ground cover vs underwood

ground cover and underwood both are nouns.

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

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

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

Other hypernyms of ground cover include underbrush, undergrowth.

ground cover (noun)	underwood (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