A member of the knotweed or buckwheat family that likes damp ground; these were growing in a swampy meadow near Wexford. It has the clusters of tiny flowers typical of the family, but the ball-shaped—almost clover-like—clusters are distinctive. The leaves are shaped like elongated arrowheads, which gives the species its name in both Latin and English.

The plant is native to the whole eastern half of North America; curiously it is also native to East Asia.