The leaves of willowherbs are predominantly opposite, occasionally whorled, and sometimes sessile. They exhibit a simple structure, varying from ovate to lanceolate in shape, generally spanning 1-5 inches (2.5-12.7 cm) in length. The leaf surface can be either hairy or smooth, displaying a noticeable green coloration. Vein patterns are typically pinnate, with a central midrib and lateral veins. The margins can be entire or slightly toothed, contributing to their distinctive appearance useful for identification.

Willowherbs flowers have four petals, which are often notched or deeply notched. They are typically small, measuring around 0.4-0.6 inches (1-1.5 cm) in diameter, and predominantly pink in color, though some species may exhibit red, orange, or yellow hues. These flowers may be arranged in a panicle formation. The blooming period usually spans from late spring to early autumn, displaying their delicate and vibrant blooms. There is generally no noticeable scent.

The stem of willowherbs is notable for its variation and distinctive characteristics. It ranges from smooth to hairy in texture, providing a unique tactile experience. These stems are winged, adding a visually striking feature that aids in identification. Typically, the color varies from green to reddish hues, with a thickness that remains relatively slender, usually around 0.1 to 0.2 inches (2.5 to 5 mm). The branching pattern is often erect but can exhibit slight variations depending on environmental conditions.

The fruit of willowherbs is a slender, cylindrical capsule, generally elongating to several inches in length. When ripe, it splits open to release a multitude of tiny seeds. Each seed is attached to fine, silky hairs known as plumes or 'coma,' which facilitate wind dispersal. The silky fluff is soft to the touch and typically white or silver, contrasting with the fruit's green to brown hues as it matures. This characteristic feature assists the seeds in catching the breeze, enabling them to travel distances from the parent plant.