Biologists consider shiitake and other mushrooms to be fungi, a group of primitive plants. Since they have no green pigments (chlorophyll), they cannot make food from sunlight as other plants, but must live by eating plants or animals. Shiitake's favorite food is dead hardwood trees. The word "shii" is derived from the shii tree (Quercus cuspidata), an oak of central and southern Japan upon which shiitake most often grow. "Take" means mushroom in Japanese (it is repetitious to say "shiitake mushroom").
The part of shiitake that we eat, the fleshy cap, is actually a primitive reproductive structure. You may have noticed a gray or beige powder on the undersurface of opened mushrooms. These are billions of microscopic spores. Like sperm and eggs (ova) of animals, the spores are sex cells. Each spore carries half the genetic information of their parent mushroom.
Mushroom spores move about the forest with the help of wind and rain. When two compatible spores get together, they fuse their cytoplasm and genetic material and, if food is available, grow into a new mushroom. This new plant is a white filimentous subterranean growth called a mycelium. In the case of shiitake, mycelium grows inside the log, using its powerful enzymes to change wood into food. After a period of time, environmental stresses such as food depletion or temperature and humidity changes cause the mycelium to form a reproductive structure - the mushroom - and the cycle is complete.
Left to their own devices, shiitake would probably rather reproduce by the sexual cycle outlined above. However, to ensure crop quality and consistency, shiitake growers inoculate their logs with the mushroom mycelium rather than spores. The so-called vegetative (asexual) method begins by growing shiitake mycelium on wood chips, paper disks, or "enriched" sawdust. Shiitake cultivators then insert these "spawn" into holes or cuts made in hardwood logs.
In early fall, as trees shed their leaves in preparation for a dormant winter, the carbohydrate level in the tree trunk rises, making an ideal food for shiitake growth. When about 10 percent of the leaves have fallen, shiitake growers fell trees and cut them into three-foot logs. Next, about twenty to twenty-five evenly spaced holes are drilled into each log. Then wood chips (plug spawn) are hammered into the holes. When sawdust spawn are used, they are placed in holes using a special transfer tool. In both cases, the holes are sealed with hot wax.
After the logs are inoculated, they are carried into a pine forest and placed in a spot where there is an ideal balance of sunlight and shade. Usually, by the following fall, the shiitake mycelium has completely penetrated the logs, and, with seasonal temperature changes, mushrooms begin to push through the bark.
From just one inoculation, logs can be expected to produce crops of shiitake every fall and spring for three to five years, until the logs are completely decayed. The variety of shiitake called donko are superior in both flavor and medicinal qualities to the variety called koshin. In their natural effort at self-preservation, donko shiitake produce a thick cap with strong viable spores to protect against harsh environmental conditions.
The harvesting time of shiitake is very important. If the mushroom is left on the log too long, it will completely open and shed its spores, producing a mushroom that is thin, flat, dark, and lacking in vitality. According to Fusataro Taniguchi, the grower of Mitoku Macrobiotic premium sun-dried shiitake, donko shiitake (picked at the right time) should not be more than 70 percent open and should have thick, fleshy, slightly rounded caps. These cost more but are prized for their excellent flavor and healthfulness.