Epilobium is a genus of flowering plants in the family Onagraceae, containing about 197 species. The genus has a worldwide distribution. It is most prevalent in the subarctic, temperate and subantarctic regions, whereas in the subtropics and tropics Epilobium species are restricted to the cool montane biomes, such as the New Guinea Highlands. The taxonomy of the genus has varied between different botanists, but the modern trend is to include the previously recognised genera Boisduvalia, Pyrogennema and Zauschneria within Epilobium. Chamaenerion, (previously Chamerion), is considered distinct, however, according to Peter H. Raven, who has extensively studied the willowherbs and merges the other segregate genera into Epilobium. Fringed willowherb (Epilobium ciliatum) is likely a cryptic species complex; apparently these plants also commonly hybridize with their congeners. Most species are known by the common name willowherbs for their willow-like leaves. Those that were once separated in Boisduvalia are called spike-primroses or boisduvalias. Those Epilobium species previously placed in the Chamaenerion group and known as fireweeds are now segregated into the genus Chamaenerion. Description Epilobiums are mostly herbaceous plants, either annual or perennial; a few are subshrubs. The leaves are opposite or rarely whorled, simple and ovate to lanceolate in shape. The flowers are actinomorphic (radially symmetrical) with four petals that may be notched. These are usually smallish and pink in most species, but red, orange or yellow in a few. The fruit is a slender cylindrical capsule containing numerous seeds embedded in fine, soft silky fluff which disperses the seeds very effectively in the wind. The genus name derives from the Greek words "epi" meaning "upon" and "lobos" meaning "lobe", with reference to position of the petals above the ovary. A number of Epilobium species with slightly asymmetrical (zygomorphic) magenta flowers and alternate leaves were placed in a Chamaenerion group, and have been separated by some authorities into the genus Chamerion. However, Chamerion has not been universally accepted<ref name=TPL>{{cite web |url=http://www.theplantlist.org/tpl1.1/search?q=Chamerion |title=The Plant List: Chamerion |access-date=22 February 2017}}</ref> and there is now emerging consensus that this group of species should be segregated into the genus Chamaenerion. Habitat and ecology Willowherbs sensu lato are typically very quick to carpet large swathes of ground and may become key or dominant species of local ecosystems. In the United Kingdom, for example, rosebay willowherb (Chamaenerion angustifolium) is widely found on mesotrophic soils dominated by false oat-grass (Arrhenatherum elatius), cock's-foot grass (Dactylis glomerata), and red fescue (Festuca