Identificaciones de mertensia

I thought this was Andropogon glomeratus, but it appears there's an introduced lookalike, A. tenuispatheus. Does anyone know how to tell the difference?

Edit: Just a note: This grass seems to be locally common in the Piedmont, AL area. FSUS says it's uncommon in the Alabama mountains, but I have seen a lot of it growing around the city, especially in eastern Piedmont near Vigo.