Arundo donax has those kind of "ears" at the base of each leaf.

here is a link to some fairly acurate pics, a german site, but you can see the typical hair at the base of each leaf.

details Phragmites (German)

could it have been transportet with soil, like a rhizome bit?

I'm David, not Dan! (though I know many distinguished Dans, and would be happy to be one!)

I lean towards Phragmites, simply because of the better likelihood of having be a seed drop. Either way, I don't want a giant aggressive non-native grass spreading in this wetland area (lowest point on my property, you see behind it a Nyssa aquatica I planted!) so it will get sprayed with the next batch of glyphosate I mix.

Oh I found this interesting document

https://bugwoodcloud.org/mura/mipn/assets/File/Educational%20Resources/Phragmites%20Field%20Guide.pdf

The leaves on mine have a definite bluish cast, so I'm sure it's the invasive form.

That's good David. Phragmites is a terrible invasive that pushes out all other plant species. I heard cutting back the reeds and applying herbicide to the areas around the cuts works well. Good luck David!

Thank you, Jay!

You're welcome David!