marsh marigold vs swamp plant

marsh marigold and swamp plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
marsh marigold	Yes	No	No	No
swamp plant	Yes	No	No	No

As nouns, swamp plant is a hypernym of marsh marigold; that is, swamp plant is a word with a broader meaning than marsh marigold:

marsh marigold: swamp plant of Europe and North America having bright yellow flowers resembling buttercups
swamp plant: a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath

Other hypernyms of marsh marigold include bog plant, marsh plant.

marsh marigold (noun)	swamp plant (noun)
swamp plant of Europe and North America having bright yellow flowers resembling buttercups	a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath