Epimedium, also known as barrenwort, bishop's hat, fairy wings, horny goat weed, or yin yang huo (Chinese: 淫羊藿), is a genus of flowering plants in the family Berberidaceae. The majority of the species are endemic to China, with smaller numbers elsewhere in Asia, and a few in the Mediterranean region.
Species of Epimedium are herbaceous perennials, growing from an underground rhizome. Their growth habits are somewhat variable. Some have solitary stems, others have a "tufted" habit, with multiple stems growing close together. There may be several leaves to a stem or the leaves may be solitary, produced from the base of the plant. Individual leaves are generally compound, often with three leaflets, but also with more. Leaflets usually have spiny margins. The leaves may be annual, making the plant deciduous, or longer lasting, so that the plant is evergreen. The inflorescence is an open raceme or panicle, the number of flowers varying by species.
Individual flowers have parts in fours. There are four smaller outer sepals, usually greenish and shed when the flower opens. Moving inwards, these are followed by four larger petal-like inner sepals, often brightly coloured. Inside the sepals are four true petals. These may be small and flat, but often have a complex shape including a nectar-producing "spur" that may be longer than the sepals. There are four stamens.
One of the common names for the genus, bishop's hat, arises from the shape of the flowers, particularly where the spurs are longer than the sepals.
The genus was given its name by Carl Linnaeus in 1753, in describing the European species E. alpinum. The name is a latinized version of a Greek name for an unidentifiable plant, epimedion, rhat is mentioned in Pliny's Natural History (xxvii.57). The meaning of the original name is unclear.