Inoculation is the act of introducing shiitake mycelium spawn into the prepared substrate (log or sawdust bag).
Incubation, also known as the spawn run, is the period after inoculation during which the mushroom mycelium grows and spreads throughout the substrate.
By the end of incubation, you will have either: fully colonized, spawn-run logs (after many months outdoors) or fully colonized, browned shiitake blocks (after perhaps 8–12+ weeks indoors).
In both cases, the mycelium is now ready and eager to reproduce by forming mushrooms. The next step is to create the right conditions to trigger fruiting and support the development of high-quality shiitake mushrooms.
Inoculation: Hardwood Logs (Outdoor)
Once your logs are cut, rested, and ready as described above, it’s time to introduce the shiitake culture to the wood. Shiitake spawn for logs comes in two common forms: plug spawn or sawdust spawn.
Plug spawn consists of wooden dowels (usually 1 inch long) that have been pre-colonized with shiitake mycelium. These plugs are simply tapped into drilled holes in the log. They are convenient for beginners and minimize mess.
Sawdust spawn is loose granular spawn, typically colonized sawdust or grain. It can be packed into drilled holes (often using a specialized inoculation tool that injects the spawn). Sawdust spawn tends to colonize logs faster than plugs because it has more points of contact and a bit more mycelial mass, but it can be slightly trickier to handle (requiring an applicator or steady hands and a plunger).
For small-scale growers, plug spawn is very easy and effective. Larger projects may prefer sawdust spawn for speed and cost. Both are widely used in the industry. Ensure you obtain high-quality shiitake spawn from a reputable supplier.
Step-by-Step Log Inoculation:
Drill inoculation holes
Using a drill, bore holes of the appropriate diameter for your spawn, usually about 5/16 inch (8 mm) for plug spawn, or 7/16 inch (11 mm) for sawdust spawn. Hole depth should be ~1 inch (2.5 cm), or just slightly longer than the plug length if using dowels.
Space the holes in a staggered pattern along the log. A common spacing is about 4–6 inches apart along the length, and 2–3 inches between rows in a diamond grid wrapping around the log. This typically yields several dozen holes on a 3-4 foot log (e.g. a 4” diameter x 40” log might get ~40 holes).
The exact pattern isn’t critical as long as you achieve a fairly even distribution. The goal is to “plant” spawn throughout so the mycelium can colonize inward and meet up.
Insert spawn into holes
If using plug spawn, simply hammer the colonized dowel into each drilled hole until it’s flush with or slightly below the bark surface. A rubber mallet works well to avoid smashing the plugs. Make sure the plug fits snugly; if a hole is too loose (plug slides in with gaps), you might double up plugs or use sawdust spawn to fill the void.
If using sawdust spawn, you will need an inoculation tool. A spring-loaded plunger inoculator is often used: you press its tip into the spawn bag to load it (it grabs a measured plug of loose spawn), then insert the tip into a drilled hole and depress the plunger to inject the spawn. Alternatively, you can manually pack sawdust spawn into holes using a small dowel as a ramrod. Whichever method, fill each hole level with the wood surface (don’t overfill mounded out, as spawn sticking out won’t be covered/protected).
Seal the inoculation sites
Immediately after filling holes, cover each hole with melted wax. Use a food-grade wax such as cheese wax or beeswax (cheese wax is commonly used because it stays a bit pliable and doesn’t crack easily).
Melt the wax in a pot (an old slow cooker or double boiler works well, typically keeping wax around 160–180 °F). Then daub or brush the hot wax over each filled hole. The wax seals in moisture and spawn, and keeps contaminants or pests out. If there were any cut branch stubs on the log or if you choose to seal the log ends, wax those areas too.
Sealing log ends is optional; it can help retain moisture but some growers skip waxing ends to allow a bit more breathing – use your judgment based on how dry your climate is. In dry regions, wax the ends; in humid regions with high rainfall, ends can be left open to allow some natural inoculation by shiitake spores.
Label the log (optional but recommended)
Mark each log with the strain of shiitake and date of inoculation (using aluminum tags or weatherproof labels stapled to the end, or just write on the waxed end with a paint marker). This helps later to track how long logs took to fruit and which strains perform best.
Your log is now inoculated, essentially “planted” with shiitake mycelium in each of those inoculation points. The spawn will begin to grow outward through the wood over the coming months.
Incubation: Spawn Run on Logs (Outdoor)
After inoculation, logs enter a long incubation period called the spawn run. This typically lasts 6 to 12 months before the logs are ready to fruit, though exact timing varies with log size, strain, and conditions.
Laying Yard Setup
Arrange your inoculated logs in a shaded, sheltered outdoor area. Shade is crucial: direct sun can overheat or dry out the logs, and UV light can kill surface mycelium. Aim for ~60-80% shade. Under a forest canopy is perfect, or you can use shade cloth (around 70% shade) over a simple frame.
