Excerpts from Jim Conrad's
Naturalist Newsletter

Prairie Three-awn Grass, ARISTIDA OLIGANTHA

from the January 6, 2013 Newsletter issued from the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA

I'd hoped that winters here would be so mild that at least a few plants could be found flowering year-round, but this week I haven't seen a single blossom. Except for the hills' wooded slopes populated with evergreen Ashe Junipers and Texas Live Oaks, which lose most of their leaves at winter's end, the landscape is dun colored, the color of straw, of dead annuals and of perennials not yet issuing spring sprouts. Though grasses are much more in evidence here than back East, possibly there's less species diversity among them. The same prairie grasses appear again and again, and in this Newsletter we've already looked at the most common ones.

However, one annual, knee-high clump grass we haven't looked at yet is shown above.

Vegetatively it's similar to several other species found here. Only when you come close enough to see the fruiting heads against a black background provided by a friend with black trousers can you see this grass's most distinctive field mark, shown below:

Prairie Three-awn Grass, ARISTIDA OLIGANTHA, grains with long, speading awns

That fruiting head bears mature, caryopsis-type grains and atop each grain there are three long, widely spreading, needlelike bristles, or "awns." Each awn is about 2½ inches long (6cm). In our area there's simply no other grass with its grains topped by three long, spreading awns.

We do, however, have other grasses whose grains are topped by three spreading awns, just that their awns aren't nearly so long. In our September 23rd Newsletter we looked at the Purple Three-awn Grass, structured very much like our present grass, except that its grains' awns were less than half as long. In fact, about 19 three-awn grass species -- 19 members of the genus Aristida -- are listed for Texas, so the present one with its very long awns is our second species for this area. Awn length in none of our other Aristida species even comes close to that of this one.

This long-awned species is ARISTIDA OLIGANTHA, in English often called Prairie Three-awned Grass, though other species of Aristida also occur in the prairies. It's also called Oldfield Threeawn, which probably is a good name because here I find it in disturbed, sometimes eroded soil like you might find in old fields. The species occurs in prairies and on limestone soils throughout most of North America and arid parts of northern Mexico.

Though Prairie Three-awned Grass's dense clumps protect against soil erosion, most folks around here don't care much about it because with those long awns its grains work into sheep wool and can injure the eyes of sheep and cattle. The awns serve the plant by helping with seed dispersal when they tangle in the hairs of passing animals.

Prairie Three-awned Grass provides nesting sites and/or material for small animals, and of course seed-eating birds have their way of feeding on its grains without bothering with the awns.