Setaria parviflora (Poiret) Kerguélen, knotroot bristlegrass. Perennial herb, rhizomatous, fibrous–rooted, several–stemmed at base, ± cespitose (tufted), ascending, in range to 120 cm tall; shoots typically bent (geniculate) at each node; rhizomes to 40 mm long, knotty with dormant buds or spheric swellings at base of active shoots; adventitious roots from rhizomes and at lower nodes of shoot.
Stems (culms) ± cylindric to elliptic in ×–section, to 3 mm diameter, tough, sometimes with groove along entire length of internode, glabrous; internodes narrowly hollow.
Leaves alternate distichous, simple with sheath; sheath open, 45—130 mm long, angled along midvein, margins narrowly membranous and generally overlapping basally, V–shaped at throat, without lobes (auricles) at top, glabrous; ligule truncate, 0.5—1.3 mm long, membranous on lower 1/2—2/3 and densely ciliate–hairy on margin; blade linear, 35—200(—250) × 2—4.5(—8) mm, keeled, long–tapered above midblade, V–shaped in ×–section at base becoming inrolled (involute) toward tip, entire at base to minutely toothed on margins above midblade, narrowly prowl–like at tip, dull, parallel–veined, upper surface sparsely long–pilose at base with hairs 2—6 mm long.
Inflorescence spikelets, in terminal, spikelike panicles, panicle cylindric, densely flowered, with conspicuous bristles (= reduced branchlets) > spikelets, in range 25—70 mm long, 3.5—4.5 mm diameter (excluding bristles) and 5—7 mm diameter (including bristles), spikelet with 2 florets, lower one sterile (staminate) and upper one bisexual, bracteate, lacking awns; principal rachis ridged, pubescent with erect or ascending hairs mostly along ridges; lateral branches ± pseudowhorled, each with 1 spikelet at tip to several spikelets below, with a glossy, axillary pulvinus; stalk of spikelet < 0.3 mm long, expanded and saucerlike at tip, bearing (4—)6—7(—12) bristles in 2, often unequal, clusters at base of stalk; bristles scabrous with upward–pointing barbs (antrorse), ascending at anthesis and in fruit, wavy or arched, (1.3—)2—7.5 mm long, pale yellowish green to yellowish tan.
Spikelet compressed–ellipsoid, 2.2—2.5 mm long, glabrous, easily deciduous, breaking below glumes leaving a whitish disc ca. 0.3 mm across; glumes 2, unequal, dimorphic, broadly ovate and acute to obtuse at tip, lower glume sheathing, membranous, 1.1—1.5 × 0.8—1 mm and 3–veined, broadly obtuse at tip, upper glume 1.7—1.8 × 1.1—1.2 mm and 3—5–veined, glumes pale green and semi–transparent becoming whitish and opaque with green veins, sometimes purplish red at tip; lower lemma (sterile floret) similar to upper glume in texture, ovate, 2.2—2.5 × 0.9—1.2 mm, flattened on back, with inrolled margins (clasping upper floret), 5–veined, ± with short point at tip; lower palea membranous and ovate, 1.8—2.1 × 0.8—1.2 mm, 2–veined, sharply infolded at margins; upper lemma (fertile floret) ovate, 2—2.2 × 0.9—1.3 mm, with inrolled margins and clasping palea, translucent–green but light green or often purplish red on exposed portion, netlike throughout and conspicuously transversely ridged below midpoint (appearing reptilean); upper palea 1.6—2 mm long, 2–veined, netlike and with transverse ridges between veins, smooth and infolded outside of veins and eventually clasping fruit.
Flowers bisexual or staminate; perianth (lodicules) 2, trapeziform (broad at top), 0.25—0.4 × 0.2—0.35 mm, glossy, colorless, truncate and sometimes also notched at tip; stamens 3, free; filaments threadlike, 2—2.5 mm long, translucent–white; anthers exserted, versatile, dithecal, 0.9—1 mm long, orangey brown, sacs linear–ellipsoid and free at ends, longitudinally dehiscent; pollen light yellow to light yellow orange; pistil 1; ovary superior, ovoid, ca. 0.4 × 0.3 mm, translucent–white, glabrous, 1–chambered with 1 ovule; styles 2, ca. 1 mm long, translucent–white, smooth; stigmas exserted near tip of lemma, 1.2—1.5 mm long, reddish to purplish red, conically branched (plumose).
Fruit achene (caryopsis) tightly enclosed by hardened lemma and palea, ovoid compressed top–to–bottom, 1.5—1.7 × 1 mm, dull grayish brown; lemma and palea brownish to orangey brown.
A. C. Gibson & B. A. Prigge