This annual polyploid (2n = 50, 100) is closely related to frankii. Both have ranges centered in east-central states, but capillaris is slightly more southern and typically occurs on drier, poorer soils, often with much sand or gravel. Compared to frankii, capillaris can be distinguished based on its deeply grooved seeds. Also, its panicles are larger (ca. 10-50 cm long versus 4-20 cm), forming most of the plant's height (versus less than half), with longer pedicels (4-25 mm versus 1.5-5 mm). Plants are pilose along sheath margins (versus mostly glabrous), but stems lack glandular pits as found below nodes in frankii.