sweet flag vs marsh plant

sweet flag and marsh plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet flag	Yes	No	No	No
marsh plant	Yes	No	No	No

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

sweet flag: perennial marsh plant having swordlike leaves and aromatic roots
marsh plant: a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath

Other hypernyms of sweet flag include bog plant, swamp plant.

sweet flag (noun)	marsh plant (noun)
perennial marsh plant having swordlike leaves and aromatic roots	a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath