Purple threeawn (Aristida purpurea) is a somewhat variable grass. It may range in height anywhere from 6 to 30 inches. Its stems usually prefer to grow outward rather than upward, branching and spreading to form large, low bunches.
The leaves may be straight or curved, rolled inward or flat, and anywhere between 1 and 12 inches long. Consistently, however, they are less than 1/10 inches wide. The top surface is rough to the touch.
The infloresences may be tight or loose and open, and range anywhere between 1-1/2 to 12 inches in length. They usually droop gently forward and are purplish in color, which accounts for the first part of the common name of this species. As for the second part, “threeawn,” that comes from the fact that the plant grows spikelets on a slender pedicel, each spikelet having one floret and each floret having three long bristles protruding from it.
Purple threeawn is a plant of western North America, growing northward from northern Mexico as far as Kansas and Colorado. In the former state, it is most common in the western two thirds, although it has managed to establish itself in Riley County, as well.
This species primarily occupies disturbed sites, such as roadsides, railroad rights-of-way, overgrazed pastures, and abandoned fields. However, where conditions are favorable, purple threeawn can become established on undisturbed ground, particularly in grasslands, but also in forests and shrubby areas.
The preferred soil of purple threeawn is dry sand. However, it can tolerate any other soil type provided that moisture levels are not too high. This plant does best in full sunlight.
Purple threeawn is a perennial grass. It can flower any time from May through August, being quite drought-tolerant. It spreads readily from seed.
The preference of this grass for disturbed areas is a testament to its ability as an early successional species, preparing the ground for native wildflowers to move in.
While purple threeawn is almost never grazed by wildlife, it does attract butterflies, and its seeds are eaten by some songbirds, such as the junco. Game birds will nest in purple threeawn.
Purple threeawn may be used in mixes to provide erosion control on stream banks.
A few gardeners appreciate purple threeawn for its attractive color, modest space requirements, and ability to thrive without supplemental watering. However, be aware that purple threeawn will quickly take over the landscape if allowed to go to seed. Clip seedheads off before they mature to prevent your ornamental plants from becoming a nuisance.
Purple threeawn is typically considered an undesirable weed. For one thing, it can compete quite successfully with other plants during short-term droughts. For another thing, its awns will stick to socks and fur, spreading the seeds far and wide.
This species is particularly problematic in the pasture. It makes a poor forage plant due to its low protein content. Furthermore, its awns can injure the eyes, noses, and mouths of grazing animals. To deal with purple threeawn in the pasture, preventing overgrazing is the most important step to take. Also, summer burning can be helpful, as it will set the plant back at a time when it is trying to reproduce.
These two related species can be difficult to tell apart. Some technical terminology is required here, so please refer to the labeled diagram of a grass spikelet in our grass plant glossary. The lower glume of an arrowfeather threeawn spikelet is usually more than 75% as long as the upper glume, while the lemmas are 4 to 8 mm long. In contrast, the lower glume of a purple threeawn spikelet is usually less than 75% as long as the upper glume, and the lemmas are 7 to 13 mm long.
Parts of a Grass Plant: A Glossary
Definitions for technical terms used in this post.