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.

Fringed willowherb (Epilobium ciliatum) is likely a cryptic species complex; apparently these plants also commonly hybridize with their congeners.

[6] A number of Epilobium species with slightly asymmetrical (zygomorphic) magenta flowers and alternate leaves are placed in a Chamaenerion group and are further separated by some authorities into a genus Chamerion.

[7][8][9] 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 rubra), while great willowherb (Epilobium hirsutum) is found in mesotrophic grassland with stinging nettle (Urtica dioica).

Broad-leaved willowherb (Epilobium montanum) is found characteristically, though not abundantly, in the mesotrophic grasslands with meadowsweet (Filipendula ulmaria) and sometimes the uncommon Greek valerian (Polemonium caeruleum).

Its young leaves, roots, and shoots are edible (if somewhat bitter), and rich in provitamin A and vitamin C. The Dena’ina found them also useful as food supplement for dogs and applied sap from the stem to wounds, believing it to have antiinflammatory properties.