sweet flag vs bog plant

sweet flag and bog plant both are nouns.

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

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

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

sweet flag (noun)	bog 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