Chamaenerion angustifolium is a perennial herbaceous flowering plant in the willowherb family, Onagraceae. It is known in North America as fireweed and in the British Isles as rosebay willowherb. It is also known by the synonyms Chamerion angustifolium and Epilobium angustifolium. It is native throughout the temperate Northern Hemisphere, including large parts of the boreal forests.
The reddish stems of this herbaceous perennial are usually simple, erect, smooth, 0.5–2 metres (1+1⁄2–6+1⁄2 feet) high with scattered alternate leaves. The leaves are spirally arranged, entire, narrowly lanceolate, and pinnately veined, the secondary leaf veins anastomosing, joining together to form a continuous marginal vein just inside the leaf margins.[4]: NQ
The inflorescence is a symmetrical terminal raceme that blooms progressively from bottom to top, producing a gracefully tapered shape. The flowers are 2 to 3 centimetres (3⁄4 to 1+1⁄4 inches) in diameter, slightly asymmetrical, with four magenta to pink petals and four narrower pink sepals behind. The protruding style has four stigmas.
The upright, reddish-brown linear seed capsule splits from the apex and curls open. It bears many minute brown seeds, about 300 to 400 per capsule and 80,000 per plant. The seeds have silky hairs to aid wind dispersal and are very easily spread by the wind, often becoming a weed and a dominant species on disturbed ground. Once established, the plants also spread extensively by underground roots, an individual plant eventually forming a large patch.
