INOCULATION

The process of inoculation includes drilling fresh holes into the sapwood of the log and inserting shiitake spawn into the hole, followed by sealing the hole with wax. The process of inoculating is conceptually similar to seeding a crop, where the ground (the log) has been freshly tilled to be clear of weeds (freshly cut and not yet invaded by other fungi), and the seedbed prepared (holes drilled) and seeded (spawn inserted and wax-sealed) with enough seed to be competitive with weeds (the inoculation rate).

Your inoculation “station” should ideally be sheltered from rain, have access to electricity, and in a location that allows for uncomplicated access to your bolts. Inoculation station setups do not need to be complicated, but think ahead about workflow, to avoiding things that could be hazardous or inefficient (see Figure 1). Plan for a way to keep logs sheltered from wind and direct sun until the logs can be placed into the forest for spawn run. It’s good to leave logs in the rain before and after you’ve inoculated, but you you’ll need to allow the bark of your logs to be dry for at least a short period while you’re inoculating (explained below).

Drilling

The drilling station is best set apart and facing away from the inoculation and waxing areas, because wood debris is flung outward during drilling. Be sure to protect your eyes from flying wood debris during drilling. You will need something like a sawbuck to hold your log in place during drilling (Figure 2). Although conventional hand drills will accomplish the task of drilling holes in the log for inoculation, angle grinders fitted with modified 12-13.5mm bits have become a standard go-to for commercial forest-cultivated shiitake producers. These angle grinder setups offer high-speed (≥10,000rpm) that drill a hole near instantaneously, and the bits have integrated depth-stoppers that eliminate the need to regulate your drilling depth for each hole.

Holes are drilled in rows along the length of the log, with every row offset by 50% from the other in a diamond pattern around the log. Recommendations for inoculation rates and patterns vary according to the source. US-based sources recommend 2-3” spacing between rows, and 4” between holes in the row. Japanese literature claims that shiitake grows at a lateral:vertical rate of 6:1, so that is why the lateral rows of holes down the length of the log are spaced closer together than the spacing between holes within a row. In Japan, shiitake being produced outdoors for dry shiitake markets use a lower inoculation rates with ~1.5” between row spacing and ~9-10” within rows, whereas shiitake being produced on logs for fresh market will use a higher inoculation rate. There is a cost/benefit dynamic with inoculation rate that has been yet to be in the PNW. The inoculation process is likely the most labor-intensive part of investing in a forest-cultivated shiitake operation, but higher rates of inoculation can 1) help guarantee strong, thorough colonization of the log, and 2) mushrooms produce most readily from the inoculation hole. On the other hand, higher inoculation rates are more labor-time intensive, and a higher rate also means more holes are cut into the bark of the log, which is counter to preserving the integrity of the bark. All trials to date in the PNW have used a 2” between-row spacing and 4” within-row spacing, which is a similar rate/spacing to what is used in Japan for fresh market shiitake.

A drilling guide with marks of alternating colors drawn onto it every 2” can be fastened to your sawbuck to let you know where your holes should be in the row (see Figure 2), starting with one color, and then the next row’s holes will be drilled at the marks of the other color, which are offset 2” from the first row’s holes, and so on. Patterns can also be made to pin onto the end of your logs to help guide you in where your rows should be, according to the girth of the log (see Figure 3). Each log’s average girth can be determined by measuring its circumference at the center of the log with a sewing tape. Larger-girth logs will be able to accomodate more rows and vice versa; a link to an inoculation rate chart for determining the number of rows according to circumference along with row pattern guides is here: INOCULATION RATE CHART AND ROW PATTERN GUIDE.

Plugging spawn

Spawn typically can be purchased in two forms- small wooden dowels, or sawdust that has been colonized with shiitake fungus. In Japan, wooden dowels are largely used for outdoor production for dry shiitake markets, whereas sawdust spawn is generally used for shiitake being produced for fresh market. The latter is used because the sawdust spawn method 1) promotes a faster spawn run, and 2) the holes drilled for sawdust spawn facilitate fruiting from the holes and a well-formed (aesthetically pleasing) shiitake mushroom. Material cost-wise, sawdust spawn is more cost-effective than plug spawn (hardwood dowels) when inoculating larger numbers of logs, although waxing is not used with plug spawn. To date, all of the shiitake production trials in the PNW for commercial production have used sawdust spawn; this method is likely more regionally-relevant because most PNW forest-cultivated shiitake growers are likely to be producing for fresh market, because a fast spawn run is favorable, and because the waxing part of this method likely helps retain critical moisture during dry PNW summers. For 4’-long bolts with an inoculation rate of 2” between rows and 4” between holes in the row, expect to use ~0.37 to 0.46 lbs of sawbust spawn per bolt.

