Poa arida (Plains Bluegrass)
|Also known as:|
|Habitat:||sun; dry to wet; prairies, swales, shores|
|Fruiting season:||June - August|
|Plant height:||6 to 30 inches|
|Wetland Indicator Status:||GP: FAC MW: FAC NCNE: FAC|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Slender panicle to 4+ inches long, the branches usually erect, sometimes ascending, rarely spreading, with 1 to 5 branches (commonly 2 or 3) per node and only 1 to 3 branches on the lowest node. Spikelets (flower clusters) are short-stalked, yellowish green to purple-tinged, flattened, lance-elliptic in outline, 3.2 to 7 mm (to ~¼ inch) long with 3 to 7 florets; the uppermost floret may be sterile.
At the base of a spikelet is a pair of bracts (glumes) that are both lance-oblong, hairless, awnless, keeled, 3-veined, smooth to slightly rough along the keel, light green with transparent whitish edging, 2.5 to 4.5 mm long, the lower glume as long as or slightly shorter than the upper glume. Surrounding a floret is a pair of bracts (lemma and palea), the lemma 2.4 to 4.5 mm long, blunt to pointed at the tip, thin and whitish on the edges especially near the tip, keeled sometimes obscurely so, 5-veined, with long white hairs on the veins and usually between them on the lower half of the lemma, the hairs sparse to dense; the palea is slightly shorter and much narrower than the lemma, 2-veined, rough or hairy along the veins. The thickened base of the floret (callus) usually lacks any crinkled hairs; when present they are only about ¼ as long as the lemma.
Leaves and stems:
Leaves are mostly basal, the stem leaves few, alternate, widely spaced, up to 4 inches long, stiff and erect, 1 to 4 mm wide, hairless, mostly smooth, boat-shaped at the tip, usually folded; basal leaves are up to 12 inches long and flat or folded. Color is commonly silvery-green, at least when young.
The sheath is hairless and the edges fused on the lower 1/10 to 1/5 (closed sheath). The ligule (membrane where the leaf blade joins the sheath) is 1.5 to 4 mm long, usually pointed at the tip, and lacks a fringe of hairs. Nodes are smooth. Stems are unbranched, single or a few from the base, round to slightly compressed, mostly erect, sometimes prostrate from the base and rising at a lower node (geniculate). Plants form colonies from spreading rhizomes, but can also form loose clumps.
Spikelets turn straw-colored to light brown with maturity, the individual florets dropping away leaving the glumes behind persisting on the stalk. The grain (seed) is greenish, linear, 2.1 to 3.2 mm long.
Plains Bluegrass is a cool-season grass common in the Great Plains, but uncommon in Minnesota where it reaches the eastern edge of its range. While the typical habitat in most of its range is dry grasslands, in Minnesota it is usually found in moist to wet saline prairies in our western counties, with a single 1939 record from exposed granite on the shore of Rainy Lake in Koochiching County. Where it's been introduced farther east it is likely to be found along salted roadsides.
Plains Bluegrass is recognized by its colony-forming habit; unbranched stems; short, erect stem leaves to 4 inches long, usually folded, with boat-shaped tips; leaves commonly silvery-green when young; sheaths closed on the lower 1/10 to 1/5; ligule up to 4 mm long, usually pointed at the tip; panicle branches usually erect, only 1 to 3 branches on the lowest node; spikelets with 3 to 7 florets, lemmas to 4.5 mm long usually with dense white hairs on the major veins and sparse to dense hairs between the veins on the lower half of the lemma. The callus usually lacks any crinkled, cobwebby hairs, but when present, they are only about ¼ as long as the lemma.
Somewhat similar is Canada Bluegrass (Poa compressa), which is also rhizomatous but has distinctly flattened stems, sparse hairs on lemma veins and none between them, and usually has at least a few long, crinkly hairs on the callus.
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Photos by K. Chayka and Peter M. Dziuk taken in Lac Qui Parle County.
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