It is very easy to mismatch Phalaris arundinacea or aquatica with common weeds. Search some images of these species if you don't believe me.

Agropyron repens
Alopecurus myosuroides
Apera spica-venti
Avena fatua
Bromus secalinus
Cynodon dactylon
Digitaria sanguinalis
Echinochloa crus-galli
Poa annua
Setaria glauca
Setaria viridis
Sorghum halepense

These are all common weeds (at least for Europe) and not worth a damn lot
Hope you are having better luck then i am. Cheers!