Hordeum pusillum

Hordeum pusillum, the little barley, is a diploid annual grass native to the United States (except the westernmost parts), which arrived via multiple long-distance dispersals of a southern South American species of Hordeum about one million years ago. Its closest relatives are therefore not the other North American taxa like meadow barley (H. brachyantherum) or foxtail barley (squirreltail grass, H. jubatum), but rather Hordeum species of the pampas of central Argentina and Uruguay. It is less closely related to the Old World domesticated barley, from which it diverged about 12 million years ago.