Willowherbs are perennial or annual herbs with opposite leaves below the inflorescence and alternate above. The four-merous white, pink or purple flowers arise singly from the upper leaf axils, forming a terminal, raceme‑like cluster. Flowers have 8 stamens and a short style with a deeply 4‑lobed stigma. Hypanthium is prolonged beyond the inferior ovary as a slender tube. Fruit is a linear, dehiscent capsule with sepals soon decidious, and the seeds bear a tuft of hairs at one end.