ground cover vs underbrush

ground cover and underbrush both are nouns.

Word	Noun	Adjective	Verb	Adverb
ground cover	Yes	No	No	No
underbrush	Yes	No	No	No

As nouns, underbrush is a hypernym of ground cover; that is, underbrush is a word with a broader meaning than ground cover:

ground cover: small plants other than saplings growing on a forest floor
underbrush: the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest

Other hypernyms of ground cover include undergrowth, underwood.

ground cover (noun)	underbrush (noun)
small plants other than saplings growing on a forest floor	the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest
low-growing plants planted in deep shade or on a steep slope where turf is difficult to grow