Protection from wind is also helpful to prevent drying. Common ways to stack logs for incubation include crib stacks (logs crossed in tiers), lean-to stacks (one end of logs on ground, other on a raised support, forming a lean), or “raft” stacks (logs laid parallel slightly elevated off ground). The exact stacking method can vary; the key points are to keep logs off the soil, allow some airflow between them, and maintain shade.
Moisture Management
During incubation, logs should maintain internal moisture above about 30%. Shiitake spawn cannot effectively colonize wood that drops below ~23% moisture. Ideally, logs stay 35%+ moisture for vigorous growth. In practical terms, this means if natural rainfall is infrequent, you need to water the logs.
Check the logs periodically: the bark should feel firm and slightly damp, not brittle. If you cut a “cookie” off the end of a sacrificial log and weigh/dry it, you can gauge moisture scientifically, but most growers go by feel/experience. If you get an extended dry spell, especially in summer heat, soak or sprinkle the logs to keep them from drying out.
A light watering with a sprinkler for a few hours can rehydrate surface moisture. Some growers submerge logs for 1–2 hours mid-summer, if needed, but avoid soaking too long as that can trigger premature fruiting. Aim to just keep logs moist, not to induce a flush at this stage.
Temperature
Shiitake mycelium grows well in a broad range, roughly 50–80 °F (10–27 °C) inside the logs. Optimal might be around 70 °F for fastest colonization, but outdoor logs will experience ambient swings.
In cold winter climates, mycelium will go nearly dormant below about 40 °F. That’s fine, it will resume in spring. Extremely high sustained internal log temperatures above 90 °F could be harmful, but shading usually prevents this. Snow cover in winter actually can protect logs from desiccation, just be sure logs are inoculated well before the first freeze so mycelium is established.
Wide-range shiitake strains can colonize across seasonal temperature changes, whereas specialized cold-weather strains may colonize more slowly in heat and warm-weather strains slow down in cold. In general, just keep logs in a naturally cool, shaded spot.
Colonization Progress
Shiitake mycelium will gradually spread from each inoculation point. You might notice, after a few months, white mycelial patches appearing at the ends of logs or even around inoculation sites under the wax. This is a good sign that the spawn run is advancing. Some mycelium may be visible under loose bark or at cracks.
By 6–12 months, typically the log is fully colonized. One indicator is the appearance of a white film on the log ends, and sometimes a brownish staining under the bark, which indicates the fungus has permeated the wood. At this point, the log is ready for fruiting induction.
Thinning and Pest Notes
During the long spawn run, try to minimize disturbance to the logs. They’re doing their quiet work. Keep an eye out for pests: squirrels or deer might occasionally kick around logs, and insects like bark gnawers or moisture-loving pests might show up. If you maintain good moisture and shade, serious issues are rare during incubation.
One competitor to watch for is Trichoderma (green mold) on the log ends. If you see patches of green mold on the cut ends, it means that area dried out and got infected. You can scrub off any green mold with a brush and re-wax that spot to contain it. As long as shiitake is well-established, it usually out-competes such invaders inside the log.
After incubation, typically by the next growing season if inoculated in late winter/early spring, your logs will be primed to produce shiitake mushrooms.
Inoculation: Sterilized Substrate Bags (Indoor)
For indoor shiitake cultivation on sawdust blocks, inoculation must be done under sterile conditions to prevent contamination of the sterilized substrate. This typically means working in a clean-air environment, either under a laminar flow hood (which provides HEPA-filtered air) or in a glove box / still-air box that has been sanitized. Personal hygiene (clean hands, gloves, face mask) and sterilized tools are important.
Remember, once you sterilize the substrate, it’s a blank slate that can be easily colonized by any airborne mold or bacteria, so you need to carefully introduce only the shiitake spawn.
Spawn for Indoor Cultivation
Spawn for inoculating sawdust bags is usually grain spawn or sawdust spawn. Grain spawn is common for its high nutritional content and ease of mixing. You can purchase spawn from commercial labs; make sure to choose a strain appropriate for indoor cultivation.
Popular commercial shiitake strains for sawdust include ones like 3782 and CS2 are often bred for bag culture. If it arrives refrigerated, let it equilibrate to room temp before use. Don’t let it over-incubate and brown in the bag, you want actively growing mycelium.
Step-by-Step Inoculation for Sawdust Bags
Prepare your inoculation space
Wipe down surfaces with 70% isopropyl alcohol or bleach solution. Sterilize tools (scalpel, spawn containers) and ensure still air if using a glove box, or run your laminar flow hood for 15+ minutes before to purge particulates. You will need an impulse sealer on hand or strong zip ties to seal bags after inoculation.
Open bags aseptically
In front of the flow hood, or inside your SAB, cut open the top of the substrate bag using a flame-sterilized or alcohol-wiped knife. If the bag has a plug or collar system, remove that to open access.
