Plug spawn, also known as dowel spawn, consists of small wooden dowels fully colonized by mushroom mycelium. This colonized wood is the material used to inoculate hardwood logs and stumps for outdoor mushroom growing, a method commonly employed for species like Shiitake, Oyster, or Lion’s Mane. Creating your own spawn offers significant cost savings over purchasing pre-made plugs. It also allows a grower to propagate specific, high-performing strains from a liquid culture or agar plate. This initial stage of preparation ensures the biological material is robust and ready to thrive when introduced to the log substrate.
Required Materials and Pre-Sterilization
The preparation phase for making plug spawn centers on hydrating the wooden dowels and ensuring they are completely sterile before inoculation. You will need small hardwood dowels, typically measuring 5/16-inch in diameter and 1-inch in length, as the substrate for the mycelium. Mason jars with modified lids—either featuring a self-healing injection port or a filter patch for gas exchange—are the preferred containers for sterilizing and incubating the dowels. The inoculant must be a healthy, contaminant-free source of mushroom culture, such as a liquid culture syringe or agar wedges.
A still air box (SAB) or a laminar flow hood is necessary to maintain an aseptic environment during the transfer process, which significantly reduces the risk of contamination. Other tools include 70% isopropyl alcohol for surface sanitization and a burner to flame-sterilize inoculation loops or scalpel blades. The dowels must first be hydrated by soaking them in water for a minimum of 24 hours to provide the necessary moisture content for mycelial growth.
After soaking, pack the dowels loosely into the glass jars, leaving some headspace, and place the sealed jars into a pressure cooker. Sterilization must be rigorous because wood is a dense material that requires thorough heat penetration to eliminate all competing organisms. Sterilize the jars at 15 pounds per square inch (PSI) for a minimum of 90 to 120 minutes, which is the necessary duration to kill heat-resistant bacterial endospores. Once the cycle is complete, allow the pressure to drop naturally and the jars to cool completely to room temperature before handling.
The Plug Spawn Inoculation Procedure
Inoculation is the delicate process of introducing the pure mushroom culture into the sterilized dowels while preventing airborne contaminants. This step must be performed inside an aseptic workspace, such as a still air box or flow hood, which filters the air and provides a clean working environment. Sanitize the dowel jars, the culture source, and all tools with 70% isopropyl alcohol and allow them to air dry before beginning.
If using a liquid culture, employ a sterile syringe to draw up the mycelium-rich solution. Insert the needle through the injection port and distribute approximately 1 to 3 milliliters of liquid culture over the dowels. For agar inoculation, use a flame-sterilized scalpel to cut small, square wedges from a fully colonized agar plate. Drop these active mycelium pieces into the jar and shake gently to distribute them among the sterile dowels.
It is helpful to add a small layer of sterilized, nutrient-rich material, like hydrated wood pellets or grain, on top of the dowels before sterilization to give the culture a quick energy source. The mycelium establishes itself quickly on this material, accelerating initial colonization. After inoculation, gently shake or swirl the jars to ensure the culture is spread across the dowel surfaces, maximizing the points of contact for colonization.
Incubation and Monitoring for Full Colonization
Following inoculation, move the jars to an incubation area providing optimal conditions for the mycelium to colonize the wooden plugs. The ideal environment is dark and maintains a consistent temperature, typically between 70°F and 75°F (21°C to 24°C) for common hardwood species like Shiitake and Oyster. Temperature stability is important because fluctuations can stress the mycelium or encourage mold growth. Darkness prevents premature fruiting, ensuring energy is directed toward colonizing the wood.
Colonization usually takes three to six weeks, depending on the mushroom species and the density of the wood. The visual cue for success is a thick, white, and often slightly fluffy coating of mycelium completely covering the dowels, making the wood grain largely invisible. This white growth should appear uniform and healthy throughout the jar.
Monitoring for contamination is a daily task, as molds and bacteria easily outcompete the slower-growing mycelium. Green, blue, or black powdery patches often indicate molds like Trichoderma, while slimy, foul-smelling areas can indicate bacterial blotches. Any jar showing signs of growth must be immediately isolated and discarded, preferably by opening it outdoors, to prevent spores from contaminating clean spawn.
Storage and Shelf Life of Finished Spawn
Once the wooden plugs are fully colonized and exhibit a healthy, dense mycelial coating, they are ready for use or storage. To preserve the spawn’s viability until log inoculation, slow the mycelium’s metabolic rate by storing the jars in a refrigerator. The best way to achieve this is by storing the fully colonized jars in a refrigerator.
An ideal storage temperature is between 34°F and 40°F (1°C to 4°C), which induces a state of semi-dormancy. Keep the jar lids sealed to prevent the dowels from drying out, ensuring the gas exchange port remains functional. Under these cool, dark conditions, the plug spawn maintains high viability for six to twelve months. Using the spawn within six months is recommended for the most vigorous colonization.