cotton grass vs cotton rush

cotton grass and cotton rush both are nouns.

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

As nouns, cotton grass and cotton rush are synonyms defined as:

cotton grass and cotton rush: any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes

cotton grass (noun)	cotton rush (noun)
any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes	any sedge of the genus Eriophorum; north temperate bog plants with tufted spikes