- Written also: Water-pepper
- Family: Dock Family – Polygonaceae
- Growing form: Annual herb.
- Height: 20–70 cm (8–28 in.). Stem erect–ascending.
- Flower: Regular, approx. 3–5 mm (0.12–0.2 in.) long. Perianth consists of 4 (or 5) segments united from base to halfway, and with white or pinkish margins and greenish base. Pistil formed from 3 fused carpels, basally united styles 3. Stamens usually 6. Inflorescence a nodding spike.
- Leaves: Alternate. Almost stalkless, usually with short-hairy margins. Blade with entire margins, narrow-ovate, tapering but with blunt apex. Stipules fused into a stem-enclosing sheath (an ochrea) that is loose and fringed at the mouth.
- Fruit: Deep brown, oval, flat, 3 mm (0,12 in.) long achene.
- Habitat: Ditches, ponds, puddles, shores, and other damp sites with lush vegetation.
- Flowering time: July–September.
Water pepper flowers relatively late, and the last nuts drop on the snow to be dispersed by wind. Human-induced changes in waterside ecosystems, e.g. eutrophication, enhance the spreading of this species. Water pepper is almost cosmopolitan, although it is still absent from most of Africa.
Water pepper is an annual which has been used as a medicine and seasoning. Its scientific name (hydropiper) means water pepper, just like the English name. Its leaves have a burning peppery taste. Water pepper has been used against many different maladies. It can also be encountered as an ingredient of Japanese spice mixes. When prepared by soaking in wine, it has been thought to determine the sex of unborn children in favour of boys. Water-pepper has also been used to give wool a yellow colour.