Rosebay willowherb or fireweed

Appearance

The reddish stems of this herbaceous perennial are usually simple, erect, smooth, 0.5–2.5 m 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.: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 cm in diameter, slightly asymmetrical, with four magenta to pink petals and four narrower pink sepals behind. The protruding style has four stigmas. The floral formula is ✶/↓ K4 C4 A4+4 or 4+0 Ğ.

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.

Naming

It is also known by the synonyms "Chamerion angustifolium" and "Epilobium angustifolium".

Two subspecies are recognized as valid:
⤷  "Chamaenerion angustifolium" subsp. "angustifolium"
⤷  "Chamaenerion angustifolium" subsp. "circumvagum" Hoch

Habitat

Fireweed is often abundant in wet calcareous to slightly acidic soils in open fields, pastures, and particularly burned-over lands. It is a pioneer species that quickly colonizes open areas with little competition, such as the sites of forest fires and forest clearings. Plants grow and flower as long as there is open space and plenty of light. Fireweed reaches its average peak colonization after 5 years and then begins to be replaced as trees and brush grow larger. Seeds remain viable in the soil seed bank for many years. When a new fire or other disturbance occurs that opens up the ground to light again, the seeds germinate. Some areas with heavy seed counts in the soil can, after burning, be covered with pure dense stands of this species and when in flower the landscape is turned into fields of color.

Fireweed is an effective colonizer; it may not be present until after a fire has moved through a landscape. Because of its very high dispersal capacity, "propagule pressure" from its regional presence will let it quickly colonize a disturbed area. Once seedlings are established, the plant quickly reproduces and covers the disturbed area via seeds and rhizomes. It is somewhat adapted to fire as well and so can prevent the reintroduction of fire to the landscape. Fireweed is well adapted to seed in severely burned areas as well, because the mineral soil that is exposed due to the removal of organic soil layers provides a good seedbed.

In Britain the plant was considered a rare species in the 18th century, and one confined to a few locations with damp, gravelly soils. It was misidentified as great hairy willowherb in contemporary floras. The plant's rise from local rarity to widespread abundance seems to have occurred at the same time as the expansion of the railway network and the associated soil disturbance. The plant became locally known as "bombweed" due to its rapid colonization of bomb craters in the second world war.