Eastern willowherb is a perennial member of the evening primrose family. The plant can grow quite tall, often one to three feet, but its flowers are small and delicate in comparison. It is found across the eastern half of the United States and into southern Canada. One of its common names, “cinnamon willowherb,” refers to the mature seeds, which carry tufts of brownish hairs that help them travel on the wind. It is also known as “purpleleaf willowherb,” a nod to the purple tinges that often color the leaves and stems. The leaves are narrow and lance‑shaped with numerous small teeth. Lower leaves are hairless, while upper leaves are covered with short, incurved hairs.

The flowers are small, about a quarter to a third of an inch across, with four pale pink or white petals that are shallowly notched at the tip. Each flower is solitary in the axils of the upper leaves, and together they appear as a loose, terminal cluster near the top of the stem. They often nod slightly, especially when newly opened. The four sepals are narrow and frequently tinged with pink or purple. At the center of each flower is a slender white style with a slightly enlarged stigma, surrounded by eight stamens. Behind the petals is a long, slender inferior ovary, a characteristic feature of willowherbs. After pollination, this ovary elongates and matures into the narrow seed capsule.

This species is strongly tied to wetlands, most often growing in shallow water, saturated soils, marsh edges, seeps, and muddy streambanks, though it can occasionally appear in other persistently moist places. It spreads primarily by seed, carried long distances by wind on the silky coma, and also by short rhizomes that allow small clonal patches to form in stable wet soils.