quack grass or couch grass, Old World perennial grass (Agropyron repens), now widely distributed and in the United States a troublesome weed. It somewhat resembles a beardless wheat and has creeping, yellowish rootstalks, the joints of which, even though detached, are capable of producing new plants; thus quack grass is a good soil binder but extremely difficult to eradicate. The dried sweetish rootstalks have been used medicinally, and the foliage is useful for forage. Quack grass is classified in the division Magnoliophyta, class Liliopsida, order Cyperales, family Gramineae.