Flower:

Loose branching cluster 4 to 10 inches long at the top of the stem. Branches are ascending to spreading, with 1 to 4 spikelets (flower clusters) per branch. Spikelets are short-stalked, single-flowered, often reddish-purple, the body 10 to 15 mm (to ~½ inch) long, narrowly lance-elliptic in outline.

At the base of a spikelet is a pair of bracts (glumes), both similar, thin and papery, hairless, 1-veined, narrowly lance-shaped tapering to a pointed tip with an awn extension, 20 to 30 mm (to 1.2 inches) long including the awn, the upper glume slightly longer than the lower glume and both much 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 10 to 15 mm long, mottled gray or brown, hairless, narrowly lance-linear with a 3-branched awn at the tip, the branches all about the same length, their bases tightly twisted into a stiff erect column 8 to 12 mm long, then loosely coiled a few times before angling out, the free portion of the awns 3 to 5 cm (1¼ to 2 inches) long (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, 3 to 10 inches long, 1 to 4 mm wide, hairless, lance-linear, flat or the edges rolled in (involute). The sheath has thin, papery edging and is mostly hairless except for (usually) a fringe of tangled hairs at the tip, sometimes with sparse long 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 above the base, erect to ascending, and forming small clumps.

Fruit:

Spikelets are light brown at maturity, the florets shedding individually, leaving the glumes behind on the stalk. Grains (seeds) are dark chestnut brown at maturity, narrowly lance-linear, 8 to 10 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 tuberculosa is a species of dry, sandy prairie and savanna where vegetation is sparse and competition relatively low. Minnesota is at the extreme northwest edge of its range, where it is restricted to a few locations in our east central and southeastern counties. According to the DNR, while typically found in the driest and sandiest of sites, this has not prevented the decline or conversion of these lands by agriculture or development or competition from invasive species. It was listed as Special Concern in Minnesota in 1984 but due to continuing declines, it was elevated to Threatened in 2013. It is distinguished from the other five Aristida species by its three awns tightly twisted into a long column before making several loose loops and then angling widely away, though this twisting is not evident until fruit starts forming.