Crab and Quack are too totally different grasses.
Quack is perennial and spreads by seeds and rhizomes.
Crab , the type that is a lawn weed, is annual and reseeds itself like annual Rye Grass.
Now Quack, can turn into a lawn but if some one wants a mono-culture lawn, it sticks out like a sore thumb.
The problem is Quack will move out of the lawn and into the garden, if not by rhizomes by letting go to seed and mowing thereby blowing the seeds into the garden.
