Flower:

Branching cluster 2 to 6 inches long at the top of the stem, loose and open or erect and contracted. Branches are erect to spreading, occasionally curving or U-shaped, with 2 or more spikelets (flower clusters) per branch. Spikelets are short to long-stalked, single-flowered, often purplish, the body 8 to 24 mm (1/3 to ~1 inch) long, narrowly lance-elliptic in outline.

At the base of a spikelet is a pair of bracts (glumes), both thin and papery, hairless, 1-veined, narrowly lance-shaped tapering to a pointed tip or with a short awn (1 mm) extension, the lower glume body 8 to 12 mm long, the upper glume 14 to 25 mm long, about twice as long as the lower glume and slightly longer than the floret. Florets are surrounded by a pair of bracts (lemma and palea), the lemma thicker than the glumes, 3-veined, the body 6 to 17 mm long, whitish to purplish, hairless, narrowly lance-linear with a 3-branched awn at the tip, branches all near the same length, 2 to 10+ cm (¾ to 4+ inches) long, all angled at the base and not coiled (awns are all initially straight and twist with age); the palea is obscure and mostly enclosed by the lemma. The thickened base of the floret (callus) is covered in straight, white hairs.

Leaves and stems:

Leaves are basal and alternate on the lower stem, up to 6½ inches long, 1 to 2 mm wide, thin, wiry, hairless, lance-linear, the edges usually rolled in (involute). The sheath has thin, papery edging and is hairless or with a few long soft hairs near the tip. The ligule (membrane where the leaf joins the sheath) is fringed with short hairs less than 1 mm long. Nodes are smooth. Stems are hairless, few to many branched at or near the base, erect to ascending or spreading from the base and rising at the lower node (geniculate), and forming clumps.

Fruit:

Spikelets are light brown at maturity, the florets shedding individually, leaving the glumes behind on the stalk. Grains (seeds) are chestnut brown, narrowly lance-linear, 6 to 15 mm long.

Notes:

The Aristida genus is a fairly large group of small to medium sized, clump forming grasses that are mostly associated with dry, sandy soils. They are commonly called three-awn grasses due to the three-parted awn at the tip of the lemma. Structurally there is a central and two lateral awns and the relative differences in length and degree of twisting of these awns is a primary diagnostic to their identification; many are readily distinguished on this characteristic alone, but the twisting may not be distinctive until maturity. There are six species found in Minnesota, and while three of the six are state listed as rare, from a field encounter perspective, the other three are relatively uncommon but for a few specific sites.

Aristida purpurea is a rather variable species with seven recognized varieties; distinguishing characteristics are combinations of: whether panicle branches are stiff or flexible, length of the lower glume relative to the upper glume, length of the awns, width of the lemma at both the base and tip, length of the panicle, length of leaf blades, and color of the glumes and lemmas. It's complicated. While the ranges of all these varieties overlap, only one is found in Minnesota: var. longiseta, which is the most common var. Its leaves are up to 6½ inches long, sometimes mostly basal, sometimes mostly on the stem; panicle branches are mostly straight and stiff, erect to ascending but sometimes drooping; lower glumes are 8 to 12 mm long and about half as long as the upper glume; awns are up to 4+ inches long.

At the extreme eastern edge of its range, it is limited to drier soils in our western counties and was likely never widespread in our state. According to the DNR, extensive conversion of even marginal lands to agriculture—either crop land or pasture—has made it quite rare with continuing threats of gravel mining, invasive species and development. It was listed as a Special Concern species in 1984. Of Minnesota's six known Aristida species, it is distinguished by its very long untwisted and angled awns, and also it is perennial where all our other three-awn species are annual. It is also the most robust of our Aristida species and has the longest awns.