sedge vs cotton grass

sedge and cotton grass both are nouns.

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

As nouns, cotton grass is a hyponym of sedge; that is, cotton grass is a word with a more specific, narrower meaning than sedge:

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

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