Plants diurnal

Purple Threeawn

Aristida purpurea

A shimmering jewel of the dry prairie, Purple Threeawn transforms backyards into a sea of swaying burgundy. Its iconic three-pronged seeds catch the sun, providing both dramatic beauty and essential habitat for small wildlife.

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Quick Identification

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Size

Height: 30-75 cm (12-30 inches); Spread: 15-45 cm (6-18 inches)

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Colors

Reddish-purple to burgundy seed heads; grayish-green foliage turning straw-colored in winter

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Key Features

Three distinct long awns branching from each seed
Purplish-red seed heads that shimmer in sunlight
Dense, tufted 'bunchgrass' growth habit
Extremely narrow, rolled leaf blades

When You’ll See Them

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Activity pattern diurnal

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Peak hours 9 AM - 6 PM (Best visibility in late afternoon sun)

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Season May-September (During blooming and seeding)

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Diet Photosynthetic; thrives in nutrient-poor, well-drained sandy or rocky soils

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Habitat Dry plains, rocky slopes, desert grasslands, and suburban xeriscapes

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Behavior

Purple Threeawn is a perennial bunchgrass that plays a vital role in the early stages of prairie recovery and soil stabilization. Unlike many turf grasses, it grows in distinct, tight clumps, leaving space between plants for wildflowers and wildlife to navigate. It is known for its 'dancing' movement in the wind, which is amplified by the long, hair-like awns that give the species its name.

As the plant matures, its behavior is centered on seed dispersal. The three awns act as a sophisticated delivery system; they are sensitive to humidity and can twist or untwist, effectively 'drilling' the seed into the soil or hitching a ride on the fur of passing animals. While not a primary food source for large livestock due to its sharp seeds, it provides critical structural complexity in dry ecosystems.

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Camera Tips

Capturing the beauty of Purple Threeawn on a backyard or trail camera requires a focus on lighting rather than movement. To truly highlight the 'purple' in its name, position your camera to the west of the plant, facing east. This creates backlighting during the 'golden hour' of late afternoon, which causes the translucent awns to glow with a brilliant reddish-purple hue. Without this specific lighting, the grass can appear as a muted brown or gray on camera.

Because this species is highly reactive to the wind, it is a frequent cause of 'false triggers' on PIR (Passive Infrared) motion sensors. If you are using a camera to monitor the plant itself, set your sensitivity to 'Low' or use a 'Time Lapse' mode rather than motion detection. However, if you are using the grass as a backdrop for wildlife, utilize the 'Zone' setting on your camera to exclude the moving seed heads from the trigger area while keeping the ground beneath it active.

For those interested in the 'micro-wildlife' that visits the grass, mount your camera on a low stake just 6-10 inches off the ground. Purple Threeawn is a magnet for small lizards and ground-feeding birds like sparrows, who use the dense base for cover. Using a 'Macro' lens or a camera with a close focal distance (within 3-5 feet) will allow you to see the incredible detail of the three-pronged seeds, which are often missed by the naked eye.

In the winter months, don't move your camera! While the purple color fades to a golden-straw hue, the structure of the grass becomes even more important for wildlife. Use a high-shutter speed setting during winter storms to capture the dramatic way this grass captures frost and snow, creating high-contrast images that look like natural sculptures.

Similar Species

Species that look similar or are commonly confused with Purple Threeawn.

Mexican Feather Grass

Finer, softer texture with single, long silky awns rather than the distinct three-pronged structure of Purple Threeawn.

Little Bluestem

Turns a deep mahogany-red in winter rather than purple in summer, and has wider, flatter leaf blades.

Sideoats Grama

Has distinctive seeds that hang off only one side of the stem, looking like tiny flags, whereas Threeawn seeds are arranged in a more terminal, airy cluster.

Frequently Asked Questions

The plant displays its namesake color during its blooming and seeding phase, typically from late spring through late summer. The color is most intense in the 'awns' (the hair-like structures on the seeds) before they dry out and turn a tan or straw color in the fall.

Simply planting it in clusters provides excellent cover for ground-nesting birds, lizards, and beneficial insects. Small birds like juncos and sparrows are often attracted to the area to forage for insects living in the bunchgrass clumps.

Yes, it is extremely drought-tolerant and thrives in poor soil where other plants might fail. It requires very little water once established and is a favorite for sustainable, low-maintenance 'xeriscape' gardens.

The three awns help the seed disperse by catching the wind and allow it to latch onto animal fur. They also help the seed orient itself correctly on the ground and can even help 'screw' the seed into the soil as they react to changes in moisture.

Look closely at a single seed head. If it has three distinct, long 'hairs' (awns) branching out from the tip of the seed, it is likely an Aristida species. The reddish-purple color during summer is the most common visual giveaway for this specific variety.

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