Sp. Pl. 1: 347. 1753.
Herbs usually robust and rank, sometimes woody near base, with thick, ropelike stolons to 1 m with scattered cataphylls and, often, terminal leafy rosette. Stems erect to ascending, often clumped, terete, 25–120(–250) cm, unusually thick, 3–9 mm diam., well branched mainly in distal 1/2, densely long-villous throughout, usually mixed glandular puberulent distally, rarely sparsely villous or densely white-tomentose. Leaves opposite proximal to inflorescence, alternate distally, sessile and ± clasping stem; blade elliptic-lanceolate to narrowly obovate or elliptic, 4–12(–23) × 0.3–4(–5) cm, base cuneate to attenuate, margins serrulate, 15–50 teeth per side, veins 6–9 per side, apex acute to acuminate or obtuse proximally, surfaces ± densely villous; bracts moderately reduced. Inflorescences erect racemes or panicles, usually densely villous and glandular puberulent, rarely tomentose. Flowers erect; buds 5–9 × 1.8–4.5 mm, sometimes beaked; pedicel 3–11 mm; floral tube 1.3–2.9 × 2.2–4 mm, conspicuous ring of spreading hairs near mouth inside; sepals oblong-linear, often keeled, 6–12 × 1–3 mm, abaxial surface densely pubescent; petals bright pink to rose-purple, rarely white, broadly obcordate, 9–20 × 7–15 mm, apical notch 1–3 mm; filaments white or pink, those of longer stamens 5–10 mm, those of shorter ones 2.5–6 mm; anthers cream, 1.5–3 × 0.6–1.2 mm; ovary 15–34 mm, densely villous and glandular puberulent; style white or pink, 5–12 mm, usually glabrous, stigma deeply 4-lobed, 1.8–2.2 × 3–5.5 mm, lobes recurved or spreading, exserted beyond anthers. Capsules often flushed purple, 25–90 mm, surfaces usually densely villous and glandular puberulent, rarely glabrescent; pedicel 5–20 mm. Seeds narrowly obovoid, 0.8–1.2 × 0.3–0.6 mm, chalazal collar inconspicuous, dark brown, surface coarsely papillose; coma easily detached, tawny or dull white, 7–10 mm. 2n = 36.
Phenology: Flowering Jun–Sep.
Habitat: Low wet areas along streams, rivers, ponds, and lakes, roadside ditches, along railroad tracks, marshes and swampy areas.
Elevation: 0–150[–3000] m.
Introduced; B.C., N.B., N.S., Ont., P.E.I., Que., Colo., Conn., Ill., Ind., Ky., Maine, Md., Mass., Mich., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Utah, Vt., Wash., W.Va., Wis., Eurasia, Africa.
Epilobium hirsutum is very widespread in cool temperate Eurasia and montane regions, occurring throughout Europe except in the far north (P. H. Raven 1968), through the Caucasus and central Asia (E. I. Steinberg 1949) to Nepal (Raven 1962), China (Chen C. J. et al. 1992), and Japan (A. W. Lievens and P. C. Hoch 1999). It occurs as well along the Mediterranean coast of Africa, through East Africa to southern Africa, and in the Canary and Cape Verde Islands (Raven 1967).
Epilobium hirsutum exceeds almost all other species of the genus in stature, so its size, very large flowers, and densely villous aspect make it easy to identify. R. L. Stuckey (1970) provided a detailed account of the introduction and spread of E. hirsutum in North America, noting the earliest known collection (July 1829) was from Newport, Rhode Island. Most early collections appeared in waste areas, particularly near harbor ballast piles, although some may have been grown in gardens. By the 1890s this species was well established along the Atlantic coastal region from New Jersey and Philadelphia through New England, and around Niagara Falls in the Great Lakes region. During the twentieth century, E. hirsutum spread extensively in southern Ontario and Quebec, south along the Atlantic coast to Maryland, and to all of the states along the southern shores of the Great Lakes, most recently including Wisconsin (1970), and Indiana (1972). It occurs in much the same habitat as that of another, more widely publicized invader, Lythrum salicaria, and sometimes is recorded as a companion species. The earliest known collection in western North America was made in 1933 in Bingen (Klickitat County), Washington. Whether from that introduction or others, E. hirsutum is now naturalized and widespread in the Pacific Northwest. It also was reported recently from the Denver region in Colorado and near Midway in Utah.
Epilobium grandiflorum F. H. Wiggers and E. grandiflorum Allioni are illegitimate names that pertain here.