Phragmites australis, common reed, commonly forms extensive stands (known as reed beds), which may be as much as 1 square kilometre or more in extent. Where conditions are suitable it can also spread at 5 m or more per year by horizontal runners, which put down roots at regular intervals.[citation needed] It can grow in damp ground, in standing water up to 1 m (3 ft 3 in) or so deep, or even as a floating mat. The erect stems grow to 2–4 metres (6 ft 7 in–13 ft 1 in) tall,[3] with the tallest plants growing in areas with hot summers and fertile growing conditions.