The fringed willowherb is a softly hairy perennial in the evening primrose family, growing on slender, branching stems up to about 4 ft tall. Its flowers are small and easily overlooked, sometimes mistaken for members of the pink family, though pinks have five petals rather than the four characteristic of willowherbs.

This species is widespread across northern and western North America, occurring throughout most of the United States except the Deep South, and extending well into Canada and Alaska. It became established in parts of Europe in the early 20th century.

Leaves are opposite below and often alternate above, lanceolate and finely toothed, with modestly visible venation. Stems and leaves are softly hairy throughout, lacking bristly or glandular hairs.

At the ends of the stems, the upper leaf axils bear numerous small pink, pale purple, or white flowers that together form a loose, terminal‑looking cluster. Each flower has four petals that are distinctly notched to form two small lobes, often with darker veining. The style is slender and white, ending in a slightly enlarged, 4‑lobed stigma. The long structure behind the petals is the inferior ovary, sometimes mistaken for a fused corolla tube. The fruit is a slender, hairy capsule up to 10 cm long, containing many downy seeds that disperse widely on the wind.

This species is primarily a wetland plant but also occupies a wide range of other habitats, including ditches, streambanks, moist meadows, disturbed ground, and roadsides. It spreads both by seed which are dispersed widely by wind on the silky coma, and by short rhizomes that allow local clonal expansion. It is also known as American willowherb, hairy willowherb, or northern willowherb. Three subspecies are recognized, but only Epilobium ciliatum subsp. ciliatum occurs in Pennsylvania.