- Name also: Fringed Willow-herb, Northern Willowherb, Slender Willowherb, Hairy Willowherb, Purple-Leaved Willowherb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Thick-leaved overwintering basal rosette and remains of withered leaf rosette.
- Height: 30–100 cm (12–40 in.). Stem unbranched–branched, erect-branched, bristly, lower parts glabrous, upper part with glandular hairs, often reddish.
- Flower: Corolla regular, white (often light reddish), 3–4 mm (0.12–0.16 in.) long; petals 4, with notched tips, as long as calyx. Sepals 4. Stamens 8. Pistil formed from two fused carpels, stigma club-like; inferior ovary (below tepals), with glandular hairs. Inflorescence a lax, leafy raceme.
- Leaves: In basal rosette and opposite on stem (alternate in inflorescence), very short-stalked. Blade ovate, round-based, short-tapered, with serrated margins, glabrous, bright green.
- Fruit: Tubular, 4-valved, 5–8 cm (2–3.2 in.) long capsule. Seeds ridged, plumed.
- Habitat: Gardens, yards, flower-beds, shores, ditches, roadsides, logging areas, wasteland.
- Flowering time: July–August.
- Harmfulness: Harmful invasive species.
In Finland there are 18 species of willowherb, if 5 established newcomers are included. Fringed willowherb is originally native to North America, but it spread with people to Europe and eventually through Sweden to Finland. It was first recorded in Finland in Lohja in 1915. American willowherb (E. adenocaulon) and purple-leaved willowherb (E. glandulosum) also established themselves in Finland at this time. Suddenly Finland had a group of new willowherbs which greatly resembled each other and it took some time before it was learned how to differentiate between them.
North American willowherbs have spread with exceptional speed into new areas, especially American willowherb which has been the pioneer of the early 20th century arrivals. Fringed willowherb has been slightly slower in its conquest of Finland. The species’ greater need for water and weaker ability to push its way into wild vegetation have made it somewhat rarer and harder to find than its relative, except in the south. It probably still hasn’t achieved a very secure position along its northern border, but it has at least established itself as a part of Finnish nature in the centre and south of the country. Care should be taken with American willowherbs because plants that arrive from similar climates to Finland can thrive here so well that they become a threat to the native species.
Fringed willowherb can be differentiated from American willowherb by its smaller size and lighter shade of the corolla, and also by the identifying markers on the seeds. Fringed willowherb is often erect-branched and looks clearly different from the more fan-like branches of American willowherb. Additionally its leaves are smaller, blunter, with smaller teeth, and of a purer green colour. On the other hand, identifying plants that have not yet flowered can still be very difficult.