A member kindly provided the relevant part of the key to the species:

Culms robust, 2-4 m. (or more) tall; leaves 15-30 mm. broad; panicles 8-30 cm. long, usually more than 15 cm.; lemmas alike; outer bristles 5-6 mm. long, inner ones longer and sparsely short-hairy at base; one longer and stouter than all the others……………………………………………….P. purpureum Schumacher (Elephant Grass; Napier Grass)

Culms usually less tham 2.5 m. tall; leaves 3-18 mm. wide, panicles 10-25 cm. long, usually about 12 cm.; lemmas unlike; outer bristles 3-4 mm. long, inner ones plumose-villous at base with silky hairs, one longer but several others nearly as long ……………..
Spikelets 3.5-4 mm. long; infloresence about 1 cm. thick; perennials ………………..P. setosum (Swartz) L.C. Richard
Spikelets 5 mm. long; inflorescence 1.3-2.5 cm. thick; annuals……….P. polystachyum (L.) Schultes (Mission Grass).

It was thought to be P. purpureum (Elephant grass).