Plants perennial; cespitose and shortly rhizomatous. Culms (6)25-100 cm, 0.5-3.5 mm thick, spreading to weakly ascending, usually freely branching; nodes pilose, hairs spreading to ascending; internodes pilose, with papillose-based hairs, or sparsely hispid. Sheaths rounded, glabrous, margins shortly ciliate distally; ligules 0.6-4 mm, of hairs; blades(3)6-15 cm long, 1-5 mm wide, spreading, abaxial surfaces sparsely hirsute, adaxial surfaces more densely so, hairs papillose-based, midribs prominent and not white, margins involute, bases cuneate, apices subulate. Terminal panicles 3-35 cm long, about 1/2 as wide, shortly exserted; axillary panicles smaller, not fully exserted; rachises scabridulous; primary branches usually solitary, sometimes paired, divergent and widely spaced, secondary branching mostly on the distal 1/3 of the primary branches; pedicels 1-4 mm, spreading to appressed, confined to the distal portions of the secondary branches. Spikelets 2.1-2.9 mm long, 0.8-1 mm wide, glabrous. Lower glumes 1-1.2 mm, to 1/2 as long as the spikelets, attenuate, 5-9-veined, veins anastomosing apically; upper glumes and lower lemmas similar, extending about 0.5 mm beyond the upper florets, 11-13-veined; lower florets sterile; lower paleas 1-1.3 mm; upper florets 1.5-1.8 mm long, 0.6-0.8 mm wide, smooth. 2n = 36.
Panicum diffusum grows along river banks, ditches, and disturbed areas in wet, loamy or clayey soils. Its range extends from Texas to the Caribbean and northern South America.