Originally from Eurasia, exotic Phragmites australis ssp. australis (Figure 1) invades high quality and disturbed wetlands alike, reducing biodiversity by excluding other plant life and disrupting ecological function. It’s spread has been rapid, and land managers are hard-pressed to keep up.
In contrast, native Phragmites australis ssp. americanus has co-evolved with other native flora and fauna, has existed in Wisconsin for thousands of years, and does not typically reduce biodiversity or cause ecological disruption where it occurs. It most often forms either loose or localized colonies, which allow for the co-occurrence other species. The growth form of native Phragmites lends a unique element of physical structure to wetlands and the tissues of the plant support a number of insect species.
For these reasons, resources need to be directed towards the eradication of the exotic subspecies and not wasted eradicating stands of the native subspecies. The eradication of native Phragmites may simplify wetland food webs and the associated disturbance may create opportunities for truly harmful invaders.
While some may think otherwise, efficient identification of native and exotic subspecies isn’t especially difficult, if you know what to look for. Here I focus on characteristics most easily observed during the fall and winter. Fall and winter are arguably the best time to identify the native and exotic subspecies, because some of the most reliable characteristics for identification are most clearly observed on dormant plants.
Investigation of several characteristics is strongly advised, because interpretations of certain more qualitative characteristics can vary from person to person, and some plants may be ambiguous or atypical with respect to any single characteristic. However, if one considers several characteristics the determination will almost always be clear and direct. Hybrids between native and exotic subspecies would be problematic for identification, but hybrids are exceedingly rare (documentation in North America is in the low single digits).
Below I use material from a patch of exotic Phragmites australis ssp. australis and a patch of native Phragmites australis ssp. americanus, both from Scuppernong Springs in Waukesha County, Wisconsin, to illustrate winter differences between the subspecies.
1) The lower stems (culms) of the exotic subspecies are tightly covered in a leaf sheath that remains well into winter (Figures 2). In contrast, by late summer or early fall, the lower culms of native plants are without sheaths or any remaining dry sheaths are loose and very easily removed (Figure 3). This distinction should be a component of any identification taking place late enough in the growing season that remaining leaf sheaths have dried and are no longer green (middle/late summer through winter).
2) The lower stems (culms) of the native species TEND to be shinier in appearance and smoother in feel than the exotic (Figure 4). In the exotic, parallel ridges running up and down exposed culms give a slightly rough or textured feel. While these ridges are present in the native, they are less obvious.
3) Occasionally, the culms of the native will have distinct black spots formed by a fungus (Figure 5). This fungus is not known to colonize the exotic. The absence of these spots indicates nothing about whether the Phragmites is native or exotic, but the presence of spots indicates that the Phragmites is native. It is worth investigating several culms in a given patch for spots. These very distinct spots should not be confused with dark / black blotches caused by mildew (Figures 4 and 5) or injuries caused by insects, which may leave spots that are brown in color and / or irregular in shape.
4) Certain floral parts in the spikelets of the inflorescence tend to differ in average size between the native and exotic subspecies. The native tends to have first (lower) glumes between 3 mm and 6.5 mm in length, with most longer than 4 mm (Figure 7) and the exotic tends to have first (lower) glumes between 2.5 mm and 5 mm in length, with most shorter than 4 mm (Figure 8). Always measure several first glumes, ideally from a few different seed / flower heads) and take an average to avoid basing a decision on a potential outlier. This practice also helps ensure that you’ll notice if structures on one particular spikelet are damaged or missing in a potentially misleading way.
5) The native also tends to have longer second glumes than the exotic. Its second (upper) glumes tend to be between 5.5 mm and 11 mm in length, with most longer than 6 mm (Figure 9), and the exotic tends to have second (upper) glumes between 4.5 mm and 7.5 mm long, with most shorter than 6 mm (Figure 10). In Figure 10, the lower glume on the exotic spikelet is ambiguous, because it measures almost exactly 6 mm and falls within the range of both subspecies! This illustrates why, for second glumes just like first glumes, we should always measure several, ideally from a few separate seed / flower heads) and take an average to avoid basing a decision on a potential outlier. This plant happened to have about 50% glumes that were almost exactly 6 mm in length, but the remainder were 4-5.5 mm, so I considered the second glumes to be consistent with the exotic subspecies, particularly in the context of the first glumes, sheaths, and culms shown above.
Care needs to be taken to measure the glumes and not the lemmas, which are often still present in fall and winter. In addition, other floral parts may be present in the spikelet above glumes and lemmas, particularly in the fall. The first glume is nearest the base of the spikelet, and the second glume attaches immediately above and opposite the first glume. I recommend use of a hand lens to ensure measurement of the correct parts. The discussion of grass morphology in a field guide, such as Judziewicz and others’ Field Guide to Wisconsin Grasses should be helpful for learning these parts. Learning them in order to identify Phragmites will also expand your ability to identify grasses in general. Figure 11 shows exotic and native spikelets side by side.
6) The native tends to form loose stands in which other species of plants are able to grow (Figure 12). The exotic tends to form dense stands that are nearly monocultures or that greatly reduce the growth and vigor of co-occuring species (Figure 13). This is why the exotic is a problem. Apply this characteristic with caution, however, because exceptionally nutrient-rich sites, nutrient poor sites, and sites that generally cause the plants stress can lead to altered stand density and vigor.
Don’t judge the density of Phragmites patches from far away. A large patch of native Phragmites australis ssp. americanus can appear to be more dense than it truly is when viewed from the side and from a distance (Figure 14). Instead, get up close.
The below table summarizes and lists, from what I consider to be the most reliable (top) to least reliable (bottom), the characteristics for fall and winter identification of Phragmites discussed above. The size of the ligules is another distinguishing characteristic, but ligules are better used in summer, because they have largely deteriorated or the leaves they are on have been shed by late fall and winter, so that’s another post for another season.
Finally, some might be familiar with the tendency of the native to develop strikingly red lower culms during the growing season after the sheaths split open and / or fall off (Figure 15). This red color disappears when plants go dormant in fall, and it only develops in the first place as a consequence of the sheaths falling off and exposing the culms to sunlight. Therefore, I would stress observing the sheaths above all else, because the red coloration may be less apparent if the lower culms are shaded, and because, even with sheaths adhering tightly, the exotic strain can have reddish coloration in small exposed areas between the sheaths.
All of this for two subspecies of one species. Identification really does become quite simple with practice. After a while, you’ll feel like you can make the ID from a speeding car on the freeway. Resist that temptation. It’s usually best to get up close and personal the plants. Good luck!
Price, A.L., J.B. Fant, and D.J. Larkin. 2013. Ecology of Native vs. Introduced Phragmites australis (Common Reed) in Chicago-Area Wetlands. Wetlands DOI 10.1007/s13157-013-0504-z
Saltonstall, K., P.M. Peterson, and R. Soreng. 2004. Recognition of Phragmites australis subsp. americanus (Poaceae: Arundinoideae) in North America: evidence from morphological and genetic analyses. Sida 21(2):683-692.
Swearingen, J. and K. Saltonstall. 2010. Phragmites Field Guide: Distinguishing Native and Exotic Forms of Common Reed (Phragmites australis) in the United States. Plant Conservation Alliance, Weeds Gone Wild. http://www.nps.gov/plants/alien/pubs/index.htm