- Latin synonym: Polygonum amphibium
- Name also: Water Knotweed, Longroot Smartweed, Water Smartweed, Willow Grass
- Family: Dock Family – Polygonaceae
- Growing form: Perennial herb, having both aquatic and terrestrial forms.
- Height: 10–150 cm (4–60 in.)
- Flower: Regular, approx. 3.5 mm (0.14 in.) long. Perianth consists of 5 pink segments united from base to halfway. Pistil consists of 2 fused carpels. Styles 2, united from base to halfway. Stamens 5.
- Leaves: Alternate. Terrestrial leaves short-stalked and hairy, blade usu. 8–15 cm (3–6 in.) long, narrowly oblong or ovate, sometimes reddish, and with rounded or tapered base. Aquatic leaves long-stalked and hairless, floating blade approx. 10–15 cm (4–6 in.), oblong, and with rounded or cordate base. Stipules fused into a stem-enclosing sheath (an ochrea) that on aquatic individuals also covers most of the leaf-stalk.
- Fruit: Black or brown, glossy, roundish but flattened, approx. 3 mm (0.12 in.) long achene.
- Habitat: Shallow waters along lake-shores, brackish water bays, rivers, ditches, roadsides and fields.
- Flowering time: July–August.
Amphibious bistort is a perennial that can grow both in water and on dry land. It reproduces chiefly vegetatively, and often forms extensive pure stands when growing in water. Terrestrial and aquatic shoots differ in habit even if they grow from the same rootstock. In water, amphibious bistort can grow at a depth of 2.5 metres (8 ft.). In Russia and Central Asia amphibious bistort has been used as fodder and medicine.