Flower:

Flowers are single at the top of the stem and arising from leaf axils in the upper half of the plant. Flowers are usually white, sometimes pinkish, about 1/3 inch across with 4 notched petals. In the center is a white club-shaped style surrounded by 8 stamens of varying lengths.

The 4 sepals cupping the flower are lance-triangular and shorter than the petals. Between the flower and stalk is a slender ovary over an inch long and slightly wider than the stalk. The sepals, stalk and ovary are all moderately to densely covered in short curved hairs, usually mixed with sparse gland-tipped hairs.

Leaves and stems:

Leaves are opposite or alternate, linear to narrowly elliptic, ¾ to 3 inches long, about ¼ inch (1 to 7 mm) wide, blunt to pointed at the tip, stalkless or minutely stalked, often with small leaf clumps (fascicles) in the axils. The upper surface is evenly covered in short curved hairs, the lower hairy along the midrib and hairless or sparsely hairy on the surface. Edges are toothless and slightly to strongly rolled under (revolute).

Stems are usually single, sparsely to much branched, green to reddish, and covered in short, curved hairs, denser in the upper plant and more sparse towards the base; sometimes the hairs are in lines, especially towards the base. Horizontal stems (stolons) end in a vegetative bud (turion) which detaches and produces a new shoot the following spring.

Fruit:

The ovary elongates up to about 3 inches as it matures, drying to brown then splitting lengthwise from the top down in 2 to 4 segments, the sides curving away and releasing the numerous seeds. Seeds are elliptic, brown, about 2 mm long with a tuft of long white hairs at the tip to carry them off in the wind.

Notes:

Bog Willowherb is the most common Epilobium species in Minnesota, found in wetlands across the state. It is not very particular about soil type and grows almost anywhere with moist to wet soil, in full sun or part shade. The Minnesota Willowherbs can be split into 2 groups, based on whether leaves are toothed or not. The toothless group includes Bog Willowherb (E. leptophyllum), Marsh Willowherb (E. palustre) and Downy Willowherb (E. strictum). We've spent more time in the field looking at this species, sometimes scratching our heads over the other two. A hand lens can be helpful.

Bog Willowherb is distinguished by the short, curved hairs on stems and leaves, and linear leaves that are evenly hairy on the upper surface and rolled under (revolute) along the edges; it can be spindly and few-branched or more bushy. Marsh Willowherb is most similar, but leaves are hairless on the upper surface and mostly flat, not revolute, and it also has the distinction of the stem tip strongly nodding when flowers are still in bud, where Bog Willowherb is erect. Downy Willowherb has straight, spreading hairs throughout, not curved. Both tend to have broader leaves than Bog Willowherb.