Oyster mushrooms (Pleurotus ostreatus) are popular for home cultivation due to their rapid growth cycle and adaptability to various materials. This fungus is known for its delicate flavor and meaty texture, making it suitable for many culinary applications. Growing them indoors allows for year-round production with simple equipment and minimal space requirements. This guide offers a step-by-step approach to cultivating these mushrooms, from substrate preparation to the final harvest.
Preparing the Growing Medium
The foundational step is selecting and preparing the substrate, the material that provides nutrients for the fungus. Oyster mushrooms thrive on lignocellulosic waste materials like chopped straw, hardwood sawdust pellets, or spent coffee grounds. The material must first be hydrated to a moisture content of approximately 60-70%, allowing the mycelium to access water and nutrients.
Once hydrated, the substrate must undergo pasteurization to eliminate most competing molds and bacteria. This process involves maintaining the temperature between 60°C and 80°C for one to two hours, often using a hot water bath or steam. Pasteurization leaves behind beneficial microbes that help the mycelium resist aggressive contaminants. The substrate must then be cooled quickly to below 30°C before inoculation to prevent thermal shock and contaminant proliferation.
Inoculation and Mycelial Colonization
Inoculation involves mixing the prepared substrate with mushroom spawn, which acts as the fungal seed. The spawn, typically grain or sawdust colonized by mycelium, is mixed into the cooled substrate at a concentration of 3-10% of the total substrate weight. This mixture is packed into sealed cultivation bags or containers to maintain high carbon dioxide levels and humidity.
The colonization phase requires a stable, warm environment, typically 20°C to 27°C, depending on the mushroom variety. During this period, the white, thread-like mycelium consumes the substrate, forming a dense, interwoven network. The sealed container is kept in darkness or very low light, as light is not required for vegetative growth.
Metabolic activity within the substrate generates heat, which must be monitored to ensure the internal temperature does not exceed 33°C. Colonization usually takes two to four weeks, concluding when the entire substrate block is covered in a thick, uniform layer of white mycelium. High carbon dioxide levels must be maintained during this time, signaling the fungus to remain in its vegetative state.
Inducing and Managing Mushroom Formation
To transition the fully colonized substrate to fruiting, the environment must be manipulated to mimic natural external conditions. This shift is triggered by three changes: a drop in temperature, the introduction of light, and increased fresh air exchange. Lowering the air temperature to 13°C to 24°C, depending on the strain, signals the mycelium that conditions are right for development.
The mycelium requires light to properly orient the developing fruiting bodies and form healthy caps. Exposure to indirect sunlight or a standard fluorescent or LED light for 8 to 12 hours daily is sufficient to induce the formation of mushroom “pins.” The most significant change is the introduction of fresh air exchange, which lowers the carbon dioxide (CO2) concentration surrounding the block.
Oyster mushrooms produce a large amount of CO2, and if the concentration remains high (above 1,000 ppm), the developing mushrooms will become deformed, resulting in long, spindly stems and underdeveloped caps. The growing area must be fanned several times daily or equipped with continuous ventilation to refresh the air. This constant air exchange makes maintaining high humidity challenging.
The air surrounding the substrate needs a relative humidity of 85% to 95% to prevent the pins from drying out. Growers often use a humidity tent or an ultrasonic humidifier to achieve this moist environment, while also misting the mushrooms multiple times a day. Balancing fresh air exchange and high humidity is the most difficult aspect of home cultivation. Once conditions are met, small mushroom formations (pins) will appear at the openings on the substrate block within a few days.
Harvesting and Encouraging Future Yields
Oyster mushrooms grow rapidly once pinning occurs, often doubling in size daily, making the harvest window short. They are ready for picking when the edges of the caps begin to uncurl and flatten, but before they release extensive white, powdery spores. Harvesting before spore release is recommended, especially for indoor growing, to maintain air quality.
To harvest, the entire cluster should be gently twisted and pulled away from the substrate block, or cut cleanly at the base. Removing the entire cluster ensures no residual organic matter is left to decompose and invite contamination. After the first harvest (the “first flush”), the substrate block can be rehydrated by soaking it in cold water for several hours. This replenishes the moisture required for the mycelium to produce subsequent yields, often a second or third round of mushrooms.