Panicum maximum is a perennial, tufted grass with a short, creeping rhizome. The stems of this robust grass can reach a height of up to 2 m. As the stems bend and nodes touch the ground, roots and new plants are formed. The leaf sheaths are found at the bases of the stems and are covered in fine hairs. It remains green until late into winter. The leaf blades are up to 35 mm wide and taper to a long fine point. The inflorescence is a large multi-branched, open panicle with loose, flexuous branches. The lower branches of the inflorescence are arranged in a whorl. The lower floret is usually male with a well-developed palea (upper bract enclosing flower) (Gibbs Russell et al .1991). The fertile (female) upper lemma is pale. Spikelets are green to purple and flowering occurs from November to July.

This species varies in size and hairiness and may also vary to a lesser extent in growth habit. There are distinct forms of Panicum in South Africa, but the transversely wrinkled upper floret or seed of P . maximum, distinguishes it from all other Panicum species.