps neutralize the natural acidity of the peat moss, aiming for a pH between 7.0 and 7.5. Apply a layer of about one to two inches of the moist casing material gently over the fully colonized substrate. The layer is not mixed in, but rather patted lightly to create a uniform surface. This casing layer creates a microclimate that holds moisture and provides the physical stimulus needed to initiate the reproductive cycle.
The transition to fruiting is triggered by a temperature drop and increased fresh air exchange (FAE). The temperature must be lowered to the cooler range of 55–65°F (13–18°C) to signal the mycelium to begin forming mushrooms. Simultaneously, the carbon dioxide levels must be reduced by introducing daily fresh air circulation, which is a key signal for the formation of pins, the tiny mushroom primordia. High humidity, maintained by misting the casing layer without soaking it, is continuously required during this phase. Pinning can be expected within one to two weeks after the environmental changes are implemented.
Harvesting and Subsequent Yields
Portobello mushrooms are ready for harvest when their caps have fully expanded, revealing the dark gills underneath, but before the edges curl excessively or the gills begin to drop spores. Timing the harvest correctly ensures the mushrooms have reached their full size and rich flavor profile. The best technique for harvesting involves gently twisting the mushroom off the substrate at the base of the stem, which minimizes damage to the underlying mycelial network. Alternatively, the mushroom can be cut cleanly at the base, leaving a small piece of stem tissue behind. Care must be taken to remove any leftover tissue or aborts from the harvested area to prevent decay.
Following the first harvest, known as a “flush,” the substrate needs a period of recovery and rehydration to prepare for subsequent growth cycles. The casing layer should be kept moist, and the fruiting conditions, including the cooler temperature and high humidity, must be maintained. The substrate can typically produce two to three more flushes, with each subsequent flush yielding a slightly smaller amount of mushrooms.