crab grass vs crabgrass

crab grass and crabgrass both are nouns.

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

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

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

Other synonyms of crab grass include finger grass.

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