Mushroom cultivation involves managing precise environmental conditions to encourage fungal growth and fruiting. While typically performed in climate-controlled indoor spaces, a greenhouse offers a viable alternative for many growers. A standard greenhouse, built for sun-loving plants, presents unique challenges, particularly concerning light exposure and temperature control. Converting this space requires specific modifications to create the dark, humid, and regulated atmosphere fungi demand. This guide details the necessary adaptations and techniques to transform a plant-focused greenhouse into an efficient mushroom production facility.
Adapting the Greenhouse Environment
Unlike plants, most cultivated fungi require near-total darkness during colonization and deep shade during fruiting. Direct sunlight is detrimental, as it desiccates the mycelium and inhibits growth or causes malformed fruits. To achieve low light levels, high-density shade cloths (up to 90%) must be layered over the structure. For complete darkness during initial incubation, growers use blackout tarps or specialized polyethylene sheeting to block all external light.
Temperature stability is paramount, as different mushroom species and growth stages have specific thermal requirements. Greenhouses naturally experience significant temperature swings, necessitating both cooling and heating systems to maintain narrow ranges. Evaporative coolers or simple misting systems can reduce internal temperatures during warm seasons. Conversely, electric heaters or propane burners, coupled with thermostat controls, are necessary to maintain the incubation temperature, often around 70–75°F (21–24°C), during colder periods.
Fungi require high relative humidity, typically between 80% and 95%, especially during the pinning and fruiting stages, to prevent the caps from drying out. A simple watering can is insufficient for maintaining this saturation level. High-pressure fogging systems or ultrasonic humidifiers are effective tools for aerosolizing water into a fine mist. Wetting the floor and walls of the greenhouse regularly also helps to create a stable, high-moisture environment through passive evaporation.
Mushrooms release carbon dioxide (CO2) as they grow, and high concentrations inhibit cap development, causing elongated, spindly stems (“legginess”). Constant fresh air exchange (FAE) is necessary, primarily during the fruiting phase. Exhaust fans near the ceiling and passive air intakes near the floor create a gentle, continuous air flow. This system ensures CO2 levels remain below 1,000 parts per million (ppm) while replenishing the oxygen supply needed for healthy fruiting.
Selecting Mushroom Species and Substrate
The inherent environmental fluctuations of a greenhouse make selecting resilient, fast-colonizing species advisable for beginners. Oyster mushrooms (Pleurotus species), such as Blue or Pearl Oysters, are highly forgiving of minor temperature and humidity variations. Certain strains of Shiitake (Lentinula edodes) are also suitable, provided their specific temperature requirements can be met during fruiting. Highly sensitive gourmet varieties, like Lion’s Mane, often require stricter environmental controls that are difficult to maintain consistently in a modified greenhouse setting.
The substrate serves as the nutrient base upon which the mycelium grows and produces mushrooms. Common materials include hardwood sawdust, straw, or wood chips, depending on the species. Before inoculation, this material must undergo heat treatment to eliminate competing molds, bacteria, and insect pests. Pasteurization involves heating the substrate to 140–180°F (60–82°C) for several hours, killing most contaminants while preserving some beneficial microbes. Sterilization—heating the substrate above boiling point—is necessary for sensitive species to remove all potential competitors and ensure a successful spawn run.
The Cultivation Cycle
After the substrate is prepared, inoculation introduces the mushroom spawn—grain colonized by mycelium—into the bulk material. This mixture is packed into bags or containers and moved to the greenhouse for the spawn run phase. During this period, the mycelium spreads throughout the substrate, consuming nutrients and forming a dense, white network. This phase requires stable temperatures, often 70–75°F, and typically complete darkness to encourage rapid and uniform colonization.
The spawn run duration varies by species, but once the substrate block is fully colonized, it is ready for fruiting. This transition requires an environmental “shock” to signal the fungus to shift from vegetative to reproductive growth. This shock involves dropping the air temperature and significantly increasing fresh air exchange to lower the CO2 concentration. Simultaneously, relative humidity must be raised near 95% to encourage the formation of primordia, or “pins.”
These small, dense structures rapidly develop into mature mushrooms over the next few days, provided the environmental parameters remain consistent. The grower must continuously monitor temperature, humidity, and airflow to ensure the developing mushrooms receive adequate oxygen without drying out. Maintaining slightly higher light levels during fruiting, achieved through low-intensity fluorescent bulbs or minimal ambient light, can help some species achieve proper cap color and orientation.
Harvesting is performed just before the mushroom cap fully flattens and begins to drop spores, which can inhibit subsequent growth and create contamination risks. The mushrooms are typically twisted or cut cleanly at the base, ensuring no residual stem material remains on the substrate block. Proper removal encourages the mycelium to conserve energy and prepare for the next production cycle, often called a “flush.”
Managing Pests and Contamination
The high-humidity environment of a mushroom greenhouse is attractive to various insect pests, particularly fungus gnats and mites. Fungus gnat larvae feed directly on the mycelium, causing significant crop damage, especially during the spawn run. Low-impact control measures include the use of yellow sticky traps to capture adults and the application of Bacillus thuringiensis israelensis (BTI), a biological larvicide, to the substrate or floor.
Contamination from competing molds, like the common green mold Trichoderma, is a frequent challenge in non-sterile settings. These molds appear as patches of green, blue, or orange growth that rapidly overtake the white mycelium, rendering the substrate unusable. Proper substrate pasteurization is the first line of defense against these competitors. Immediately removing any substrate block showing signs of contamination prevents spores from spreading throughout the greenhouse, protecting the remaining healthy crop.