Definition of water marigold in US English:

water marigold

noun

  • 1Marsh marigold, Caltha palustris, or any of several other plants of the genus Caltha (now rare).

  • 2A yellow-flowered aquatic bur marigold of North America, Bidens beckii, which is highly sensitive to water pollution and hence used as an indicator species.

Origin

Early 19th century; earliest use found in John Sinclair (1754–1835), agricultural improver, politician, and codifier of ‘useful knowledge’.