Stems stout and softly hairy, 2 to 5 feet high from perennial roots, propagating by underground shoots. Leaves usually opposite, sometimes alternate, oblong-lanceolate to lanceolate, sessile or even clasping the stem at the base, pointed at the apex, sharply but finely toothed on the margins, 1 to 4 inches long, one-third to one-half of an inch wide, thin and pubescent. Flowers rose-purple, three-fourths to 1 inch broad, in the axils of the upper leaves; calyx tube linear with four deciduous lobes or sepals at its summit; petals broadly obovate and notched at the apex, pubescent within at the base. Stigma deeply four-lobed; stamens eight. Fruiting capsule 2 to 3 inches long and very slender, with numerous small seeds each provided with a tuft of whitish hairs.
A native of Europe which, like the Purple Loosestrife, is thoroughly naturalized in marshes, swamps and ditches throughout the eastern states, especially about the larger cities, towns and ports. Flowering from July to September.
Memoir 15 N. Y. State Museum
A. Great Hairy Willow-Herb - Epilobium hirsutum
The other species of Epilobium in New York are chiefly inconspicuous, small-flowered marsh herbs, two of them very rare Alpine species of the higher Adirondacks, the other four being inhabitants of swamps and bogs at lower altitudes. Of these, the commonest in most localities is the Northern Willow-herb (Epilobium adenocaulon Haussknecht).