Common couch is characterised by having creeping, white underground stems (rhizomes), shoots with small auricles and a short ligule at the base of the leaf blade.
The heads (spikes) superficial resemble perennial rye-grass, but the spikelets sit flat against the stem rather than with the narrow, rounded side adjacent to the stem as in rye-grass.
Black bent also has rhizomes but has a long ligule, no auricles and the heads are large, loose panicles. Creeping bent is somewhat similar to black bent, although mature plants are shorter and produces creeping stems (stolons) on the soil surface, not underground rhizomes. Creeping soft-grass produces rhizomes, has conspicuously hairy nodes on the stem and a compact panicle.
Onion couch looks like false-oat grass with large loose panicles producing relatively large seeds with awns. However, the stem bases are swollen to form several round, onion-like corms, up to 1 cm in diameter, hence the common name.