Tentative ID, I did not expect this to be so difficult in winter but this is from a known population in the area. Photos 1-5 show the plant in comparison to a large invasive phragmites plant which I cut maybe 6 inches from the base. To help prevent confirmation bias I also compared this plant to two smaller invasive phragmites plants (photos 6-9) of which I cut at the base. In these photos the tentative P. australis americanus is the right-hand plant.

All features I noticed were different were consistent across each comparison. The most distinctive difference was that at the jointed area of plants (nodes?), invasive phragmites was smooth whereas americanus was not and had a slight lip. The other was that the base of the plants was noticeably redder than the comparatively tanner/browner invasive phragmites. Note the photo comparing size between the two. Invasive phragmites was several feet taller. The sheaths seemed a bit looser but it was tough to be sure, most were missing from the suspected americanus. The suspected americanus seemed to be a bit shinier whereas the other seemed to have a slightly duller sheen, but it definitely wasn't a day and night difference between the two in the diffuse, overcast light.

The one thing that made me a little more confident on the ID which was unfortunate is there seems to be some sort of arthropod which is specific to americanus damaging this patch and causing >95% to fail to fruit. At first I thought it had been cut or browsed but my third to last picture shows webbing and frass. This damage was not present in any invasive phragmites so far as I could tell. My last two photographs show the extent of this damage.

Though I cut several of the invasive stalks to compare it to, I did not cut or otherwise damage the suspected native variety, and was careful to not step on any plants.