slough-grass

from The Century Dictionary.

• noun A stout subaquatic perennial grass, Beckmannia erucæformis, found in Europe and Asia, and in North America from Ontario to the Pacific, reaching north to Alaska. It grows in sloughs and along streams, and in some places in the northwest makes an important part of the forage of lowlands. It bears narrow one-sided spikes suggesting the rattle of a rattlesnake, whence sometimes called rattlesnake-grass. Also called nit-grass and wild timothy.