Flower:

Open panicle 4 to 10 inches long, usually about half as wide as long, lance to egg-shaped in outline, the main branches slender, spreading to ascending, with forking branchlets on the upper half to 2/3 of the branch. Spikelets (flower clusters) are single at branchlet tips, light green to yellowish at flowering time, 1.8 to 3.2 mm long, and have a single floret.

At the base of a spikelet is a pair of bracts (glumes) that are keeled, 1-veined, taper to a pointed tip, the lower glume 2.1 to 3.2 mm long, the upper glume usually somewhat shorter. Surrounding the floret is a pair of bracts (lemma and palea), the lemma papery thin, 5-veined, pointed to jagged to straight across at the tip, 1.2 to 2.3 mm long, as long as or somewhat shorter than the upper glume, and rarely with a short, slender awn arising from the middle of the back; the palea is minute (less than .1 mm long) or more often absent altogether. The thickened base of the floret (callus) is sparsely to densely covered in hairs .1 to .3 mm long.

Leaves and stems:

Leaves are mostly alternate with a few basal leaves that wither away by flowering time. The 3 to 10 stem leaves are 2 to 8 inches long, 1.5 to 5 mm (less than ¼ inch) wide, flat, hairless, smooth to slightly rough on both surfaces. Leafy vegetative stems are common.

Sheaths are hairless, smooth to slightly rough. The ligule (membrane where the leaf joins the sheath) is 1.5 to 7 mm long, the top edge pointed to straight across, often jagged or shredded, and lacks a fringe of hairs. Nodes are smooth. Stems are smooth, unbranched, single or a few from the base forming loose clumps, erect, stout in sunny locations but weak and tend to flop over in shadier habitats, and sometimes root at the lower nodes.

Fruit:

Spikelets turn straw colored to pale brown at maturity, the florets dropping away leaving the glumes behind persisting on the stalk. The grain is elliptic, 1 to 1.9 mm long.

Notes:

Autumn Bentgrass is occasional in a variety of habitats, but most often in shaded floodplains and creek banks. We ran across it in a wooded area along the St. Croix River in Franconia, where we go fishing. The widely forking branches of the panicle were what caught my eye (reminds me of a chemical diagram) but the divergence isn't always quite so pronounced. It is otherwise distinguished by the loosely clump-forming habit (lacking rhizomes or stolons); 3 to 10 stem leaves 2 to 5 mm wide, the lowest usually withered by flowering time; panicle branchlets along the upper half to 2/3 of a branch (where other Agrostis species have branchlets and spikelets more concentrated near branch tips); single-flowered spikelets 1.8 to 3.2 mm long, lower glume usually longer than the upper; lemmas thin and translucent, 1.2 to 2.3 mm long, rarely awned; paleas usually lacking; usually short hairs on the callus (magnification needed). While Agrostis is considered a cool season grass, Autumn Bentgrass is a late bloomer, not usually flowering until later in the summer or even early autumn.

Of the 5 Agrostis species known to be in Minnesota, A. gigantea and A. stolonifera both have paleas about half as long as the lemmas, where the others have paleas that are minute at best and more often lack them altogether (magnification recommended). Autumn Bentgrass most closely resembles Rough Bentgrass (A. scabra), which also lacks rhizomes, stolons and paleas, but its leaves are only 1 to 2 mm wide and panicle branches are very rough, where Autumn Bentgrass is smooth or nearly so.