Prairie Three-Awn
Aristida oligantha
Grass family (Poaceae)

Description: This native grass is a summer annual that is tufted at the base, producing several culms up to 2' long. The culms can be erect or sprawl across the ground, branching at each node. Each slender culm is light greyish green to pale purple, somewhat flattened, and hairless. Relative to the orientation of each culm, the blades of the alternate leaves are erect to slightly spreading. These leaf blades are up to 3" (9 cm.) long and up to 1/8" (1.5 mm.) across; they are linear in shape, light greyish green to pale purple, and hairless. The leaf sheaths are light greyish green to pale purple, hairless, and finely ribbed. The ligules lack significant hairs or papery membranes. The nodes are hairless and often reddish.

The upper branches of each culm terminate in racemes or panicles of spikelets up to 8" long. Each raceme or panicle produces relatively few spikelets. Each spikelet consists of a pair of awned glumes and an awned fertile lemma. The glumes are about –1" long, linear in shape, and keeled; each glume has a short straight awn. The fertile lemma is about –" long and linear in shape; it has 3 awns that bend sharply near the base and they are about 1–2" long. The spikelets are nearly sessile or they have slender pedicels up to 1/3" (1 cm.) long. These spikelets are initially greyish green to pale purple, but they become various shades or grey or brown as they mature. Later, the entire plant (culms, leaves, and spikelets) becomes straw-colored. The blooming period occurs during the late summer and early fall. Each spikelet produces a single grain. The awned lemmas can be blown about by the wind, which helps to distribute the grains to new areas. The root system is fibrous. This grass reproduces by reseeding itself.

Cultivation: The preference is full sun, dry conditions, and a barren soil that consists of gravel, sand, or clay. This grass does best in areas with scant vegetation and is quite drought resistant.

Range & Habitat: Prairie Three-Awn is a fairly common grass that occurs in most areas of Illinois (see Distribution Map). However, it is even more common in drier areas to the west of the state. Habitats include dry gravel prairies, sand prairies, glades, overgrazed pastures, abandoned fields, roadsides, areas along railroads, and barren waste areas. In Illinois, Prairie Three-Awn is primarily a railroad weed.

Faunal Associations: In Illinois and the lower Midwest, wildlife makes little use of Aristida spp. (Three-Awn grasses). Some leafhoppers that feed on Three-Awn grasses include Flexamia picta and Flexamia pyrops. Small rodents eat the seeds in the desert southwest, but they have access to better sources of food in the lower Midwest.

Photographic Location: Along a railroad in Urbana, Illinois.

Comments: This is the most common Aristida sp. (Three-Awn grass) in Illinois. Some of the less common Three-Awn grasses in the state are often found in dry sand prairies. The most striking feature of these grasses are the bent awns of their fertile lemmas. Prairie Three-Awn differs from many other Three-Awn grasses by the long awns of its fertile lemmas (at least 1" long). Other Three-Awn grasses have awns up to " long (in some species, this applies only to their lateral awns, which are shorter than the central awns). An exception is Aristida tuberculosa (Needle Grass), which has awns on its lemmas that are about as long as those of Prairie Three-Awn. However, the awns of Needle Grass coil tightly around each other to form a basal column about " long, after which they bend away from each other in the manner of other Three-Awn grasses. The awns of Prairie Three-Awn and other Three-Awn grasses are free to the base, or they coil around each other to form a short basal column that is only 1/8" long.

Return