- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Thick-leaved overwintering rosette and withered remnants of leaf rosette at base.
- Height: 50–100 cm (20–40 in.). Stem unbranched–upper part scantly branched, with curved and glandular hairs almost to the bottom.
- Flower: Corolla regular, purple, 6–8 mm (0.25–0.35 in.) long; petals 4, with notched tips. Sepals 4. Stamens 8. Pistil formed from two fused carpels, stigma club-like; ovary below tepals, with glandular hairs only. Inflorescence a compact, large-leaved raceme.
- Leaves: In basal rosette and opposite on stem (alternate in inflorescence), stalkless, almost amplexicaul. Blade quite broadly ovate, broad-based, short-tapered, wrinkled, shiny, dark green.
- Fruit: Tubular, 4-valved, 5–7 cm (2–2.8 in.) long capsule. Seeds ridged, plumed.
- Habitat: Ditches, roadsides, waste ground.
- Flowering time: July–August.
Rosebay willowherb (Epilobium angustifolium) and hairy willowherb (E. hirsutum) are tall, large-flowered plants that are easily noticed by even casual naturalists. A whole bunch of shorter and smaller-flowered willowherbs grow in the Finnish wild too, however, all with their own interesting features. Genus Willowherb’s most common representatives in Finland include three American established aliens: northern willowherb, American willowherb (E. adenocaulon) and fringed willowherb (E. ciliatum). All three species can grow side by side in the same habitat and they can also cross breed, so the group might also contain intermediate forms. Northern willowherb flowers a bit later than its relatives, seedlings later still than plants that have grown from overwintering buds. Northern willowherb is sturdier than its relatives, but a definite classification can be made by inspecting the hair on its tubular ovary. Northern willowherb’s scientific name glandulosum, ‘with glandular hairs’, refers to the quality of its hairs, but the prevailing type of hairs on the species is curved hairs.
Northern willowherb is very urban in Finland. The first observation of the species at the end of the 1920s was made in Helsinki and the species probably established itself in Finland via gardens in northern Helsinki. It then gradually started to spread out in different directions from its original centres. The clear border of its current habitat could be a demonstration that it is still spreading equally in every direction. Its core area is still Helsinki, although the species is naturally present near Turku and here and there across Uusimaa province (in which Helsinki is located) outside the capital too. The species seems to favour sea air and as such it hasn’t penetrated very deeply inland anywhere else in the Nordic countries either. The best place to find northern willowherb is in very rich ditches and in damp clayey land.