Epilobium ciliatum is a fibrous-rooted perennial with simple to branched, erect stems from 100-190 cm high. The herbage consists of fine, short hairs, often arranged in vertical lines with the hairs often oriented downwards. The leaves are opposite especially below, or perhaps slightly offset or alternate above. The blades are ovate to lanceolate with sparsely toothed margins and sessile leaf bases. The blades vary from 3-12 cm long and 0.5-5.5 cm wide.
The inflorescence consists of terminal racemes at the tip of the erect stem and side branches. The racemes are often covered with lines of gland-tipped hairs and numerously flowered. The flower stems are 2-15 mm long in fruit. The floral tube is 0.5-2 mm long while the 4 reddish sepals are 2-6 mm long. The 4 petals are white, pink or rose-purple in color and range from 2-10 mm long. The stigma is not lobed, but instead club-shaped or subcapitate. The fruit is a capsule from 3-10 cm long.
Variety ciliatum: Plants open and freely branched when developed. Leaves lanceolate to lance-ovate. Petals white to pink, ranging from 2-6 mm long. Inflorescence not leafy. Found along roadsides and other open, disturbed sites. Turions not present.
Variety glandulosum: Plants simple or sparsely branched when developed. Leaves ovate or narrowly ovate with prominent veins. Petals rose-purple and rarely white, ranging from 6-10 mm long. Inflorescence fairly leafy and narrow. Found in cool, moist montane habitats such as streams, wet meadows and seeps. Turions present.
Variety watsonii: Plants freely branched when developed. The inflorescences are flat-topped. Petals pink to purple. Turions not present.
Bog willow herb is found in wet meadows and along wet streams and rivers at middle to higher altitudes in the mountains.
Bog willow herb is found in mountainous areas from the Arctic south through much of Canada and the United States (except the southeastern US) to Mexico and Central America. It is found across the Aleutian Islands to the Kamchatka Peninsula and south to Japan & Korea.