Each rachis of water paspalum (Paspalum hydrophilum) lacks the wings characteristic of brook paspalum and more closely resembles those of knotgrass.
Water paspalum, like brook paspalum (P. acuminatum), has glabrous (hairless) nodes. This separates them from knotgrass (P. distichum).
Leaves of water paspalum have a wide, distincly white midvein. None of the other key perennial grasses infesting rice fields have this feature. The midvein of brook paspalum leaves is sometimes white too, but not as conspicuously.
One of the most distinctive features of water paspalum is its hollow stem.