Flower:

Tight clusters of a few to several short-stalked flowers at the top of the stem and arising from leaf axils in the upper plant. Flowers have 8 to 19 pale yellow to creamy white rays (petals) that have a few teeth at the tip; when fully open the flower is bell or trumpet-shaped, ~½ inch diameter. The styles are longer than the rays and have divided, curled tips.

Surrounding the base of the flower, forming a tube, are 2 sets of bracts, typically light green to yellowish at flowering time often turning purple in fruit. The outer bracts are short and narrowly triangular; the inner bracts are lance-linear, blunt to pointed at the tip and up to ~2/3 inch (8 to 15 mm) long. Bracts and flower stalks are moderately to densely covered in bristly hairs.

Leaves and stems:

Leaves are alternate, the lowest leaves largest, up to ~4 inches (to 11 cm) long and 2 inches (5 cm) wide, somewhat spatula-shaped, rounded to pointed at the tip, tapering at the base to a winged stalk. Lower leaves usually wither away by flowering time. Leaves become smaller, more elliptic and stalkless as they ascend the stem and are reduced to bracts in the flower clusters. Surfaces are rough-textured, edges are toothless or irregularly toothed. Color is green to gray-green.

Stems are single, erect, unbranched, green to purple or purple spotted, and rough-textured from short, bristly hairs, more so on the upper stem.

Fruit:

Fruit is a dark brown seed 5 to 6 mm long, linear to elliptic, with a tuft of yellowish or light brown hairs to carry it off in the wind.

Notes:

Rough Rattlesnake-root, a.k.a. Prenanthes aspera, is an uncommon species in Minnesota, where it reaches the northern edge of its range, primarily found in open prairie and rock outcrops in late summer. The DNR has noted its decline in the state and has been tracking it for possible inclusion in the rare species list. It is currently Endangered in Wisconsin.

Rough Rattlesnake-root is distinguished by the yellowish flowers, mid-stem leaves lance-elliptic and stalkless, lowest leaves usually withered away by flowering time, and rough-hairy all over, more so in the upper plant. The flower cluster can appear spike-like but is made up of small racemes or panicles in the leaf axils.

Most similar is Purple Rattlesnake-root (Nabalus racemosus), a more common species also found in prairies, which has pale pink to purplish flowers, smooth stems below the flowers, smooth-textured leaves, mid-stem leaves are clasping, and the lower leaves usually persist through flowering time. The other two related species in Minnesota are White Rattlesnake-root (N. albus) and Nodding Rattlesnake-root (N. crepidineus), both of which are (usually) taller plants, have larger branching clusters of flowers and more variable shaped leaves; N. albus is the only one of this group that has hairless floral bracts..

Note the genus has gone back and forth between Nabalus and Prenanthes; since 2010 Nabalus has more often been the accepted genus.