Flora of North America species comparison

Potentilla nana has most often been considered a race of P. hyparctica or as the priority name for P. hyparctica. As now defined, P. nana occurs in an arc across southern Alaska, from the Aleutian and Pribilof islands to the Alaskan panhandle and adjacent British Columbia. T. Wolf (1908) treated it as a dwarf form of P. fragiformis; J. Soják (1996) interpreted P. nana as a hybrid species originating from cross(es) between P. fragiformis and P. hyparctica. Although the hybrid hypothesis has morphologic support, P. nana has a unique combination of morphologic features and has a distinct range from both P. fragiformis and P. hyparctica. The species are also ecologically segregated: P. nana is a coastal heath and gravel slope species, like P. hyparctica; P. fragiformis is strictly maritime coastal.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The Potentilla recta species group is among the most complex in Europe and western Asia, with diverse taxonomic circumscriptions. The most conservative is adopted here (also used by P. W. Ball et al. 1968; J. Soják 2004), in which most variation is included in a broadly defined P. recta. An alternative is provided by A. Kurtto et al. (in J. Jalas et al. 1972+, vol. 13), who recognized six subspecies of P. recta and three additional species. The phase found in North America is rather uniform, corresponding to the pale-flowered sulphurea variant that Kurtto et al. included in subsp. recta. However, B. Boivin (1952, 1966b) accepted two additional varieties in Canada, locally naturalized in Ontario and Saskatchewan. Nomenclaturally these correspond to subsp. obscura and subsp. pilosa as recognized by Kurtto et al., but other than 5-foliate leaves in var. obscura (Boivin 1967–1979), the morphological basis for Boivin's taxonomic conclusions is unclear.

The earliest North American collection of Potentilla recta seen by the authors was gathered by C. W. Short as a weed in a garden in Montreal in 1852 (MO). It was encountered initially as a naturalized plant in New York in 1879, and by 1900 it was fairly widespread in New England before being found in Ontario, Canada. From there it spread rapidly throughout much of the northern United States and southern Canada (K. Zouhar, http://www.fs.fed.us/database/feis/). It was in British Columbia by 1914, and in a Forest Service pasture in eastern Utah by 1924. The species is listed as a noxious weed in much of North America, spreading aggressively in both natural and disturbed habitats.

The species is often confused with the native, widespread Potentilla gracilis (sect. Graciles), but differs from the latter in having ephemeral basal leaves (often replaced by a new flush in later summer), pale yellow petals, and shorter styles. The long, stiff, spreading, tubercle-based hairs on stems and petioles of P. recta, overtopping a layer of short hairs and glands, also serve to distinguish this aggressive species from native P. gracilis at any stage of the life cycle. The leaves are also sometimes mistaken for Cannabis. The name P. recta is sometimes confused with P. erecta (Linnaeus) Raeuschel, a much different plant in sect. Potentilla.

(Discussion copyrighted by Flora of North America; reprinted with permission.)