This photo gives a good indication of the density, uniformity, and beauty of a typical patch of Epilobium hirsutum.
Epilobium hirsutum is an herbaceous species that reaches 1 to 1.5 meters in height. It has woolly leaves and stems, hence the name hirsutum (Latin for hairy). The common name for this species, great willow-herb, is inspired by its long, narrow leaves that are similar in appearance to the leaves of willow species (Salix spp.). Epilobium hirsutum produces many showy flowers that are purple and bell-shaped. It has long white stigmas that ripen at the same time as the anthers, making the flowers capable of self-pollination. Before resorting to self-pollination, Epilobium hirsutum flowers hold their stigmas prominently, so that an insect may cross-pollinate them. Should this fail to happen, the stigma on the fading flower curls backwards to touch the ripe pollen held on the stamens. These flowers then form long narrow capsules that contain small seeds attached to long white hairs.
Great willow-herb is native to Eurasia, but is now a common weed around the world. It forms dense, monotypic stands in wetlands and waterways. It is associated with another wetland species, purple loosestrife (Lythrum salicaria). Epilobium hirsutum and Lythrum salicaria strike a seasonal balance – in autumn, the shorter days and colder temperatures favour Epilobium hirsutum while in spring Lythrum salicaria grows more quickly, making up for lost ground. Together, these two species form bright purple, colourful flower displays. Both Epilobium hirsutum and Lythrum salicaria were originally planted as ornamental garden species. Despite their beauty, they should not be planted outside of their native range, as both can aggressively crowd out other wetland species.