Flower:

Flowering head is an open panicle 6 to 15 inches (15 to 40 cm) long, erect or nodding to one side, the branches spreading to ascending but typically drooping toward the tip, with tufts of hair in the branch axils. Spikelets (flower clusters) are stalked, lance-oblong in outline, slightly flattened, 5 to 10 mm long, usually purple, occasionally yellow, and have 4 to 8 florets; the uppermost floret may be sterile.

At the base of a spikelet is a pair of bracts (glumes), both narrowly egg-shaped, blunt to pointed at the tip, often with a minute, sharp point at the apex (mucronate), keeled, 1-veined, the lower glume 2.5 to 3 mm long, the upper glume 3.5 to 4 mm long. Surrounding a floret is a pair of bracts (lemma and palea). Lemmas are elliptic, 3 to 5 mm long, 3-veined, hairy along the veins for about half their length, 2-toothed at the tip, the midvein extending between the teeth for about .5 mm. The palea is as long as the lemma or nearly so. The thickened base of the floret (callus) is hairy.

Leaves and stems:

Leaf blades are ascending to spreading to drooping, flat, 4 to 20 inches (10 to 50 cm) long, up to 3/8 inch (1 cm) wide, surfaces usually hairless but rough-textured. There is often a slight constriction or wrinkle across the blade at about 1/3 its length. Leaves become dramatically smaller as they ascend the stem.

Sheaths are hairless except at tip, the collar surface covered in white hairs. The ligule (membrane where the leaf joins the sheath) is a fringe of hairs about .5 mm long. Nodes are smooth. Stems are multiple from the base in a loose clump, mostly erect, hairless, usually unbranched but occasionally an auxiliary panicle may arise from a lower node.

Fruit:

Mature florets drop off individually, leaving the glumes behind on the stalk. Grains (seeds) are oblong, somewhat flattened, dark brown at maturity, about 2 mm long.

Notes:

Purpletop is native to the US but is likely introduced in Minnesota, to-date only seen in a few locations. As of this writing, there are no herbarium records under Tridens flavus or its synonym Triodia flava, and the earliest record I found was a report on iNaturalist from 2018 in Dakota County, where it may have been introduced in a restoration planting. It is salt-tolerant and apparently has a preference for disturbed soils, commonly found along roadways and railroads, but also in old fields and open woods. It is a late season bloomer, just starting to flower in September. The dark purple panicle can be striking, especially in a patch of multiple plants. An uncommon form has yellow spikelets.

Purpletop is distinguished by the large, open, somewhat droopy panicle blooming in late summer to early fall, panicle branches hairy in the axils; purple (or yellow) spikelets with 4 to 8 florets, lemmas hairy along the veins and around the callus; flat leaves often with a crinkle at about 1/3 their length; leaves and sheaths hairless except on the collar surface. The hairy collar is probably the most distinctive trait. There are 2 recognized vars: var. chapmanii has panicle branches more stiffly erect and straight as well as hairy all around the base, and is present mostly in the southern and southeastern US; var. flavus has more lax panicle branches, is as described above, ranges as far north as southern Canada is present in Minnesota.