Also known as Rowdy Lamb Herb, Barrenwort, Bishop's Hat, Fairy Wings, Horny Goat Weed, or Yin Yang Huo, is a genus of about 60 or more species of herbaceous flowering plants in the family Berberidaceae. The large majority are endemic to southern China, with further outposts in Europe, and central, southern and eastern Asia.

Epimedium species are hardy perennials. The majority have four-petaled "spider-like" flowers in spring.

Many species of Epimedium are alleged to have aphrodisiac qualities. According to legend, this property was discovered by a Chinese goat herder who noticed sexual activity in his flock after they ate the weed. The over-exploitation of wild populations of Epimedium for use in traditional Chinese medicine is having potentially serious consequences for the long-term survival of several species, none of which are widely cultivated for medicinal purposes.

The "active ingredient" in Epimedium is icariin. Icariin is purported to work by increasing levels of nitric oxide, which relax smooth muscles.

From the gardeners point of view Epimedium flowers comprise two main parts, the inner sepals, which are petal-like and four in number (the four outer sepals are small, insignificant, and rapidly shed as the flowers open) and the petals, which are held within the sepals. In some species these petals have developed long spurs and in such plants they greatly exceed the size of the surrounding sepals, producing a flower shaped like an inverted crown, and also giving rise to the one of the common names for Epimedium- bishops hat. In other species, however, (such as E. perraldianum) the petals are reduced to tiny spurs, and it is the greatly enlarged and highly coloured sepals that have expanded to catch the attention of the wandering insect.

from: Wikipedia