A monotub is a modified plastic storage container used in mycology to create a controlled microclimate for growing mushrooms. This enclosed system functions as a fruiting chamber, balancing the atmospheric conditions needed for the fungal network, or mycelium, to produce mushrooms. The design allows for passive fresh air exchange (FAE) and high humidity retention, which are key factors for successful cultivation. The monotub simplifies the cultivation process, making it a popular choice for both new and experienced growers seeking consistent yields.
Essential Materials and Tools
The construction of a monotub begins with a clear or translucent plastic storage tub, typically between 50 and 80 quarts in size, including a secure-fitting lid. You will need a standard electric drill and a two-inch hole saw attachment to create the ventilation openings. A measuring tape and a permanent marker are needed for accurately plotting the location of the holes before drilling.
For the filtration system, use a breathable material like poly-fil (polyester stuffing) or specialized adhesive filter patches. Accessories include fine-grit sandpaper or a utility knife to smooth rough plastic edges, and a sanitizing agent like 70% isopropyl alcohol or diluted bleach. These materials allow the container to be modified to manage airflow and maintain a contamination-free environment.
Modifying the Tub Structure
The modification process requires careful measurement to position the air exchange ports correctly, which is fundamental to the monotub’s function. The lower set of holes should be placed three to four inches up from the base, just above where the bulk substrate will reach. This placement ensures that heavier carbon dioxide gas, a byproduct of mycelial respiration, can passively exit the chamber while facilitating the intake of fresh, oxygen-rich air.
A second set of holes is placed higher up on the tub, typically three to five inches from the top rim or near the lid. These upper ports allow for the exit of warmer, moisture-laden air, which helps regulate internal humidity and prevents air from becoming stagnant.
Using the two-inch hole saw attachment, drill the holes slowly to prevent cracking the plastic; using a reverse setting can melt through the material more cleanly. A common configuration is drilling two holes on each long side and one hole on each short side, totaling six ports. After drilling, remove any sharp plastic remnants with sandpaper or a knife to create a smooth surface, ensuring a better seal when filters are applied.
Installing Air Exchange and Filters
The newly created holes must be outfitted with a filter material to allow for gas exchange while preventing airborne contaminants, such as mold spores, from entering the chamber. Poly-fil is a common choice because its density can be adjusted to regulate the rate of fresh air exchange (FAE) and humidity. For the lower holes, the poly-fil is typically packed tightly to slow down the air exchange and help retain moisture near the substrate surface.
The upper holes, which act as exhaust ports, may be fitted with a slightly looser plug of poly-fil or a filter patch to encourage a steady, gentle flow of air. Alternatively, specialized adhesive filter patches or micropore tape can be used, which provide a consistent filtration level without the need to manually adjust the poly-fil density. The correct balance of FAE is important because the growing mycelium consumes oxygen and releases carbon dioxide; a buildup of carbon dioxide can cause mushrooms to develop long, spindly stems instead of healthy caps.
Final Sanitization Procedures
Before the modified monotub is introduced to the mushroom spawn and substrate, a thorough sanitization process is necessary to eliminate competing microorganisms. Contamination is a significant risk in mushroom cultivation, as unwanted bacteria and molds can quickly outcompete the desired fungal culture. The first step involves washing the tub and lid completely with hot water and dish soap to remove any dust or manufacturing residue.
Following the initial wash, all interior surfaces should be wiped down with a chemical agent to reduce the microbial load. A solution of 70% isopropyl alcohol is effective for sanitizing plastic surfaces, as it evaporates quickly and leaves no residue. Alternatively, some growers use a diluted bleach solution (a 1:10 ratio with water), which is a powerful disinfectant. The tub must then be allowed to air dry before introducing any substrate or spawn, ensuring a clean environment for the mycelium to thrive.