Grifola frondosa is a polypore mushroom that grows in clusters at the base of trees, particularly oaks. The mushroom is commonly known among English speakers as hen-of-the-woods, ram’s head and sheep’s head. It is typically found in late summer to early autumn. In the United States’ supplement market, as well as in Asian grocery stores, the mushroom is known by its Japanese name maitake (“dancing mushroom”). Throughout Italian American communities in the northeastern United States, it is commonly known as the signorina mushroom. G. frondosa should not be confused with Laetiporus sulphureus, another edible bracket fungus that is commonly called chicken of the woods or “sulphur shelf”. Like all polypores, the fungus becomes inedible when older, because it is then too tough to eat.
The fungus is native to the northeastern part of Japan and North America, and is prized in traditional Chinese and Japanese herbology as a medicinal mushroom, an aid to balance out altered body systems to a normal level. It is widely eaten in Japan, and its popularity in western cuisine is growing, although the mushroom has been alleged to cause allergic reactions in rare cases.