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 and Eurasia, but it is known on most continents as an introduced species and sometimes a noxious weed. It grows in many types of wet habitat, such as ponds, streams, and marshes. It is a rhizomatous perennial herb which takes a variety of forms and is quite variable in morphology. It may be an aquatic plant, growing submerged or floating in water bodies, it may grow in muddy and wet areas which are periodically inundated, and it may grow in moist spots on land, such as in meadows. Dry-land and fully aquatic plants are sometimes considered different named varieties of the species.
The plant 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 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 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 long.
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