3. Prenanthes altissima Linnaeus, Sp. Pl. 2: 797. 1753.
Tall rattlesnakeroot, prenanthe élevée
Nabalus altissimus (Linnaeus) Hooker; Prenanthes altissima var. cinnamomea Fernald; P. altissima var. hispidula Fernald
Plants 40–250 cm; taproots thickened, knotty, tuberous. Stems erect, greenish to purplish, glabrous proximally, often tomentulose distally. Leaves: proximal present at flowering; petioles winged; blades usually ovate or triangular, 4–15 × 2–16 cm, thin, bases truncate to hastate or cordate, margins entire or shallowly dentate, often deeply 3-lobed, faces glabrous or with scattered hairs on veins; distal reduced in size and lobing. Heads in narrow or spreading, paniculiform arrays. Involucres cylindric, 9–14 × 2–3 mm. Calyculi of 4–6, blackish, triangular bractlets 1–4 mm, glabrous. Phyllaries (4–)5(–6), pale green, often blackish at bases and apices, linear to lanceolate, 10–12 mm, glabrous or sparsely hairy. Florets (4–)5(–6). corollas pale yellow to greenish yellow, 7–15 mm. Cypselae brown to tan, subcylindric, subterete, 4–5 mm, indistinctly 5–10-ribbed; pappi usually whitish or pale yellow, sometimes reddish brown, 5–6 mm. 2n = 16.
Flowering Aug–Nov. Open deciduous hardwood or mixed woods, shaded slopes, bluffs, disturbed areas, roadsides; 50–800 m; N.B., N.S., Ont., P.E.I., Que.; Ala., Ark., Conn., Del., Ga., Ill., Ind., Ky., La., Maine, Md., Mass., Mich., Mo., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va.
Prenanthes altissima is recognized by its narrow involucres with 5 pale green, glabrous phyllaries, (4–)5(–6) florets, and pale yellow to greenish yellow corollas. Pappi in this species are most commonly whitish or pale yellow. Specimens with reddish brown to orange pappi have been recognized as var. cinnamomea, found in Arkansas, Louisiana, and Missouri. Specimens with densely hairy stems and pale yellow pappi have been recognized as var. hispidula, found mostly in New York, New England, and adjacent Canada.