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Water pepper or smartweed (Polygonum hydropiper) is a plant in the family Polygonaceae. It is native to temperate Europe and Northern Hemisphere, and also Australia. Water pepper is used as a spice due to its pungent flavor.
Water pepper grows in damp places and shallow water. It contains several active ingredients, such as polygodial (an unsaturated dialdehyde), waburganal (which is responsible for its pungent taste), and rutin (a source of bitter taste).
In Japan, the leaves of water pepper cultivars are used as a vegetable. The wild type of water pepper has a far more pungent taste than the cultivars, and produces oils that can cause skin irritation. The formic acid in the leaves makes it unpalatable to livestock. The seeds of water pepper is sometimes added to wasabi.
