Agrostis perennans (Autumn Bentgrass)

Plant Info
Also known as: Upland Bentgrass
Genus:Agrostis
Family:Poaceae (Grass)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; moist to dry soil; floodplain forest, creek banks, wooded slopes, thickets, seeps, rock outcrops, cliffs
Fruiting season:August - September
Plant height:8 to 30 inches
Wetland Indicator Status:GP: FAC MW: FAC NCNE: FACU
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

Pick an image for a larger view. See the glossary for icon descriptions.

Detailed Information

Flower: Flower shape: indistinct Cluster type: panicle

[photo of flowering panicle] 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.

[close-up of spikelet] 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: Leaf attachment: alternate Leaf type: simple

[photo of leafy vegetative shoot] 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.

[photo of sheath, ligule and node] 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: Fruit type: seed without plume

[photo of mature florets] 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.

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More photos

Photos by K. Chayka and Peter M. Dziuk taken in Chisago County and in Wisconsin.

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