quack grass vs quackgrass

quack grass and quackgrass both are nouns.

Word	Noun	Adjective	Verb	Adverb
quack grass	Yes	No	No	No
quackgrass	Yes	No	No	No

As nouns, quack grass and quackgrass are synonyms defined as:

quack grass and quackgrass: European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed

Other synonyms of quack grass include Agropyron repens, couch grass, dog grass, quick grass, witch grass, witchgrass.

quack grass (noun)	quackgrass (noun)
European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed	European grass spreading rapidly by creeping rhizomes; naturalized in North America as a weed