Arguably one of the most common and widespread plants on the planet is Common Reed (Phragmites australis), a large grass (up to 15 feet tall) found in marshy areas throughout all but the most northern reaches of North America, as well as in temperate and tropical regions around the rest of the world. Known for its dense, erect growth form and showy inflorescence plumes, Common Reed is the target of land stewards across the continent, as it rapidly spreads by rhizomes, creating monocultures and outcompeting native vegetation. It is especially prevalent in areas with high nutrient levels, in areas of high salinity (such as roadsides), and in places where the soil has been disturbed. The rhizomes by which is spreads are particularly impressive; I've seen the plant coming up through asphalt under which its roots have spread. It was once thought that Common Reed did not spread by seed, but only by these rhizomes; we now know that Common Reed does in fact spread to new locations by seed, and that it then reproduces rapidly at that new location through vegetative reproduction.
As a result of recent research, three subspecies of Common Reed have been described. One of these, Phragmites australis ssp. australis, is the strain native to Europe that was introduced in ballast material in the late 18th century. This subspecies has since invaded wet areas across North America. Phragmites australis ssp. americanus is native to the United States and Canada, though it is being replaced in the eastern United States by the introduced European subspecies. The third subspecies, P. australis ssp. berlandieri, is known from Florida to California and south, but its nativity in the United States portion of its range is in question. It is definitely thought to be introduced in Arizona and California.
In most of North America, we only have to deal with two of the subspecies, P. australis ssp. australis and P. australis ssp. americanus. There are several morphological characters that can be used to distinguish between these two.
In the non-native P. australis ssp. australis (above), the inflorescences are dense, bushy, and often purplish or golden in color. The native P. australis ssp. americanus (below) has inflorescences that are more sparse and diffuse.
There are additional floral characters used to distinguish between the two subspecies, including the length of the upper and lower glumes. The native subspecies has longer glumes than the non-native subspecies (lower 3.0-6.5 mm, upper 5.5-11.0 mm in the native; lower 2.5-5.0 mm, upper 4.5-7.5 mm in the non-native).
Examining the culm can also help to tell if you are looking at the native or non-native strain of Common Reed. The non-native P. australis ssp. australis (above) has dull, slightly ridged culms that are mostly green. It also never has the characteristic black spots (formed by a native fungus that has adapted to the native subspecies) that are sometimes seen on the native subspecies. The native P. australis ssp. americanus (below) has shiny, smooth culms that are often pigmented red.
The sheaths of P. australis ssp. australis remain tight on the plant when the plant senesces, whereas those of P. australis ssp. americanus become loose and fall off of the stem rather easily. Stems of the non-native subspecies are often in dense colonies, and they persist through the winter into the next season. Stems of the native subspecies, conversely, are often more sparse and intermixed within other native vegetation, and they have a tendency to not persist into the following spring.
Leaf color can be used in conjuction with other characters to distinguish between the native and non-native subspecies of Common Reed. The non-native strain (above) has leaves that are more commonly dark green, whereas the leaves are more commonly yellow-green in the native strain (below). Ligule length is a consistent character, with longer ligules (more than 1.0 mm) on P. australis ssp. americanus and shorter ligures (less than 1.0 mm) on P. australis ssp. australis.
As additional research is conducted, there will likely be changes to the Latin names of these subspecies, and additional new subspecies may be described.
For more information, be sure to check out Phragmites Field Guide: Distinguishing Native and Exotic Forms of Common Reed (Phragmites australis) in the United States by Jil Swearingen and Kristin Saltonstall.