10. Balsamorhiza macrophylla Nuttall, Trans. Amer. Philos. Soc., n. s. 7: 350. 1840.
Balsamorhiza hookeri Nuttall var. idahoensis (W. M. Sharp) Cronquist; B. macrophylla var. idahoensis W. M. Sharp
Plants 30–45(–100) cm. Basal leaves green, ovate to lanceolate, 20–50+ × 8–15 cm (pinnatifid, lobes lanceolate, 20–80+ × 10–40 mm, entire or ± dentate), bases ± cuneate, ultimate margins usually entire (plane or weakly revolute, ciliate), apices obtuse to acute, faces scabrous or piloso-hirtellous to pilose (at least abaxial usually gland-dotted as well). Heads usually borne singly. Involucres ± hemispheric, 20–30 mm diam. Outer phyllaries lance-ovate or lanceolate to lance-linear, 12–30(–40) mm, equaling or surpassing inner (margins ciliate), apices acute to attenuate. Ray laminae 35–50+ mm. 2n = 100 ± 2.
Flowering May–Jul. Deep soils, rocky meadows, sagebrush scrublands, conifer forests; 1000–2400 m; Idaho, Utah, Wyo.
Balsamorhiza macrophylla is a high polyploid; it occurs sympatrically with B. sagittata. It evidently arose from hybridization between B. sagittata and B. hispidula. Balsamorhiza macrophylla has the multi-branched caudices and massive taproots of the former, and the leaf dissection of the latter. No hybrids with other species are known. Presumably, the high-polyploid chromosome complement precludes interbreeding. Plants of var. idahoensis are smaller, are known only from southwestern Idaho and northeastern Utah, and differ from var. macrophylla by being pilose, with strongly shaggy-pilose involucres. More study may determine that var. idahoensis merits specific rank. The Utah populations are not well understood and deserve attention.
Helton, N., D. Wiens, and B. A. Barlow. 1972. High polyploidy and the origin of Balsamorhiza macrophylla. Madroño 21: 526–535.