Beach knotweed (Polygonum paronychia) is a native perennial found at low elevations amid the coastal dunes and sandy beaches along the Pacific coastline. These specimens were growing at Crissy Field State Recreation Site near Brookings OR in late October.
The plant is semi-shrubby, prostrate and spreading. The brownish stems are covered with the membranous remains of torn stipules (appendages at the base of leaf stalks) called ochrea.
Beach knotweed leaves are narrowly oblong and rolled under at the margins giving the leaves a linear appearance. They are alternate and mostly bunched near the tips of the stems. The midrib on the leaf underside is prominently bristly.
The flowers of beach knotweed are clustered in the upper leaf axils. White to pink, the flowers are short-stalked with five petal-like tepals (structures that are not clearly sepals or petals). There are 8 stamens and a single, superior, single-chambered ovary.
Beach knotweed fruits are three-angled achenes containing a single smooth, shiny, black seed.
P paronychia is also commonly known as dune knotweed or black knotweed. Knotweed refers to the thickened joints on the stems. This trait is also reflected in the genus name, Polygonum, which derives from the Greek words “polys” meaning many and “gonu” meaning knee or joint.
In Greek paronychia is an infection of the finger, particularly near the nail (a whitlow). Poultices made from these plants were used to treat whitlows, thus the species designation.