Polygonum hydropiper), also known as water pepper, marshpepper knotweed, arse smart[2] or tade, is a plant of the family Polygonaceae.

A widespread species, Persicaria hydropiper is found in Australia, New Zealand, temperate Asia, Europe and North America.

The perianth of each tiny flower consists of four or five segments, united near its green base and white or pink at the edges.

[7] P. hydropiper is an annual, and prefers damp environments for optimal growth; it will readily grow in riparian zones on the banks of streams and rivers, but can also grow in other areas where water collects, such as on the banks of canals, tyre and hoof tracks in woodlands, waterlogged soil, and around gates in fields.

Two bicyclic sesquiterpenoids are present, polygodial (tadeonal, an unsaturated dialdehyde with a drimane backbone), and warburganal, which gives it its pungent taste.

Water pepper contains an essential oil (0.5%) which consists of monoterpenoids and sesquiterpenoids: α-pinene, β-pinene, 1,4-cineol, fenchone, α-humulene, β-caryophyllene, trans-β-bergamotene.

A study has proved that P. hydropiper leaves possess noteworthy anthelmintic efficacy and justify their use in traditional medicine against intestinal worms.