water pepper

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two smartweeds that grow in moist habitats, Persicarea hydropiper, which has acrid-tasting leaves, or P. hydropiperoides, which has milder leaves.

from The Century Dictionary.

• noun The smart-weed, Polygonum Hydropiper. The mild water-pepper is P. hydropiperoides.
• noun Same as waterwort, 1.

from the GNU version of the Collaborative International Dictionary of English.

• Smartweed.
• Waterwort.