A tall, coarse, perennial grass with stout rhizomes. It grows in dense clumps or nearly solid stands. Leaves are smooth, 6-20 inches long, and have a white or light green midvein. Stems are pink to rusty red near the base. Panicles (flower clusters) are large, loosely branched, purplish, and hairy. Spikelets (the small flowering units) occur in pairs or threes, and each has a conspicuous awn. Seeds are reddish-brown and nearly 1/8 inch long. Blooms June through November.
Similar species: Two native grasses resemble Johnson grass vegatively. The stems of gama grass (Tripsacum dactyloides) are usually flattened at the base, whereas Johnson grass stems are rounded. Silver plume grass (Erianthus alopecuroides) has long silky hairs on the upper surface of its leaves where the leaves meet the stem, while Johnson grass leaves are smooth.