One of the specialized tools you will need to inoculate bolts with sawdust spawn at a production scale is an inoculation tool. The simplest ones are relatively inexpensive and consist of a metal tube that has the same inner diameter as the hole you drilled (so make sure your drill bit diameter matches the diameter of the inoculation tool!) which has a handle around the upper portion, and a spring loaded plunger that is activated by pushing on a button on the end of the tool, either with your thumb or hitting with your palm (see Figure 3). When the plunger is retracted the end of the metal tube can be stuffed with sawdust spawn, and then can be conversely pushed out of the tube and into the hole in the log by pushing the plunger down. There are also inoculation tools that can simultaneously insert styrofoam plugs into the hole as an alternative to the waxing step, but they are more expensive. All trials in the PNW to date have used the more simple inoculation tool followed by waxing.

It is common to use the bag that the spawn came in as container for holding spawn during inoculation. An alternative is to fabricate a long, narrow spawn tray that is the length of the logs; this is intended to reduce the distance between the spawn bag and the log during inoculation of many logs to reduce arm fatigue in the long run. It also helps if two people are plugging the same bolt side by side. The example shown in see Figure 4 for this type of container is made from a plastic gutter and two gutter endcaps. The gutter container should be secured to the inoculation table in some way to avoid spilling (or catapulting) spawn during inoculation. Spawn is crumbled from its compressed form in the bag into the tray; breaking up the spawn also helps with packing the inoculation tool uniformly, and getting the right amount into the hole in the log.

You will want a sturdy table ( you can often build one easily from wooden pallets) with something fashioned to keep the log in place for the plugging portion of inoculation, as the logs are heavy and are frequently jostled throughout the process. Some growers use a set of four caster wheels to hold the log in place and to also help with spinning the log around as needed to access and fill all the holes. When plugging, you should aim for packing the hole with enough spawn to fill up to where the bark meets the sapwood. This maximizes effective contact between the spawn and sapwood, and leaves a small reservoir for the wax to sit in.

When you’re not actively plugging spawn, your spawn should be stored in a container that is breathable but also retains moisture, and kept in a cool, humid place (ideally in a refrigerator) until you are ready for inoculation. The earlier you use your spawn the better. Suppliers often discourage use of spawn that has been in the refrigerator for over six months.

Waxing:

The standard wax used for shiitake bolts is food-grade cheese wax, as mushrooms often come in contact with the wax when they fruit through the inoculation holes. One lb. of wax will cover approxamately five 4’ bolts, on average. It is very helpful to use a variable temperature electric pot/pan of some sort to have some precise control of the temperature of the melted wax used to seal the inoculation hole. It may take some experimentation with temperature to figure out the optimal temperature for waxing according to the temperature of the air, the log, the tool being used, and how fresh the wax is. Wax will gradually volatilize and become thicker the longer it’s heated. Food grade cheese wax appears to begin to hit the smoke point around 250°F, whereas 200°F appears to be a favorable temperature to default to. Also note that:

• It is always a good idea to secure your wax melting device firmly to the table, as it can easily be spilled and may cause burns to whomever may be working nearest to it.

• The wax is flammable, so open flame burners used to heat the wax add an extra risk of fire danger to the process.

• Colder weather, cold logs , and wax that has not heated to a high enough temperature will cause the wax to solidify much more quickly, both on the log, and on the tool being used to apply the wax.

• Overly cool wax also does not stick to logs as well, AND

• *Wax will not stick on wet logs*- this is a very easy mistake to make. Plan to have your logs out of the rain for several days before inoculating.

Wax can be applied in a variety of ways, and some are more wax-efficient than others. There are specialized wax applicators that can be purchased which claim increased efficiency, but spoons, simple 1” foam or fiber-bristled paintbrushes (poly-bristled bushes will melt) or daubers (aka “wool daubers”) are likely the cheapest options that work reasonably well. They can be a bit less efficient with amounts of wax used. A stainless steel measuring cup can be used as a working melted wax reservoir along with with applicators like brushes and spoons, so that you don’t have to keep going back to the pot for each application of wax. Another option is to use stainless steel basters or modified marinade injectors. With basters, some people insert a 16-penny nail with the pointed half cut off into the tip of the baster; the nail acts like a “valve” when the nail is pushed against the spawn in the hole, but otherwise closes when the baster is lifted and the nail drops back down to where the head of the nail restricts the wax from flowing out. Marinade injectors can also be very fast and wax-efficient once you get accustomed to using them. Whichever your chosen tool is, wax is applied to the row of holes that is facing straight up/perpendicular to the table; the way that wax flows, any holes that you try to fill at an angle are very prone to developing bubbles that will leave some of your spawn exposed. After you finish a bolt, it is a good idea to quickly scan the log to make sure no holes were missed, especially if you are waxing alongside another person working on the same bolt.