Water Smartweed Sometimes, weeds make great dramatic images. This water smartweed, captured in the shadows at the edge of a small lake, might be one such image. Persicaria amphibia is a species of flowering plant in the knotweed family also known as longroot smartweed, water knotweed, water smartweed, and swamp smartweed. It is native to parts of North America, Asia, Europe, and Africa. This perennial plant, sometimes considered a weed, has a terrestrial form and an aquatic form. It may be an aquatic plant, growing submerged or floating in water bodies such as ponds, streams, and marshes. It may be a terrestrial plant grow in muddy and wet areas as well as moist spots on land like meadows.