A monotub provides a self-contained, high-humidity environment for cultivating large quantities of mushrooms, making it a popular choice for home growers seeking scalability and ease of use. This modified plastic storage container allows for the management of humidity and air exchange within a single vessel, simplifying the cultivation process compared to smaller, less integrated methods. The monotub system automates many steps, creating conditions that encourage the mycelium—the vegetative part of the fungus—to transition into producing the visible fruiting bodies known as mushrooms. Success in this method depends on careful preparation, consistent environmental control, and patience through the distinct phases of fungal growth.
Preparing the Monotub and Substrate
The foundation of a successful grow begins with the careful preparation of the container and the bulk substrate. A standard plastic tub must be modified by drilling or melting holes near the substrate level and near the top, which are temporarily covered to manage air flow. These holes will later be used for passive air exchange, often stuffed with polyfill or covered with specialized filters to block contaminants while allowing gas exchange. Maintaining a clean, sanitized environment during this construction phase is important for preventing the introduction of competing molds or bacteria.
The bulk substrate, which provides the water and nutrients, is typically a mixture of materials like coco coir, vermiculite, and gypsum (CVG). Coco coir is chosen for its water retention properties, vermiculite helps maintain air pockets, and gypsum provides minerals and helps stabilize the pH, ideally between 5.5 to 6.5. The substrate must be hydrated to “field capacity,” where a handful, when squeezed tightly, releases only a few drops of water. Overly wet substrate eliminates the necessary air pockets, leading to oxygen-poor conditions that stall colonization.
Pasteurization is a necessary step for the bulk substrate, involving heating it to a temperature between 150°F and 185°F (65°C to 85°C) for 90 minutes to two hours. This heat treatment reduces contaminants, such as mold spores, but intentionally leaves behind beneficial microorganisms. Unlike sterilization, pasteurization gives the mushroom mycelium a competitive advantage by eliminating the most aggressive competitors. This pasteurized substrate is then mixed with fully colonized grain spawn, which acts as the seed source, and layered into the prepared monotub.
The Colonization Phase
Once the grain spawn and pasteurized substrate are mixed and placed in the monotub, the colonization phase begins, focusing on allowing the mycelial network to spread completely. During this period, the monotub is kept closed with the air holes taped or plugged to maintain high levels of carbon dioxide and humidity, which are ideal for mycelial growth. The mycelium thrives in this high-CO2 environment, using it as a signal to rapidly colonize the substrate.
Ideal incubation temperatures for most common species fall between 70°F and 80°F (21°C to 27°C). This stable temperature encourages the white, thread-like hyphae to grow rapidly throughout the substrate, knitting the loose material into a solid mass known as a cake. Maintaining darkness or very low light is recommended, as light is not required for colonization. The tub should remain undisturbed for seven to ten days to prevent contamination and allow the mycelium to establish a strong network.
Visual monitoring is important for checking the progress of the colonization, which is complete when the entire surface of the substrate is covered in thick, white mycelial growth. Growers should look out for signs of contamination, such as green or blue patches, which indicate the presence of competing molds, or any foul, sour odors that suggest bacterial infection. If contamination is spotted, the entire tub should be removed from the growing area to prevent the spread of spores.
Initiating Fruiting Conditions
The transition from colonization to fruiting requires a deliberate shift in the environmental parameters to trick the mycelium into producing actual mushrooms, a process often called “pinning.” The primary trigger involves lowering the high carbon dioxide (CO2) levels that built up during colonization and introducing an abundance of oxygen, known as Fresh Air Exchange (FAE). This is achieved by removing the tape or plugs from the air holes, allowing the CO2, which is heavier than air, to escape from the lower holes and drawing in fresh, oxygen-rich air through the top holes. High CO2 levels can cause malformed or long-stemmed mushrooms, making FAE an important factor for healthy development.
Simultaneously, the grower must maintain a high relative humidity, ideally between 85% and 95%, to prevent the delicate surface of the substrate from drying out. This is managed by misting the walls and the inside of the lid of the monotub with a fine spray, ensuring that no standing water pools directly on the surface of the mycelium. The evaporation of this water from the surface of the substrate also serves as a secondary pinning trigger, mimicking the natural environmental changes that occur in the wild. Excessive air exchange can quickly drop the humidity, leading to dry caps or stalled growth, necessitating a balance between FAE and moisture retention.
Introducing a source of indirect light is the final step, acting as a directional cue that tells the developing mushrooms which way to grow. Ambient room light or a soft LED light set on a 12/12 hour cycle is sufficient, as the light guides the direction of the stems. The temperature should also be slightly reduced during this phase, with many species preferring cooler conditions between 60°F and 70°F (15°C to 21°C) for optimal fruiting. Within five to ten days of introducing these changes, small mushroom formations called primordia, or pins, should begin to appear on the surface.
Harvesting and Subsequent Flushes
Harvesting at the correct time is important for maximizing yield and maintaining the cleanliness of the grow area. The ideal time to harvest is just before the veil—the membrane connecting the cap to the stem—fully breaks and drops spores. Spore drop can cover the substrate, which some growers believe may reduce the vigor of later growth cycles. Once the veil is about to tear, the mushroom is at its peak size and potency.
The proper technique for harvesting involves grasping the base of the stem and gently twisting and pulling the mushroom free from the substrate. This method is preferred over cutting, as it removes the entire mushroom and minimizes the organic material left on the surface, which can potentially decay and attract contamination. Any small, aborted pins or remaining mushroom material should also be removed to clean the surface for the next growth cycle.
A “flush” refers to a distinct wave of mushrooms that grow and are harvested from the substrate. After the first harvest, the substrate cake has lost a significant amount of water. To prepare for the next flush, the cake must be rehydrated by soaking it in cold water for a period, typically between four and twelve hours. The cold water helps to shock the mycelium, which can stimulate the production of the next wave of pins. After soaking, the excess water is drained, and the monotub is returned to fruiting conditions, where the mycelium can often produce two to three substantial flushes before the substrate is exhausted.