Aristida purpurea

Purple Three-awn


Field view of Aristida purpurea or Redtop

Field view of Aristida purpurea, common name Purple Three-awn. When young and reddish purple, the awns are straight.

 Field View in Dry Habitat

In an arid area the plant looks thin and stringy.

 Close View of Redtop

This view is soon after collecting. Later, the purple color will fade and the awns will fan outward.

Awns Fanning Out

Here the  color remains on the seedhead but the awns have started to fan out in threes. The branches have diverged from the rachis.

 Seedheads by Lamplight

Usually in more difficult conditions the plants are a bit threadbare.

 Spikelets and Awns

These spikelets have diverged into the typical three-part awns. The florets with their triple-tipped awns may be pulled from their glumes.

A Single Spikelet

A single spikelet shows the unequal translucent glumes below with the single floret inside. The dark lemma extends upward and splits into three.

 Dense, Messy Base of the Plant

Branches protrude from almost the entire stem. When one spikelet touches the ground, it stands up on two of its tripod awns, giving it a headstart for drilling into the soil.

 Roots of Purple Three-awn

The typical dense, messy base of this perennial bunchgrass. You can see the long, thin leaves.

How to Identify Purple Three-awn

  Look for the three-part awns (up to 5 inches long, commonly one to three inches) and the quickly-fading purple color. The awns are straight and parallel when young but divergent when mature. However, the several Aristida species in Arizona are quite difficult to differentiate, except the much larger Spidergrass. It might be best to settle for calling all of them simply "Three-awns."


Similar Species

  The tripart awns of Purple Three-awn can be 5 inches long, though they are usually 2-3 inches. There are several other common three-awn grasses in Arizona, but only two of these have awns over an inch long. One, Aristida oligantha, confined to northeast Arizona, has 2-inch awns but is an annual grass, so it won't have the thick-based bunch grass look of Aristida purpurea. The other is Aristida californica, a desert grass, growing at low altitudes in western and southwest Arizona deserts. To conserve water, its leaves, unlike all other Arizona Aristida, are never over two inches long.