Phragmites australis is a robust, perennial reed  up to 4 m high, with long rhizomes. Culm is solitary, not tillering.
The leaf blade with a filiform apex, flexuous, 350 x 35 mm. Leaf sheaths remain on the culm after the leaves have fallen off. Inflorescence is compact, 120–400 mm long, with a ring of silky hairs at the base of lowest branches. The spikelet is 10–18 mm long; upper glume (the bracts at the base of the spikelet) 6–9 mm long and is acute; lemma (encloses grass flower) glabrous; callus hairs 6–10 mm long; anther 1.5–2.0 mm long. Flowering time: December to June.

According to the Wikipedia, Phragmites australis, can form stands  as much as much as 1 square kilometre or more in extent where conditions are favourable. It can also spread  at 5 metres  or more per year by horizontal runners. As well as growing in damp ground, it can grow  in standing water up to 1 metre  or so deep, or even as a floating mat.  It also tolerates brackish water and can grow near the sea in estuaries or salt marshes.  It can be very invasive, but can be suppressed by grazing with livestock or repeated burning.  (http://en.wikipedia.org/wiki/Phragmites: consulted 12/01/2015)