Hairy willowherb is a rhizomatous flowering plant in the evening primrose family. It normally grows as a perennial in most regions but can behave as an annual in harsher climates. Native to North Africa and Eurasia, it is considered an invasive species and a weed in parts of the northern United States. It may have been introduced through ship ballast or contaminated nursery stock, and it has also been sold as a garden plant, contributing to its spread in the Northeast and Great Lakes regions.

The hairy stems and leaves give this species its name hirsutum, Latin for 'hairy'. The leaves are mostly opposite, sessile, and lanceolate to narrowly oblong. They are coarsely toothed, with outward‑pointing teeth that create a dentate margin, becoming more finely serrulate toward the upper stem.

E. hirsutum grows up to 6 feet tall and bears pink, primrose‑like flowers. The blooms appear from late June to September and arise singly from the leaf axils along the upper portion of the stem. Each flower is about ¾ inch across.

Hairy willowherb thrives in sunny moist environments including wetlands, low areas, in ditches, along roadsides, and along stream beds. The plant can be self-pollinating or be pollinated by the many types of insects that visit its flowers. The elephant hawkmoth caterpillar feeds on its leaves.

This species reproduces both by seed and vegetatively through stolons. After flowering, it produces a slender seed capsule about 1–1.5 inches long that contains numerous oblong, flattened seeds. As the capsule dries, it splits open and releases the seeds, which are carried by the wind on silky white hairs. Vegetative reproduction occurs when a horizontal stolon forms a rosette of leaves; if this rosette becomes separated from the parent plant, it develops an aerial shoot and grows into a new individual.