Tridens flavus cupreus
Grass family (Poaceae)
Description: This native perennial grass is about 3-4' 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. The blades of the alternate leaves are up to 12" long and ½" across; they are linear, medium green, and flat. Leaf venation is parallel, while the upper surface and margins of the leaf blade are scabrous (rough to the touch). The sheath of each leaf is tightly rolled around the culm, medium green, and hairless, except for a tuft of hairs at its apex. The culm terminates in a panicle of spikelets up to 1½' long and about half as much across. This panicle becomes widely spreading and is rather lanky; its branches are very thin and wiry. Each narrow spikelet is up 10 mm. (1/3 in.) long, consisting of a pair of glumes and 2-4 pairs of lemmas, which are arranged together into 2 columnar ranks. The glumes are about 3 mm. long, ovate, convex on the outer surface, and shiny purple. The lemmas are 3-4 mm. long, oblong, convex on the outer surface, and shiny pale purple. 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); the tip of each lemma is indented. The blooming period occurs from mid-summer to early fall and lasts about 2-3 weeks for a colony of plants. After blooming, the spikelets lose their purple color and become brown. Each flower of the lemmas produces a single grain. As the grains ripen, the upper culm and the branches of the panicle become sticky. The root system is fibrous and rhizomatous.
Cultivation: The preference is partial sun and mesic to slightly dry conditions. Many different kinds of soil are tolerated, including those that contain loam, clay-loam, gravel, rocky material, and some sand. Most vegetative growth occurs during the late spring and early to mid-summer.
Range & Habitat: Purpletop is a common grass that occurs in most areas of Illinois, except the upper tier of counties, where it is apparently absent (see Distribution Map). Habitats include openings in woodlands, savannas, woodland borders, meadows in wooded areas, powerline clearances in wooded areas, limestone glades, fields, and abandoned railroads. Purpletop thrives in disturbed open areas of woodlands.
Faunal Associations: The flowers are are wind-pollinated and attract few insects. The caterpillars of several skippers feed on Purpletop, including Hesperia leonardus (Leonard's Skipper), Poanes zabulon (Zabulon Skipper), Polites origenes (Crossline Skipper), and Pompeius verna (Little Glassywing). The caterpillars of the butterfly Cercyonis pegala (Common Wood Nymph) also feed on Purpletop. Among mammals, the foliage is eaten by Deer Mice and Prairie Voles (see Cook et al., 1982); it is also palatable to livestock. Because it is a robust grass, Purpletop provides significant cover for wildlife.
Photographic Location: A powerline clearance in Busey Woods, Urbana, Illinois.
Comments: Purpletop is an attractive grass with purple spikelets during the late summer. At this time, it is an easy grass to identify. The form of Purpletop with purple spikelets, f. cupreus, is far more common than the typical form in Illinois; the latter has yellow spikelets. Another grass, Agrostis gigantea (Redtop), blooms earlier in the summer and has red spikelets. Both of these grasses produce very airy panicles that blow about in the wind. The latter species is somewhat shorter and has more slender leaves; its spikelets are also smaller, consisting of a single flower. The genus Tridens refers to a small group of grasses that have 3 lines of fine hairs along the lower halves of their lemmas. The only other species of this genus in Illinois, Tridens strictus, also has purple spikelets, but its panicles are very narrow and spike-like. This latter species is uncommon within the state. Some authors refer to Purpletop as Triodia flava; however, this genus is more commonly applied to grasses in Australia.