Sunday, July 31, 2022

How to Identify Native and Introduced Phragmites

A colony of Phragmites grasses with last year's panicles.
Phragmites australis subsp. americanus. Photo taken in early August in southern Minnesota.













Update: An abbreviated ID guide (PDF) is here

Phragmites or Common Reed, Phragmites australis, is a 12- to 18-foot tall, perennial grass of wetlands, shorelines and ditches. Two subspecies are common in the U.S. One is the native subspecies americanus; the other is the introduced subspecies australis.

Both subspecies grow in colonies, but subspecies australis is more aggressive and can dominate habitats to the near exclusion of other plants. For that reason, there is growing interest in identifying and mapping subspecies australis to determine the extent of its spread and to target those colonies for removal.

Although the subspecies look alike, there are several characteristics that, taken together, can identify one from the other. The most reliable characteristics are shown below, after a few terms used to describe grass stems, leaves and flowers.

Stems and leaves

A panel of two photos labeling the culm, blade, sheath and ligule.














The culm is the stem of a grass. Grasses have round culms that are hollow between the nodes, the swollen areas on a culm. The leaf blade is flat and ribbon-like, whereas the leaf sheath wraps around the culm below the blade. Where the blade meets the sheath, most grasses have a ligule, a membrane or hairy fringe visible when the blade is pulled away from the culm.

Flowers

Grass flowers, called florets, are specialized for wind pollination. Each floret is composed of two narrow bracts, a lower lemma and an upper palea. Stamens and feathery stigmas emerge from between them.

Florets are arranged in small spikes called spikelets. At the base of each spikelet are two bracts called glumes. Glume length differs between the subspecies of Phragmites. More on that in a following section. 

Spikelets of Smooth Brome, Bromus inermis, are shown below. On the left are spikelets with brown anthers and feathery, white stigmas emerging from individual florets. On the right is a single spikelet spread apart to see the florets and glumes. The spikelets are about  3 cm (1.5 in.) long. 

A panel of two photos showing spikelets of smooth brome, with glumes and florets labeled.


Phragmites characteristics

To identify Phragmites subspecies, it’s best to look at more than one plant in a colony and at several characteristics of each plant. Hybrids are possible, but they’re said to be rare. 

Ligules 

To see ligules, pull back a blade from the middle third of the culm. (Ligules may be immature on the upper third of the culm and degraded on the lower third.) Ligule length differs between the subspecies. 

Subspecies americanus: 1-2 mm long, brown, become darker and smudgy in late summer and fall. The photo below was taken in early August.

Subspecies australis: 0.5-1 mm long, appearing as a thin, brown line. The photo below was taken in early July.

A panel of two photos showing the ligules of subspecies americanus and australis.



Stem color and texture
In summer and fall, examine the base of the culm. Be sure to look at the culm and not the sheath, if one is present. The photos below are from early July.

Subspecies americanus: Lower culm is smooth, somewhat glossy, often red.
Subspecies australis: Lower culm is ridged, not glossy, often green fading to brown.

The smooth, red lower stem of subspecies americanus and the green, ridged lower stem of subspecies australis.


Sheaths

In late summer, fall and early winter, examine the lower stem for the presence of leaf sheaths. The photos below are from mid-November.

Subspecies americanus: Sheaths are absent or easily removed.
Subspecies australis: Sheaths are persistent and harder to remove. 

The bare stem of subspecies americanus compared to the sheathed stem of subspecies australis in November.


Glumes

In late summer and fall, measure glume length. By late fall some of the florets may be gone, but the glumes often persist. It's best to look at several pairs of glumes to get an idea of their average length. The photos below are from mid-November.

Subspecies americanus: Lower glume 3-6 mm long (most > 4mm); upper glume 5-11 mm long (most > 6 mm).
Subspecies australis: Lower glume 2.5-5 mm long (most < 4 mm); upper glume 5-8 mm long (most < 6 mm).

The glumes of subspecies americanus and australis along a metric ruler.



Panicles

In late winter, spring or early summer, look at last season's panicles, the plume-like clusters of Phragmites spikelets. The photos below are from early July.

Subspecies americanus: Panicles bare, thinner, less branched.
Subspecies australis: Panicles fuzzier, thicker, more branched.

A panel of two photos contrasting the panicles of subspecies americanus and australis.


Look-alikes

Amur Silver Grass and Reed Canary Grass are two smaller grasses that can be mistaken for Phragmites. 

Amur Silver Grass, Miscanthus sacchariflorus, is an introduced grass that has silvery-white panicles in late summer and fall. It grows 6-8 feet tall, shorter than Phragmites, and its leaf blades have a white midrib. Its ligules are a greenish-white, hairy fringe. The photos below were taken in early August.

A panel of two photos showing a colony and a ligule of Amur Silver Grass.
Amur Silver Grass plants and ligule.



Reed Canary Grass, Phalaris arundinacea, blooms in spring, not late summer, with smaller panicles that eventually contract. It's shorter than Phragmites, growing up to 5 feet tall. Its ligules are membranous and 3-8 mm long. 

A panel of two photos showing a colony and a ligule of Reed Canary Grass.
Reed Canary Grass colony and ligule.




References

Chadde, S.W. 2012. Wetland Plants of Minnesota. 2nd edition (revised). A Bogman Guide.

Judziewicz, E. J., Freckmann, R.W., Clark, L.G., and Black, M.R. 2014. Field Guide to Wisconsin Grasses. The University of Wisconsin Press, Madison.

Minnesota Aquatic Invasive Species Research Center (MAISRC). Identifying invasive Phragmites. Website accessed July 2022.

Swearingen, J., Saltonstall, K., and Tilley, D. Phragmites Field Guide: Distinguishing Native and Exotic Forms of Common Reed (Phragmites Australis) in the United States. Technical Note Plant Materials 56, October 2012. USDA Natural Resources Conservation Service, Boise, ID.





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