1. Tridens flavus (L.) Hitchc. (purpletop, tall redtop)

Pl. 156 d, e; Map 633

Triodia flava (L.) Smyth

Plants usually with knotty bases, sometimes with short rhizomes, forming tufts or small clumps. Flowering stems 60–150 cm long, circular in cross‑section or nearly so, glabrous. Leaf sheaths keeled, with a zone of dense hairs at the tip, otherwise glabrous, the ligule 0.3–0.7 mm long, the hairs much longer than the minute, membranous portion. Leaf blades 8–50 cm long, 5–13 mm wide, flat or with the margins inrolled toward the tip, glabrous or sparsely hairy, especially toward the base. Inflorescences 15–40 cm long, open, broad panicles, ovate to triangular in outline, the branches loosely ascending to spreading or drooping at maturity, mostly branched again 1 or more times, without spikelets in the basal 1/3–1/2. Spikelets 5–9 mm long, with 4–8 florets. Lower glume 2.2–4.0 mm long, shorter than the adjacent lemma, ovate, bluntly to sharply pointed at the tip, 1‑nerved, shiny. Upper glume 2.9–4.2 mm long, shorter than the adjacent lemma, ovate, bluntly to sharply pointed at the tip, 1‑nerved, shiny. Lemmas 3.4–4.8 mm long, ovate. Anthers 0.6–1.2 mm long. Fruits 1.5–1.8 mm long. 2n=40. July–September.

Common throughout the state (eastern U.S. and adjacent Canada west to Illinois, Nebraska, and Texas). Upland prairies, glades, savannas, openings of mesic to dry upland forests and bottomland forests, and margins of streams; also roadsides, railroads, ditches, old fields, and open, disturbed areas.

Plants of this species tend to secrete small globules of a greasy or sticky substance at the branch points of the inflorescence and upper stem nodes. The inflorescences are normally dark purple at maturity. For a discussion of the sterile hybrid with T. strictus, see the treatment of that species.