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Both subsp. americanus (native to North America) and subsp. australis (introduced to North America) are found at this site, sometimes growing intermixed. In the photos the former is indicated with an “N” and the latter with an “I”. On the date I visited this site, 16 September 2019, these are the differences I observed with living stems between the two subspecies:

•Relative stem height: subsp. americanus usually shorter, subsp. australis usually taller.
•Relative stem spacing: subsp. americanus usually more dispersed, subsp. australis usually denser (rarely solitary).
•Stem breaking: subsp. americanus fibrous and hard to break off, subsp. australis often breaks off easily.
•Stem color (lower/older internodes): subsp. americanus red and often shiny, subsp. australis yellow-green and not shiny.
•Stem texture (lower/older internodes): subsp. americanus smooth, subsp. australis minutely ridged.
•Stem & leaf sheath dots: subsp. americanus often present, subsp. australis absent.
•Leaf condition: subsp. americanus has lots of senescence, subsp. australis generally are healthy.
•Margins of upper leaves near panicle: subsp. americanus finely serrated, subsp. australis coarsely serrated.
•Sterile stems more abundant than fertile: subsp. americanus yes, subsp. australis no.
•Panicle branches for panicles of similar length: subsp. americanus fewer, subsp. australis more.
•Panicle branch spacing: subsp. americanus farther, subsp. australis closer.
•Flowering status: subsp. americanus finished flowering, subsp. australis flowering.

There probably are other characteristics I overlooked that distinguish these two subspecies from each other. Perhaps these characteristics listed above hold up at other sites in North America. I don’t use leaf color to tell them apart because although some clones of subsp. australis are strongly blue-green, some clones of that subspecies are more green or yellow-green. See this observation for an illustration: https://www.inaturalist.org/observations/30610594

Observation of P. australis subsp. americanus from this location: https://www.inaturalist.org/observations/32878566

Observation of P. australis subsp. australis from this location: https://www.inaturalist.org/observations/32888630

the introduced ssp. parviflora, which iNaturalist doesn't recognize. Twice as large as the native and broader habitat tolerance (various old field and disturbed vs. just rocky rivershores and saltmarsh margins).