Phragmites australis is a helophyte or marsh plant that grows partly submerged in water. It sprouts new leaves above the water, budding and rooting in the water and spreading along the edge of the water, partly on land in the damp or seasonally damp earth. It tends to invade where conditions are favourable, usurping tracts of lowland that tend to marshy conditions.
These reeds will be grazed by livestock when the foliage is young. They will do this regularly and may keep it short if allowed. Areas infested by the plant can sometimes be regained by grazing of new sprouts once old stems have been removed. This does not work in the water though (Vlok and Schutte-Vlok, 2010; Wikipedia).