Water-pepper or Water pepper (Persicaria hydropiper, syn. Polygonum hydropiper) is a plant of the family Polygonaceae. It grows in damp places and shallow water. It is a cosmopolitan plant, found in Australia, New Zealand, temperate Asia, Europe, and North America. It has some use as a spice because of its pungent flavour.
The water-pepper has several active ingredients. Two bicyclic sesquiterpenoids are present, polygodial (tadeonal, an unsaturated dialdehyde with a drimane backbone) and waburganal, which has been found responsible for the pungent taste (hence its edibility). The plant also contains rutin, a source of the bitter taste impression.
The plant 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. Carboxylic acids (cinnamic, valeric and caproic acid) and their esters were present in traces. The composition depends strongly on genetic factors.
In Japan this plant's leaves are used as a vegetable - these are from the cultivar, not the wild type which has a far more pungent taste. Wild waterpepper produces oils that cause skin irritation, and the many acids in its tissues, including formic acid, make the plant unpalatable to livestock. Young red sprouts are used as a sashimi garnish, and are known as beni-tade (紅蓼, red water pepper). Though livestock do not eat the wild type, some insects do, giving rise to the Japanese saying Tade kū mushi mo suki zuki (蓼食う虫も好き好き, Some insects eat water pepper and like it), which may be translated as “There is no accounting for taste.” or more narrowly “Some prefer nettles.”
The seeds of the water-pepper may be added to wasabi.
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- Dennis I. Morris DI (2009) Polygonaceae, version 2009:1. In MF Duretto (Ed.) Flora of Tasmania Online. 17 pp. (Tasmanian Herbarium, Tasmanian Museum & Art Gallery: Hobart). ISBN 978-1-921599-30-9 (PDF). www.tmag.tas.gov.au/floratasmania
- Jonassohn, M. (1996). Sesquiterpenoid unsaturated dialdehydes - Structural properties that affect reactivity and bioactivity. Doctoral thesis, Lund University, Sweden. ISBN 91-628-2215-2. PDF (730 KiB)
- Flora of North America
- Illinois Wildflowers
- Jepson Manual Treatment
- Australian Plant Name Index (APNI), Australian National Botanic Gardens, Australian National Herbarium: Persicaria hydropiper (L.) Delarbre 
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