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, that is mentioned in Pliny's Natural History (xxvii.57). The meaning of the original name is unclear.
Some varieties and hybrids have been in western cultivation for the last 100 to 150 years. There is now a wide array of new Chinese species being cultivated in the west, many of which have only recently been discovered, and some of which have yet to be named. There are also many older Japanese hybrids and forms, extending the boundaries of the genus in cultivation. Few genera of plants have seen such a dramatic increase in newly discovered species, primarily thanks to the work of Mikinori Ogisu of Japan and Darrell Probst of Massachusetts. The majority of the Chinese species have not been fully tested for hardiness nor indeed for any other aspect of their culture. The initial assumption that the plants would only thrive where their native conditions could be closely replicated have proven to be overly cautious, as most varieties are proving extraordinarily amenable to general garden and container cultivation.
While they can be successfully propagated in early spring, epimediums are best divided in late summer, with the aim of promoting rapid re-growth of roots and shoots before the onset of winter. Several breeders (in particular Darrell Diano Probst, Tim Branney & Robin White) have also undertaken their own hybridization programmes with the genus. Various new nursery selections are gradually appearing in the horticulture trade, the best of which are extending the colour and shape range of the flowers available to the gardener.
Hugely popular as garden plants for centuries in Japan, epimediums are only just beginning to garner attention in the West. While they vary somewhat in their respective hardiness, all are essentially dwellers of the forest floor, and, as such, all require fundamentally similar conditions of moist, free-draining, humus-rich soil and cool shade, with some shelter for the newly emerging leaves. Some of the more robust varieties are often recommended as plants for dry shade, and whilst their tough foliage and stout rhizomes can allow them to grow successfully in such conditions, (and in more open, exposed positions too, in some instances) they will certainly not give their best. Furthermore, dryness and exposure will pretty much guarantee the early death of many of the newer and more delicate species.
Given suitable conditions most epimediums will form beautiful groundcover plants, often with magnificent new leaves tinted in bronze, copper and reds combining with a huge variety of flower colours and forms in spring. Handsome and dense-growing foliage remains present for much of the year, with the leaves often turning purple, crimson and scarlet in autumn in some forms, and remaining evergreen in others. With all varieties, however, the foliage is best cut off at ground level shortly before new leaves emerge, so as to fully reveal their beauty of form and colour. Ideally, a mulch should then be applied to protect the new growth from frosts.
Epimedium wushanense contains a number of flavanoids. 37 compounds were characterized from the underground and aerial parts of the plant. Among them, 28 compounds were prenylflavonoids. The predominant flavonoid, epimedin C, ranged from 1.4 to 5.1% in aerial parts and 1.0 to 2.8% in underground parts.