Epimedium
Epimedium, also known as Barrenwort, Bishop's Hat, Fairy Wings, Horny Goatweed, or Yin Yang Huo (Chinese : ), 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 are believed to be aphrodisiacs.

# Aphrodisiac
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. It is sold as a health supplement, usually in raw herb or pill form and sometimes blended with other supplements.  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, which can be found in standardized extracts from 5% up to 60% potent. Strengths above that are usually reserved for lab use.
Icariin is purported to work by increasing levels of nitric oxide, which relax smooth muscle. It has been demonstrated to relax rabbit penile tissue by nitric oxide and PDE-5 activity . Other research has demonstrated that injections of Epimedium extract, directly into the penis of the rat results in an increase in penile blood pressure.
Like Viagra, icariin, the active compound in Epimedium, inhibits the activity of PDE-5. In vitro assays have demonstrated that icariin inhibits PDE-5 with an IC50 of around 1 micromolar, while Viagra has an IC50 of about 6.6 nanomolar (.0066 micromolar) and Levitra has an IC50 of about 0.7 nanomolar (.0007 micromolar).  Measured differently, the EC50 of icariin is approximately   4.62 micromolar, while Viagra is .42 micromolar.  The amount of oral administration of Epidemium extract necessary to achieve these relative concentrations is unclear from the literature, and may not be relevant if the herb works through multiple mechanisms, as has been suggested. Epimedium has been shown to up-regulate genes associated with nitric oxide production and changes in adenosine/guanine monophosphate balance in ways that other PDE5 inhibitors do not.

# Cultivation
Hugely popular as garden plants for centuries in Japan, Epimedium are only just beginning to garner attention in the West. Whilst 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 Epimedium will form beautiful ground cover 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.
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.
Some varieties and hybrids have been in Western cultivation for the last 100/150 years, but there are now a stunning array of new Chinese species (many newly discovered and a number which have yet to be named) and Japanese hybrids and forms arriving in the west to extend the boundaries of the genus in cultivation. The majority of the Chinese species have not been fully tested for hardiness or indeed for any other aspect of their culture. The initial assumptions that the plants would only thrive where their native conditions could be closely replicated have proven to be overly cautious, and most are proving extraordinarily amenable to general garden and container cultivation.
Whilst they can be successfully propagated in early spring, Epimedium are best divided in late August, with the aim of promoting rapid re-growth of roots and shoots before the onset of winter. Several breeders (in particular Darrell Probst, Tim Branney & Robin White) have also undertaken  their own hybridization programmes with the genus Epimedium, and various new nursery selections are gradually appearing in the nursery trade, the best of which are extending the colour and shape range of the flowers available to the gardener.

# Further reading
- William T. Stearn, The Genus Epimedium, revised edition 2002, ISBN 0-88192-543-8
-  Chiu JH, Chen KK, Chien TM, Chiou WF, Chen CC, Wang JY, Lui WY, Wu CW. Epimedium brevicornum Maxim extract relaxes rabbit cEC50orpus cavernosum through multitargets on nitric oxide/cyclic guanosine monophosphate signaling pathway.Int J Impot Res. 2006 Jul-Aug;18(4):335-42. Epub 2006 Jan 5. PMID: 16395327
-  Chen KK, Chiu JH.
Effect of Epimedium brevicornum Maxim extract on elicitation of
penile erection in the rat.
Urology. 2006 Mar;67(3):631-5.
PMID: 16527595
-  Ning, H., Xin, Z., Lin, G., Banie, L., Lue, T.F., Lin, C., et al.
Effects of icariin on phosphodiesterase-5 activity in vitro and cyclic guanosine monophosphate level in cavernous smooth muscle cells. Urology,(2006) 68(6), 1350-4.
-  Xin, Z.C., Kim, E.K., Lin, C.S., Liu, W.J., Tian, L., Yuan, Y.M., et al.  Effects of icariin on cGMP-specific PDE5 and cAMP-specific PDE4 activities.  Asian journal of andrology,(2003) 5(1), 15-8.
PMID: 12646997
-  Saenz de Tejada I, Angulo J, Cuevas P, Fernandez A, Moncada I, Allona A, Lledo E, Korschen HG, Niewohner U, Haning H, Pages E, Bischoff E, et al.  [ The phosphodiesterase inhibitory selectivity and the in vitro and in vivo potency of the new PDE5 inhibitor vardenafil.  International Journal of Impotence Research,(2001) Oct;13(5):282-90.
PMID: 12646997
-   Jiang Z,  Hu B,  Wang J,  Tang Q,  Tan Y,  Xiang J,  Liu J. et al. Effect of icariin on cyclic GMP levels and on the mRNA expression of cGMP-binding cGMP-specific phosphodiesterase (PDE5) in penile cavernosum.  Journal of Huazhong University Scientific Technology Medical Science.(2006);26(4):460-2.
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