carpet grass vs drop-seed

carpet grass and drop-seed both are nouns.

Word	Noun	Adjective	Verb	Adverb
carpet grass	Yes	No	No	No
drop-seed	Yes	No	No	No

As nouns, drop-seed is a hypernym of carpet grass; that is, drop-seed is a word with a broader meaning than carpet grass:

carpet grass: grass native to West Indies but common in southern United States having tufted wiry stems often infested with a dark fungus
drop-seed: a grass of the genus Sporobolus

Other hypernyms of carpet grass include dropseed.

carpet grass (noun)	drop-seed (noun)
grass native to West Indies but common in southern United States having tufted wiry stems often infested with a dark fungus	a grass of the genus Sporobolus