Add spawn
Determine your spawning rate. Typical inoculation rate is 5–10% of the substrate weight for shiitake on sawdust. Some sources suggest even as low as 2% can work, but higher spawn ratio generally gives faster and more reliable colonization. For example, for a 5 lb block, 0.25–0.5 lb of spawn (roughly a cup or two of grain spawn) might be used.
Pour the measured grain spawn into the open bag.
Minimize exposure and don’t allow the bag to stay open longer than necessary.
Mix spawn into substrate
With clean gloved hands or a sterile tool, mix the spawn throughout the sawdust. Break up any spawn clumps and try to disperse the grains evenly. Some growers simply shake and knead the bag from the outside to distribute spawn. The goal is an even inoculation so the mycelium has many points from which to start colonizing.
Even distribution = shorter colonization time.
Expel air and seal bag
Squeeze out excess air and seal the bag immediately using the impulse heat sealer, or tie it off tightly. The filter patch on the bag will allow gas exchange, so the bag should not be ballooned. A bit of slack is fine, but mostly you want the bag compact around the contents to discourage air pockets where contaminants could land.
Label bags
Mark the strain and date on the bag using masking tape or a marker to write on the bag. This helps monitor incubation duration and identify strains later.
Now the bags are inoculated. The shiitake mycelium will start growing outward from each grain of spawn through the sterilized sawdust.
Incubation: Spawn Run for Substrate Blocks (Indoor)
Shiitake incubates in two phases: first the rapid colonization where the white mycelium fills the bag, then a slower maturing phase where the mycelium browns and prepares to fruit. Key factors during incubation are temperature, humidity, light, and time:
Temperature
Maintain incubation temperature around 70–75 °F (21–24 °C) for optimal mycelial growth. This is warm enough for fast growth but not so hot as to encourage contaminants or stress the mycelium.
Shiitake can colonize a bit cooler too, down to about 65°F, more slowly. Try to avoid temps above 80°F for extended periods; some shiitake strains stall or bacteria may thrive. Many growers simply incubate bags at room temperature (~72°F).
If incubating a large number of bags, know that active mycelium produces heat; make sure the area is well-ventilated or use spacing so bags don’t overheat each other.
Light vs Dark
During spawn run, darkness or low light is preferable, but absolute darkness isn’t strictly required. Incubation is often done in the dark or ambient low light. Keeping the bags in a dark room or simply black plastic bins is fine. They can be on shelves close together at this stage since we are not fruiting yet.
Humidity and Gas Exchange
Since bags are sealed with a filter patch, they retain moisture well. You do not need to humidify the incubation room; normal room humidity (30–60%) is fine. The filter allows the mycelium to breathe, it releases CO₂ and intakes some O₂.
Do not open the bags during incubation. The bag keeps contamination out and CO₂ in. The mycelium benefits from the elevated CO₂ inside, which keeps it in a vegetative, non-fruiting mode.
Incubation time frame
Within a week or two, you’ll see white mycelium spreading from the spawn kernels into the sawdust. By about 4 weeks, many fast strains will have the bag mostly or fully colonized, all the substrate will turn solid white. Some strains or cooler temps may take 6–8 weeks to reach full colonization.
After the block is entirely white, do not rush to fruiting. Shiitake benefits greatly from an additional maturation phase often called “browning.” If you fruit immediately when it’s just turned white, yields will be lower. Instead, leave the fully colonized blocks to incubate further for 1 to 2 more months.
During this time, the mycelium’s surface will begin to turn brown and form a thick skin. This is a natural, desired process unique to shiitake: the mycelium, once it exhausts readily available nutrients, produces melanin and other compounds, toughening the exterior (it looks like a brown crust).
This “brown block” stage is basically the mycelium tightening its hold on the substrate and building up energy for a big fruiting. Growers often notice the formation of small bumps or knots on the brown surface; these are initials of future mushroom pins, sometimes called popcorning because of their appearance.
Browning usually happens best if you allow some indirect room light and keep the blocks in continued warm temperature for 2-4 weeks after full colonization. You’ll see the pure white gradually shift to a golden or brown color on the surface. It’s recommended to wait until at least half of the block’s surface has browned before initiating fruiting.
Contamination watch
If you followed sterile procedure, contamination should be rare. But inspect bags periodically: if any develop conspicuous green, black, or bacterial slime patches, remove them from the incubation area immediately to avoid spread.
Contaminated blocks should be discarded (or buried outside), they won’t fruit well if at all. The most susceptible time is the first 2-3 weeks. Once blocks are fully colonized, contamination risk drops since the shiitake mycelium is dominant. Browning blocks sometimes get a bit of naturally occurring benign molds on the very surface. If minor, you can wipe them off when you open the bag to fruit. But pervasive green mold inside means the block is lost.
Good sterile technique at inoculation and quality spawn are key to avoid this.