Arundo donax vs. Phragmites australis

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As explained in the Arundo page in the non-native organisms section, Arundo donax is a very large non-native grass that is invading Cuatro Ciénegas.  Its distribution in the valley is currently limited, but expanding.  Phragmites australis is a slightly smaller grass, apparently native to the area.  It was listed in the flora of Cuatro Ciénegas as being uncommon along waterways (Pinkava 1979), but it is currently very common.  In other parts of North America an invasive genotype of P. australis has had impacts similar to those that Arundo is having elsewhere. It is possible that the rapidly expanding P. australis in Cuatrociénegas may similarly represent an undocumented invasive form, but genetic analyses have not yet been done. ( More information on invasive Phragmites australis; Morphological differences between native and invasive genotypes of Phragmites australis)

In general, Arundo donax is taller that Phragmites, with longer leaves, and with a longer, sometimes more compact-appearing and more erect inflorescence, while Phragmites australis is a little shorter, with shorter and more narrow leaves, and a shorter inflorescence, that often looks more open, and leans to one side.

 

Arundo donax L. Giant Reed	Phragmites australis (Cav.) Trin. ex Steud. Common Reed
Synonyms Arundo donax L. var. versicolor (P. Mill.) Stokes Arundo versicolor P. Mill.	Synonyms Phragmites australis (Cav.) Trin. ex Steud. var.        berlandieri (Fourn.) C.F. Reed Phragmites communis Trin. Phragmites communis Trin. ssp. berlandieri             (Fourn.) A.& D. Löve Phragmites communis Trin. var. berlandieri      (Fourn.) Fern. Phragmites phragmites (L.) Karst.
Description Stout culms in large clumps, to 5 cm (2 in) thick, 2 to 6 m (to 20 ft) tall, with rhizomes.	Description Erect culms, to 2.5 cm (1 in) thick, 2 to 4 m (to 16 ft) tall, with rhizomes and stolons.
Leaf blades to 91 cm (3 ft) long, to 5 to 7 cm (2 to 3 in) wide.  Blades numerous, conspicuously distichous, evenly spaced.	Leaf blades to 60 cm (2 ft) long, 1 to 5 cm (0.5 to 2 in) wide.
Ligules large, papery, small hairy margin.	Ligules evident, thin, margin with long, soft hairs.
Panicle dense, erect, 30 to 60 cm long.  Spikelets 12 mm long.	Panicle 15 to 40 cm long.  Spikelets 12 to 15 mm long.
Hairy lemmas and smooth rachillas.	Smooth lemmas and hairy rachillas.
Click here for more photos that help distinguish Arundo and Phragmites