Paspalum setaceum (Hairy Beadgrass)

Plant Info
Also known as: Hairy Lens Grass, Thin Paspalum, Slender Paspalum
Genus:Paspalum
Family:Poaceae (Grass)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; open, sandy soil; savanna, roadsides, woodland edges, disturbed soils
Fruiting season:July - September
Plant height:15 to 40 inches
Wetland Indicator Status:GP: FAC MW: FACU 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: Cluster type: raceme Cluster type: spike

[photo of spike just past flowering] 1 to 3 (rarely more) spreading to ascending spike-like racemes, at the tip of the stem and arising singly or in pairs from the upper leaf axils. Sometimes all or part of a raceme is hidden, tucked inside the leaf sheath. Racemes are up to 5 inches long with a flat, ribbon-like central stalk (rachis). Dozens of spikelets (flower clusters) are arranged in two rows along the rachis, usually in pairs and often crowded. Spikelets are short-stalked, 1.4 to 2.6 mm long, round to oval with a slightly pointed tip, flattened on one side like a split pea, and each with a single fertile, perfect floret (both male and female parts) and one sterile floret.

[photo of spikelets] Grass spikelets typically have a pair of glumes (bracts) at the base of a spikelet, but the lower glume is absent or obscure on this species. The upper glume is thin, light green, as long and as wide as the spikelet, smooth to minutely hairy, and 3-veined. The lower floret is sterile surrounded by a pair of bracts (lemma and palea), the lemma thin and about the same size as the upper glume, with or without a distinct midvein, and the palea absent or obscure. The upper floret is fertile, the lemma and palea both thicker than the glume, round to elliptic with edges curled around the floret (see mature fruit photo below).

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaf clump] Leaves are crowded on the lower part of the stem with a few more widely spaced above. Leaves are flat, erect to floppy, up to 10 inches long and about ½ inch wide or less. Depending on the variety, color is yellowish-green or light to dark green, and surfaces and edges are variably covered in short, soft hairs and/or long, spreading hairs.

[photo of sheath, ligule and node] The sheath is open, the edges with a fringe of hairs and overlapping in the front at or near the tip. Depending on the variety, the sheath surface is hairless or variably covered in long, spreading hairs. The ligule (membrane where the leaf joins the sheath) is short and membranous with a band of white hairs behind it. Nodes are hairless. Stems are hairless and mostly erect, sometimes spreading, forming clumps of a few to several stems from short rhizomes.

Fruit: Fruit type: seed without plume

[photo of dissected mature spikelet] As spikelets mature the glume and sterile lemma become papery but the fertile lemma and palea harden, locking the grain (seed) inside. Spikelets turn straw-colored to light brown with age, the entire spikelet falling off at the tip of its short stalk, leaving a naked rachis behind.

Notes:

Paspalum is a large genus, with at least 65 species in North America, though taxonomy is more than a little confusing. Fortunately, P. setaceum is the only one of this genus in Minnesota and nothing else in the state looks quite like it. Unfortunately, there are as many as 9 varieties, not all of which are universally recognized, with some references treating some of these as separate species and others lumping them all together with no vars. Variable traits are leaf width and color, degree of hairiness and whether hairs are long or short, whether the sterile lemma has a distinct midvein, and whether stems are mostly erect or not. Of the 2 varieties recognized in Minnesota, var. muhlenbergii is the more common, distinguished by green leaves up to ½ inch wide that are more or less evenly covered in long, spreading hairs, and spikelets with a distinct midvein on the sterile lemma. By contrast, var. stramineum has yellowish-green leaves not more than ¼ inch wide that are short-hairy with few long hairs (if any), and the sterile lemma lacks the midvein. Both are said to grow more or less erect. Based on the leaf hairs all of our images should be var. muhlenbergii, but we have yet to see a distinct midvein on any population we've encountered, except perhaps on just 1 or 2 spikelets out of an entire raceme. We'll be on the lookout for it.

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More photos

Photos by K. Chayka taken in Anoka and Ramsey counties. Photos courtesy Peter M. Dziuk taken in Dakota and Wabasha counties.

Comments

Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: Kris S - Sherburne County
on: 2016-09-28 14:16:56

Came across this grass here at Sherburne NWR west of Zimmerman, MN. It was in a piece of remnant Oak Savanna. Neat seed head!

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