crabgrass vs crab grass

crabgrass and crab grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
crabgrass	Yes	No	No	No
crab grass	Yes	No	No	No

As nouns, crabgrass and crab grass are synonyms defined as:

crabgrass and crab grass: grasses with creeping stems that root freely; a pest in lawns

Other synonyms of crabgrass include finger grass.

crabgrass (noun)	crab grass (noun)
grasses with creeping stems that root freely; a pest in lawns	grasses with creeping stems that root freely; a pest in lawns