A perennial herb (Polygonum hydropiperoides) growing in marshes and bogs of the United States, having reddish stems, clusters of small greenish flowers, and acrid-tasting leaves.
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waterpepper — /ˈwɔtəpɛpə/ (say wawtuhpepuh) noun any of various plants of the genus Polygonum, growing in wet places, especially P. hydropiper … Australian English dictionary
smartweed — /ˈsmatwid/ (say smahtweed) noun a summer annual, Polygonum hydropiper, native to Europe and Asia, which grows in low lying areas, and whose reddish green stems and leaves contain an acrid juice; waterpepper … Australian English dictionary