Growing mushrooms from mushrooms utilizes the genetic material of an existing specimen, moving beyond pre-packaged kits. This method allows cultivators to select for specific traits, such as size, growth rate, or resilience, ensuring the next generation maintains those characteristics. It is a fundamental step in mycology that offers significant control over the cultivation process. By mastering the techniques of culture transfer and expansion, a grower can establish a continuous, self-sustaining cycle of production. This approach also provides considerable cost savings by eliminating the need to repeatedly purchase commercial spawn or cultures.
Obtaining the Starter Culture
The journey begins with securing the initial genetic material, which can be accomplished primarily through spore collection or tissue cloning. Spores are the microscopic reproductive units of the fungus, collected by creating a spore print. This involves placing the cap of a mature mushroom, gills-down, onto a sterile surface (like foil or glass) to allow the spores to drop. The resulting print can then be used to create a spore syringe by scraping the spores into sterile water and drawing the suspension into a syringe.
Maintaining a sterile environment is necessary to prevent contamination, often requiring a still air box (SAB) or a laminar flow hood. While spore prints contain a mix of genetics, offering variability in the resulting fungi, tissue cloning provides a direct genetic replica of the parent mushroom. This is achieved by aseptically removing a small piece of internal, clean tissue from the stem or cap.
The mushroom should be torn open rather than cut to avoid pushing outside contaminants into the sterile inner tissue. This small tissue sample is then transferred to a sterile nutrient medium, such as an agar plate. Agar plates contain necessary components like malt extract to fuel the initial growth of the fungal network, allowing the cultivator to observe and select clean, healthy mycelium.
Alternatively, the tissue can be introduced into a liquid culture, which is a sterilized, nutrient-rich broth that promotes rapid mycelial expansion. Using agar plates provides a clearer isolation of healthy mycelium and reduces contamination risks compared to working directly with spores. Once the agar or liquid culture is fully colonized with clean mycelium, it serves as the master culture for all future propagation and expansion.
Preparing the Growth Environment
The master culture must be expanded into spawn before bulk substrate inoculation. Spawn is typically created by transferring the colonized agar or liquid culture onto sterilized grain kernels, such as rye or millet. These grains provide a high-energy food source that allows the mycelium to rapidly colonize a larger mass, creating numerous inoculation points for the next stage.
Once the grain spawn is fully colonized, it is mixed with a bulk substrate, which acts as the primary food source and structural support for the eventual mushrooms. Substrate choice depends on the species; wood-loving species prefer hardwood sawdust mixed with bran, and others thrive on mixtures of coco coir, vermiculite, or pasteurized straw. The bulk substrate must be treated to eliminate competing organisms before inoculation, especially for nutrient-rich materials.
Substrates are either sterilized using a pressure canner or autoclave, heating the material to at least 250°F (121°C) to kill all organisms, including bacterial endospores. Achieving this temperature requires pressure, typically 15 pounds per square inch (PSI), and may take between one and four hours depending on the substrate mass. Pasteurization is a less intense heat treatment that kills most contaminants while preserving some beneficial microbes, and it is often sufficient for lower-nutrient substrates like straw.
After cooling, the fully colonized grain spawn is thoroughly mixed into the prepared bulk substrate within a sterile bag or container. This act of inoculation kickstarts the final colonization phase before the mycelium is ready to bear fruit. The added spawn provides numerous inoculation points, helping the desired mycelium outcompete any remaining contaminants.
Managing Incubation and Harvest
After inoculation, the substrate enters the incubation phase, where the mycelium colonizes the bulk material in dark, warm conditions. Optimal temperatures for colonization typically range from 70°F to 86°F (21°C–30°C), though this varies by species. During this period, the mycelial network spreads and binds the substrate into a solid mass, preparing it to support the formation of mushrooms.
No light or fresh air exchange is needed, as the focus is solely on internal growth. Once colonization is complete, the environmental conditions must be drastically altered to initiate fruiting, a process known as “pinning.” This involves introducing a temperature drop, increasing the relative humidity to between 85% and 95%, and providing consistent Fresh Air Exchange (FAE).
The introduction of cooler temperatures and increased oxygen acts as a signal to the mycelium that surface conditions are favorable for reproductive growth. Consistent FAE removes the carbon dioxide produced by the growing mycelium, which is necessary for the formation of proper caps and stems. High carbon dioxide levels can cause the mushrooms to grow long, spindly stems and small caps.
Mushrooms are ready for harvest just before the protective veil under the cap tears open, which maximizes flavor and prevents the release of spores onto the substrate. Spore drop onto the substrate can signal the mycelium to slow or stop production in that area, reducing the yield of subsequent flushes. After the initial harvest, the substrate block can be rehydrated (often by soaking) to prepare it for subsequent “flushes,” or additional rounds of mushroom production.