- Latin synonym: Epilobium ciliatum ssp. adenocaulon
- Written also: American Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Base thick-leaved overwintering rosette and remains of withered leaf rosette.
- Height: 30–120 cm (12–50 in.). Stem unbranched–branched, straggly branched, edged, lower part glabrous, upper part with arched and glabrous hairs, often reddish.
- Flower: Corolla regular, light purple (occasionally white), 3–7 mm (0.12–0.28 in.) broad; petals 4, with notched tips, clearly longer than calyx. Sepals 4, with arched and glandular hairs. Stamens 8. Gynoecium fused, a single carpel, stigma club-like; ovary under tepals, with arched, glandular hairs. Inflorescence long, lax raceme.
- Leaves: In basal rosette and opposite on stem (inflorescence alternate), very short-stalked. Blade narrowly ovate–ovate, round-based, glabrous, with shallowly toothed margins, dark green, often reddish.
- Fruit: Tubular, 4-valved, 5–8 cm (2–3.2 in.) long capsule. Seeds ridged, plumed.
- Habitat: Shores, stream-sides, ditches, ponds, gardens, roadsides, logging areas, wasteland.
- Flowering time: July–August.
- Harmfulness: Harmful invasive species.
As its name suggests, American willowherb grows as a native plant over almost all of the United States and southern parts of Canada. Somehow the plant managed to smuggle itself into Europe and began to spread like wildfire in areas with a similar climate to its homeland. Like its relatives, American willowherb produces an abundance of small seeds which are downy and can float long distances on the wind. The species reached Finland in the 1920s by spreading from both Sweden to the west of the country and from Karelia to reach the south-east at the same time, and it perhaps also arrived from the Baltic countries on the other side of the Gulf of Finland. Representatives of the species that have arrived from different areas look slightly different and it used to be possible to monitor how the different types spread. It is still often possible to distinguish the source of the plant, although it demands close botanical comparisons: the western population is large, almost unbranched and much more common, while the eastern population is quite low and branched from the base.
Nowadays American willowherb is fairly common south of the Vaasa–Kuopio–Joensuu line, and it grows to the north as far as the Oulun–Kajaani area. It remains to be seen if it will reach Lapland or will Oulu remain its northern limit. The species’ most common habitats are wet, dampish and even dry ditch banks, wasteland and banks, and it has also found favourable habitats in the south in places that are not inhabited such as on the banks of rich swamps, springs, streams, rivers and lakes. American willowherb is an almost frighteningly good example of a plant which has been able to carve out a niche for itself among the wild vegetation and is thriving as well as any native plant.
American willowherb’s close relative fringed willowherb (E. ciliatum) is a bright green, white-flowered alien which has spread just as well to a certain extent. Northern willowherb (E. glandulosum), which is stout-stemmed and has a dense inflorescence, grows more rarely in southern Finland. Finnish marsh willowherb (E. palustre) differs from the American aliens with regards to its broadly lanceolate, sharp-toothed leaves.