Prostrate knotweed (Polygonum aviculare) is a non-native annual in the Buckwheat (Polygonaceae) family. Cotyledons are linear in outline and are often misidentified as a grass seedling. The stem below the cotyledons (hypocotyl) is often reddish in color. Prostrate knotweed has a taproot. The lanceolate leaves are arranged alternately along the stem. Each leaf has a short petiole and is approximately 1/2 to 1 1/4 inches long and up to 1/3 inch wide. Stems are branching, growing prostrate along the ground, and range from 4 to 24 inches in length. Stems are swollen at the nodes with a thin membranous sheath (ocrea) encircling the stem at each leaf base. Flowers occur in the leaf axils, between the stems and leaves. The flower clusters contain between 1 and 5 inconspicuous, white to pinkish-white flowers. The fruit is a dark red to brown achene.