sweet flag vs swamp plant

sweet flag and swamp plant both are nouns.

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

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

sweet flag: perennial marsh plant having swordlike leaves and aromatic roots
swamp 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, marsh plant.

sweet flag (noun)	swamp 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