water mint
/ˈwɔːtə mɪnt/noun
Any of several plants of the genus Mentha (family Lamiaceae (Labiatae)), having aromatic leaves and typically growing in or close to water; specifically M. aquatica of Eurasia and northern Africa, which has rounded clusters of pale purplish flowers.
Origin
Middle English; earliest use found in Pluteus. From water + mint.