Water Management of Shiitake Bed Logs for Incubation and Mushroom Production

With spring shiitake log inoculation behind us, we now turn our attention on the farm to outdoor mushroom production and bed log management of those freshly inoculated logs.

Shiitake mushrooms growing on logs

Log Water Content Before Inoculation

The first and most obvious thing to check for is moisture content. If you have handled a lot of logs you can pretty much tell how green (and not overly dry) it is. If the log is really light in weight compared to others of similar size when you heft it, you need question the overall health of the wood. A light log may indicate the wood was already dead when you cut it. A light-weight log can also indicate over-drying and the condition can be corrected. Check the ends of the logs (Figure 1). If they are cracked and you can slip a dime into the crack, soak the logs for 24-48 hours before inoculation to rehydrate them.

Mushroom Log Water Management After Inoculation

Here at Field & Forest Products, we inoculate logs with shiitake spawn twice a year. Our spring inoculation is generally 250 to 300 logs while our fall inoculation is around 1000 logs. Both require different water management schemes after inoculation because of their very different post inoculation resting spots. Our spring inoculated logs are moved directly into the woods, or laying yard, after inoculation. Our fall inoculated logs are moved directly into a warm and humidified incubation room indoors for the winter. These fall inoculated logs are dead stacked, tarped, and managed for the next few months (Figure 2). Moisture management of shiitake bed logs indoors is more complex and will be discussed in a later blog as we continue to develop a protocol based on work done in Japan. However, the spring inoculated logs that are placed directly outdoors require a little more early attention.

With outdoor incubation, we depend on regular rainfall and the proper log stacking procedure to maintain log moisture content early on. Most problems in the fruiting cycle of mushroom cultivation are linked back to the initial spawn run phase, so it pays to give the inoculation and spawn run year lots of attention. This year we have been extremely fortunate that we have had adequate rainfall so far (knock on wood). But let's assume the natural water faucet gets turned off for an extended period of time and our green lawn turns to brown and that leafy debris in the woodlot starts to turn crunchy underfoot. This usually precedes a call to action.

How Often Do I Water My Logs

We were once told the three most important things to remember in shiitake cultivation were 1) moisture 2) moisture and 3) moisture. You get the picture; fungi need moisture in the wood in order to digest it, and if the rain isn't falling, we will need to take corrective action. This usually means irrigation. We like to see a rainfall event on our logs at least once a week that amounts to at least an inch, and if we don't see rain in a week or two, either we leave the car windows open overnight or hang the laundry out to dry. If that doesn't do it, the sprinklers are turned on. We have a rather simple system consisting of an impact sprinkler and a long length of hose (Figure 3). When called for we will turn the sprinkler on late in the day and let it run for several hours or long enough to fill a coffee can with about an inch of water (or overnight if we forget about it). We will not water again until a lack of precipitation dictates us to.

A general rule to remember is this: It is best to irrigate heavily and infrequently than lightly and frequently. The reasoning is this: With heavy, long irrigation cycles, water is able to move into the bed logs through the butt ends and somewhat through the bark (That's why we don't recommend waxing log ends!). When the water is turned off the log surfaces can dry and contamination by surface molds is minimized. With light, frequent irrigation, water never really gets a chance to penetrate into the log and log surfaces never really dry out so contamination by undesirables can become a problem.

The most important time for water management in bed logs is during their first growing season as the spawn establishes itself in the log. Once spawn run is complete, by the end of the growing season there should be no further need to irrigate. Mushroom production will benefit from a slight dry-down of the bed log as rainfall coupled with a cool-down in temperatures mimics the approach of a typhoon in shiitakes natural range, thus stimulating mushroom production.

Managing for Lack of Water

What if you don't have irrigation capabilities? First and foremost is your choice of stacking methods that will promote spawn run and slow down log drying rates (Figure 4). Imagine a line drawn from northeastern Florida to northwestern Minnesota and think about the different growing zones you would pass through. In the summer I imagine Florida to be warm and humid during the day and night with people sleeping in air conditioned comfort. In Minnesota, sometimes the summer nights require the winter quilt and a fire in the kitchen stove. Air conditioning? What's that? Along that line we can also measure log drying rates. In the warm and humid Southeast, drying rates are slow, while in cool and dry Minnesota, drying rates will be considerably faster. The Southeast has a more forgiving climate for shiitake log-based cultivation due to its warmth and high humidity levels during the growing season.

In the northwest, the wrong log stacking configuration could lead to the production of some pretty darn expensive firewood. So one can also imagine now, along that line as one would leave the humid southeast and travel to the northwest, ever decreasing log stack heights. Of course this is a very simplified explanation as local environment will also influence log stacking configurations but the general idea remains the same. The dryer the site, the lower the log elevation during incubation. By the way, consider placing your smallest logs in the stack on the bottom and largest on top. Your small logs should dry at an equal rate as the large logs this way. Right now with summer temperatures rarely reaching 80F so far in northern Wisconsin, we have our logs lying horizontal in a single layer and the winter quilts have been used quite a bit this summer.

Should You Soak Your Logs?

Lets say you are having a fairly rain free summer. What if a low stack isn't good enough to keep log moisture levels up without the usual weekly inch of rainfall? Well, there is always soaking logs in fresh water for several hours for them to rehydrate (Figure 5). We are not big fans of this as it adds more labor to an already very labor intensive crop, but if the log population is small and your back is strong here are a few suggestions:

Small diameter logs will dry out faster than large diameter logs, so soak those first. If the logs float, (We hope not!) weigh them down with concrete blocks, tractor wheel weights or some other hefty item. Several hours should be adequate to get water into the log. Do not be tempted to soak the logs longer than 24 hours as this could lead to anaerobic conditions within the log.

Do not attempt to soak logs for rehydration purposes after 4 months of spawn run have occurred. Sometimes a soak can bring on a premature effort to fruit which can weaken the running mycelium and thus overall log health. Sprinkling is a far better alternative at later stages of spawn growth.

Shiitake Strain that holds up best to dry conditions:
West Wind

West Wind is our most commonly used variety in the Western half of the country where the summers can be unforgivingly dry.

A stack of logs perfect for inoculating

stack of logs covered with pine boughs

Shiitake logs incubating indoors

mushroom logs stacked and covered with mycelium

Our simple sprinkler system

forest scene showing a row of logs with a sprinkler shooting out water

Shiitake logs placed low to the ground for moisture management

a row of shiitake mushrooms logs in a forest setting

Logs going into the soaking tank

a pallet of logs being submerged in a tank full of water

A stack of shiitake logs opened up for airflow

a pallet of logs being submerged in a tank full of water

Mycelium on the end of a log can be visible with ideal water management

an end of a mushroom log showing mycelium growth

Can You Overwater Logs?

Sometimes we forget that there is such a thing as too much moisture. For a crop that is native to regions where annual rainfall totals can be upwards to 80 inches, it is hard for many of us shiitake cultivators in the USA to imagine the possibility of overwatering. However, logs that are placed directly or nearly so on the ground can suffer from green mold competitors, even here in the Midwest. If you see fresh post-inoculation green molds in quantity on the bark during the first year of incubation, elevate and separate the logs to encourage more air flow (Figure 6). This should take care of the occasional mold problems found in shiitake logs during wet spells.
Many people, especially those who regularly get lots of summer rainfall (more than 30" annual precipitation), have the luxury of getting plenty of mushrooms without giving a thought to moisture management in the summer. By understanding the basic relationship between stacking method, rainfall and spawn run, growers everywhere can be prepared to enjoy years of plentiful harvest regardless of what Mother Nature hands us.