Phragmites is a genus of four species of large perennial grasses found in wetlands throughout temperate and tropical regions of the world.

The cosmopolitan common reed has the generally accepted botanical name Phragmites australis. (Cav.) Trin. ex Steud. About 130 other synonyms have been proposed, and some have been widely used.  Examples include Phragmites communis Trin., Arundo phragmites L., and Phragmites vulgaris (Lam.) Crép. (illegitimate name).

Recent studies have characterized morphological distinctions between the introduced and native stands of Phragmites australis in North America. The Eurasian phenotype can be distinguished from the North American phenotype by its shorter ligules of up to 0.9 millimeters (0.04 in) as opposed to over 1.0 millimeter (0.04 in), shorter glumes of under 3.2 millimeters (0.13 in) against over 3.2 millimeters (0.13 in) (although there is some overlap in this character), and in culm characteristics.

n North America, the status of Phragmites australis was a source of confusion and debate. It was commonly considered an exotic species and often invasive species, introduced from Europe. However, there is evidence of the existence of Phragmites as a native plant in North America long before European colonization of the continent. It is now known that the North American native forms of P. a. subsp. americanus are markedly less vigorous than European forms. The recent marked expansion of Phragmites in North America may be due to the more vigorous, but similar-looking European subsp. australis.

Observations Map

Phragmites australis, common reed, commonly forms extensive stands (known as reed beds), which may be as much as 1 square kilometer (0.39 sq mi) or more in extent. Where conditions are suitable it can also spread at 5 meters (16 ft) or more per year by horizontal runners, which put down roots at regular intervals. It can grow in damp ground, in standing water up to 1 meter (3 ft 3 in) or so deep, or even as a floating mat. The erect stems grow to 2–6 meters (6 ft 7 in–19 ft 8 in) tall, with the tallest plants growing in areas with hot summers and fertile growing conditions.

The leaves are long for a grass, 20–50 centimeters (7.9–19.7 in) and 2–3 centimeters (0.79–1.18 in) broad. The flowers are produced in late summer in a dense, dark purple panicle, about 20–50 cm long. Later the numerous long, narrow, sharp pointed spikelets appear greyer due to the growth of long, silky hairs. These eventually help disperse the minute seeds.

It is a helophyte (aquatic plant), especially common in alkaline habitats, and it also tolerates brackish water, and so is often found at the upper edges of estuaries and on other wetlands (such as grazing marsh) which are occasionally inundated by the sea. A study demonstrated that Phragmites australis has similar greenhouse gas emissions to native Spartina alterniflora. However, other studies have demonstrated that it is associated with larger methane emissions and greater carbon dioxide uptake than native New England salt marsh vegetation that occurs at higher marsh elevations.

Common reed is suppressed where it is grazed regularly by livestock. Under these conditions it either grows as small shoots within the grassland sward, or it disappears altogether. In Europe, common reed is rarely invasive, except in damp grasslands where traditional grazing has been abandoned.

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