Longroot smartweed (Water knotweed, Water smartweed, Amphibious bistort)

Persicaria amphibia

Persicaria amphibia (syn. Polygonum amphibium) is a species of flowering plant in the knotweed family known by several common names, including longroot smartweed, water knotweed, water smartweed, and amphibious bistort. It is native to much of North America, Asia, Europe, and parts of Africa, and it is known elsewhere as an introduced species and sometimes a noxious weed.

Persicaria amphibia produces a thick stem from its rhizome. The stem may creep, float, or grow erect, rooting at stem nodes that come in contact with moist substrate. Stems are known to reach 3 meters (10 feet) long in aquatic individuals. The stems are ribbed and may be hairless to quite hairy in texture.

Leaves are lance-shaped or take various other shapes and are borne on petioles. They may be over 30 centimeters (1 foot) in length. The inflorescence is a dense terminal cluster of many five-lobed pink flowers.

Plants may have bisexual or unisexual flowers, with some plants bearing only male or only female flowers. The fruit is a shiny brown rounded achene around 3 millimeters (0.12 inches) long.