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.