Shiitake Mushrooms: A Crop to Consider - You can farm this fancy fungus

Published on Fri, 04/28/2017 - 2:12pm

There is increased interest in “farming” mushrooms recently. For the beginner, the shiitake mushroom is one of choice. The shiitake is native to Japan and has been used in Asia for over 6000 years. “Shiitake” in Japanese means “mushroom of the shii (oak tree)” and is known for its flavor and reputed medicinal value.

Some of the nutritional and medicinal properties include antioxidants, vitamins A, B, B12, C, and protein. They are thought to help lower blood cholesterol, boost immune function, and prevent tumor growth.

Shiitake production is still fairly new in this country (since mid-1980’s), and new strains (varieties) and methods of production are being developed.

It starts with the log

Selecting the best available tree species is the first step to successfully growing shiitake. Shiitake mushrooms have been reported to grow on many trees, but we have found in Ohio is that oaks, hickory, and sugar maple are best, but other hardwood species may also work.

Logs should be cut from healthy, living trees free of any decay. Trees should be harvested during the dormant or winter season when the wood contains the maximum amount of stored carbohydrates. During log cutting it is important not to damage the bark layer. 

TIP: Trees and materials to avoid include all conifers, as they can create off-flavors, and black locust as they are very rot-resistant. Of course, avoid any trees that are unhealthy.

Log diameter is critical. It is recommended not to use logs smaller than three inches in diameter because they can dry out very quickly. Although smaller-dimension logs will produce mushrooms more quickly, they will tend to decompose more rapidly. Logs greater than six inches in diameter can produce mushrooms over a longer period of time, but require more inoculations to compensate for the greater diameter. They also may take longer to produce the first crop and have increased chances for becoming contaminated. 

Log length is not critical, so decide on a manageable length.

Traditional log curing is one to two months. Some growers inoculate as soon as possible, but trees’ natural chemical defense against invasions may still be active and can prevent successful inoculation. Generally, inoculation should occur within two weeks of felling a tree.

Getting shiitake spawn

The inoculum is called spawn and comes in two different forms. The dowel or plug spawn has material is pre-injected into small wooden dowels that are then hammered into drilled holes in the log. The sawdust spawn is usually produced in blocks that are then crumbled into small pieces and packed by hand or with a special tool into holes drilled in the log.

For first-timers attempting to grow shiitake mushrooms, wide-range strains are recommended. Their reliable, fast spawn run and fruiting allow a relatively faster return on investment. Logs inoculated in the spring and placed outdoors naturally produce a commercial crop that same autumn in southern climates, or midsummer the following year in northern climates. Wide-range strains are known for their ability to be force-fruited and recover vigor quickly after soaking.

Tip: It’s best to purchase spawn each time you want to inoculate logs.

Log preparation and inoculating

Logs should be inoculated when daytime temperatures are consistently above 50° F. Logs need to have a moisture content of at least 40 percent. You can often tell if wood is getting too dry to inoculate. Logs that have cracks (up to 1 mm) at the ends are starting to get “too dry” and need watering. Sprinkle periodically through the day and tarp the logs after sprinkling.

Log ends that have deep cracks (more than 1-2 mm) at the ends will benefit from a 24- to 36-hour soak in water two days prior to inoculation; just make sure the bark surfaces are not wet when drilling. Log ends with deep furrows and are dark gold in color indicate the log may not be usable for inoculation—try removing six inches off each end, but even then the logs may be too dry.

Inoculation is placing the spawn into the logs so that the shiitake fungus can grow through the wood. Holes are drilled into the log, filled with spawn, and then covered with wax or other material to seal in moisture and protect against contamination. Holes for plug spawn should be 5/16 inch in diameter and 3/4 to 1 inch deep (Figure 1). Plugs are inserted into the logs and hammered flush or just below the surface of the log. 

Sawdust spawn holes are generally wider and deeper, being 3/8 inch in diameter and 1-1/4 inch deep. Sawdust spawn is packed by hand or by special injector into the drill holes. Sawdust spawn typically yields better colonization compared to the plug spawn, and may reduce inoculations per log, but the sawdust spawn is more difficult to handle. You must be careful not to let the spawn dry out.

Holes should be staggered evenly around the log. Rows running the length of the log are spaced 1-1/2 to 2-1/2 inches apart. The holes within a row should be spaced six-to-ten inches apart and alternating with the holes in the adjacent row. Heavier inoculation will accelerate the growth of the fungus within the log but also represents additional investment.

Log incubation

Mushrooms will be produced after the shiitake fungus colonizes the log. The first “fruiting” will normally occur from six- to eighteen-months after inoculation and will depend on the strain, the inoculation rate, the incubation conditions, and tree species. Monitoring and maintaining environmental conditions during the incubation period is a critical point in the production process.

During the first two months, logs should be stacked closely to help maintain correct moisture content. Shiitake grows best when the moisture content of the wood is at least 35 to 45 percent. Growth declines when the moisture content falls below 35 percent or rises above 60 percent. When the moisture content becomes low, soak or continuously water the logs for 48 hours. Good air circulation is needed following watering to keep the surface of the logs dry to prevent contamination. The optimum situation is when the bark remains dry but the inside remains moist.

Shiitake spawn will grow between 40° and 90° F. but the optimum is 72° to 78° F.  Stacking logs under a canopy of trees or shade cloth which provides 60 to 70 percent shade helps to maintain moisture content while preventing the logs from becoming too warm. If the logs dry out or overheat, the shiitake fungus can be killed. Common stacking methods include the X pattern and the crisscross pattern (Figure 2). On hill slopes, the lean-to pattern can also be used effectively. Logs should be checked periodically and turned or re-stacked to keep the moisture content evenly distributed.

Tip: Log moisture content can be monitored by including several logs of known dry weight and periodically weighing them to determine their moisture content.

Mushroom fruiting

Natural fruiting of shiitake occurs under prolonged cool, moist conditions. It will usually occur within two weeks of a natural rainfall, but fruiting can be induced by soaking the logs in cool water for up to three days. Soaking time will vary depending on the difference between water and air temperatures. Soaking temperatures will also vary by strain. Growers should check with suppliers for details.

Traditionally, the logs will produce mushrooms in both spring and fall, although the fruiting period may be extended in the winter by placing the logs indoors. The fruiting area should have slightly more light and air movement than the spawn-run area, but still should be protected from winds and direct sun. Once logs begin to fruit, they will normally produce mushrooms for up to six years.

Harvest and storage

Mushrooms should be harvested on a daily basis, usually in the afternoon when the mushrooms are dry. Mushrooms are removed from the log by twisting or cutting at the base when they have opened about 60 to 75 percent. They should be put immediately into cardboard boxes and refrigerated. Refrigeration can extend the shelf life of shiitake from four to five days to up to two weeks. Mushrooms should be shipped to market within five days of harvest but preferably sooner.

Mushrooms of lower quality or freshness can be dried, packaged, and sold in retail and restaurant markets. Shiitake dry easily and reconstitute very well, so marketing by mail is also possible. Drying can be accomplished by placing the mushrooms over dry, warm air, preferably in sunlight which increases their Vitamin D content. Under artificial drying, gentle heat of 90° F. is gradually increased to 140° F. over a 10- to 14-hour period. Seven pounds of fresh shiitake yields about one pound of dried mushrooms.