Flower:

Loose panicle 1 to 3 inches long at the top of the stem, pyramidal in outline, few branched, the main branches spreading to ascending with 2 to 5 spikelets (flower clusters) per branch. Auxiliary panicles often form below the terminal cluster but are mostly hidden inside the leaf sheath, and contain mostly self-pollinating flowers that do not open (cleistogamous flowers). Spikelets are short-stalked, typically purplish, somewhat flattened, 6 to 9 mm (to ~1/3 inch) long with 2 to 4 florets.

At the base of a spikelet is a pair of bracts (glumes) that are both similar, shorter than the spikelet, lance-linear, 1-veined, hairless, weakly and irregularly toothed around the edge, the upper glume 2.5 to 3.5 mm long, the lower glume about ¾ as long. Surrounding a floret is a pair of bracts (lemma and palea), the lemma oblong, slightly longer than the upper glume, 3 veined with a dense covering of long white hairs along the veins, and notched at the tip with an awn less than 1 mm long arising from the base of the notch; the palea is about ¾ as long as the lemma and hairy along the keel. The thickened base of the floret (callus) is also covered in long, white hairs.

Leaves and stems:

Leaves are basal and alternate on the lower stem, flat or rolled in along the edges (involute), 1½ to 6 inches long, 2 to 4 mm (to 1/6 inch) wide, rough on both surfaces and often fringed in sparse, long hairs. Sheaths are hairless to sparsely covered in long, spreading hairs. The ligule (membrane where the leaf joins the sheath) is a fringe of hairs .5 to 1 mm long. 

Nodes are covered in long, erect, white hairs. Stems are smooth, mostly unbranched, multiple from the base, ascending to erect or prostrate at the base and angled up from a lower node (geniculate). Plants are clump-forming, occasionally rhizomatous.

Fruit:

Individual florets drop away independently leaving the glumes behind persisting on the stalk. The grain (seed) is dark brown, elliptic, about 2 mm long.

Notes:

In the bulk of its range, Purple Sandgrass is found in the dry sandy soils of prairies, roadsides and beaches, but in Minnesota is restricted to sand dunes and blowouts. According to the DNR, its dune habitat is rare in itself and many of the dune fields in Minnesota have been destroyed by development and other commercial ventures, or failed attempts at agriculture. The remaining dunes support only a few populations of Purple Sandgrass; it was listed as a Special Concern species in 1984. While it may resemble other grasses with few-branched panicles, such as Tufted Lovegrass (Eragrostis pectinacea), the nodes with long, erect hairs, spikelets with only 2 to 4 florets, lemmas with long hairs along the veins, and auxiliary panicles mostly hidden in the upper sheaths help distinguish Purple Sandgrass from other species. There are 2 varieties of Triplasis purpurea, though these are neither well-documented nor universally recognized; var. purpurea would be found in Minnesota and var. caribensis would not.