water pepper (Persicaria hydropiper f. purpurascens)
A variety of water pepper with reddish flowers or stems. Cross-referenced with 柳蓼 (やなぎたで), a related plant.
See also: ヤナギタデ (やなぎたで)
Benitade is an annual plant that grows in wetlands and produces reddish-purple flowers in autumn.