purpletop tridens (Plants of the Peggy Notebaert Nature Museum Grounds, Chicago, Illinois)

purpletop tridens

Tridens flavus

Summary ⁷

Tridens flavus (Purpletop, Red Top, or Greasegrass) is a large, robust perennial bunchgrass native to North America. The seeds are purple, giving the grass its common name. The seeds are also oily, leading to its other common name, "greasegrass". It reproduces by seed and tillers. The grass is often confused with Johnson grass.

Description ⁸

This perennial grass is about 3-5' tall and unbranched; plants can occur in tufts or as scattered individuals. The culm is green, glabrous, and terete (round in cross-section); the nodes on the culm are glabrous and green to reddish green. Alternate leaves occur along each culm; they are ascending to widely spreading and rather floppy. The leaf blades are up to 16" long and 10 mm. across; they are linear, medium green, and shallowly grooved to flat. The margins and upper surfaces of leaf blades are scabrous (rough to the touch). The leaf sheaths wrap tightly around their culms; they are medium green and hairless, except for tufts of hair at their apices. The culm terminates in a panicle of spikelets about 6-14" long and about one-half as much across. This panicle has an airy appearance and it is pyramidal in outline; the lateral branches occur individually or in whorls of 2-3 along the rachis (central stalk). Both the rachis and lateral branches of the panicle are very slender and glistening; the rachis is either erect or nods to one side, while the lateral branches are widely spreading and/or drooping. Pedicellate spikelets occur from the middle toward the tips of the lateral branches; their pedicels are 3-8 mm. long. The spikelets are 5-10 mm. long, oblongoid in shape, and slightly flattened. During the blooming period, they are reddish purple and shiny. Each spikelet has a pair of glumes at the bottom and 2 overlapping ranks of 4-8 lemmas above. The glumes are 2.5-4 mm. in length, convex to slightly keeled along their outer surfaces, glabrous, single-veined, and obtuse at their tips. One glume in each spikelet is slightly smaller than the other glume. The lemmas are 3-5 mm. long, convex along their outer surfaces, 3-veined, and bidentate (two-toothed) at their tips. There are 3 lines of fine hairs along the lower half of the central vein and 2 margins of each lemma (requires 10x hand lens to see). Each floret has 3 anthers, 2 stigmas, and an ovary. The blooming period occurs from mid-summer to early autumn, lasting about 1-2 weeks for a colony of plants. The florets are cross-pollinated by the wind. Afterwards, the spikelets lose their purple color and become tan, gray, or brown. Disarticulation of the spikelets is above the glumes and between individual lemmas. The slender grains are about 2 mm. long (when they are separated from the adherent lemmas and paleas). The root system is fibrous and short-rhizomatous. Cultivation