myrtle flag vs marsh plant

myrtle flag and marsh plant both are nouns.

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

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

myrtle 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 myrtle flag include bog plant, swamp plant.

myrtle 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