Plants perennial; cespitose, shortly rhizomatous. Culms 60-200 cm, often swollen at the base, rooting at the lower nodes. Ligules 2-12 mm, truncate, lacerate; blades 5-15(20) cm long, 0.5-10 mm wide. Panicles 1.5-15 cm long, 1-2.5 cm wide, usually cylindric, sometimes ovoid, occasionally lobed at the base, the spikelets borne individually, not clustered; branches not evident. Spikelets homogamous, all spikelets with a bisexual floret; florets 2-3(4), occasionally with 2 bisexual florets, occasionally the terminal floret viviparous; disarticulation above the glumes, beneath the sterile florets. Glumes 4.4-7.5 mm long, 1.2-1.5 mm wide, keels winged distally, wings 0.2-0.4 mm wide, usually entire, lateral veins conspicuous, smooth; sterile floretsusually 1, if 2, the lowest floret much the shorter, to 0.7 mm, the upper or only sterile floret 1-3 mm, pubescent; bisexual florets 3.1-4.6 mm long, 1.2-1.5 mm wide, pubescent, stramineous, acute; anthers 3-3.6 mm. 2n = 28.
A native of the Mediterranean region, Phalaris aquatica now grows in many parts of the world, frequently having been introduced because of its forage value. Even where it is native, it usually grows in disturbed areas, often those subject to seasonal flooding. It is now established in western North America, being most common along the coast, and as an invasive in disturbed wet prairies with clay soils.
Phalaris aquatica can hybridize with other species of Phalaris. The stabilized polyploid hybrid with P. minor, P. ×daviesii S.T. Blake, is cultivated as a forage grass in Australia, Africa, and South America. The hybrid with P. arundinacea, P. ×monspeliensis Daveau, is also a good forage grass. Although the name 'Toowoomba Canarygrass' has been applied to Phalaris×monspeliensis in North America, Ross (1989) states that it should be applied to P. aquatica. The English language name used here is more descriptive of the species and is the name used by the U.S. Department of Agriculture for P. aquatica.