cotton grass vs sedge

cotton grass and sedge both are nouns.

Word	Noun	Adjective	Verb	Adverb
cotton grass	Yes	No	No	No
sedge	Yes	No	No	No

As nouns, sedge is a hypernym of cotton grass; that is, sedge is a word with a broader meaning than cotton grass:

cotton grass: any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes
sedge: grasslike or rushlike plant growing in wet places having solid stems, narrow grasslike leaves and spikelets of inconspicuous flowers

cotton grass (noun)	sedge (noun)
any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes	grasslike or rushlike plant growing in wet places having solid stems, narrow grasslike leaves and spikelets of inconspicuous flowers