In an acidic graminoid fen. The large lemma tends to split on dissection into two sections surrounding the palea. Note the interesting extension of a rachilla with a tuft of hairs only at the tip creating two tufts of hairs attached to the floret. There is a tiny patch of extremely short pubescence right at the callus in between the two tufts of longer hairs. @threepogonias thought my observation of this species should be keyed out, so here it is. To me it seems quite distinct in the field. There is a nice illustration in Flora of North America of this species here: http://dev.floranorthamerica.org/w/images/3/3a/FNA24_P275_Calamagrostis_pg_715.jpeg
On a silver linden street tree, in Brooklyn, NYC.