Hordeum pusillum, also known as little barley, is an annual grass native to most of the United States and southwestern Canada. It 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 (Hordeum brachyantherum) or foxtail barley (Hordeum 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. It is diploid.

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