Flora of North America species comparison

Potentilla norvegica is considered native in both North America and Eurasia, with the American race occasionally recognized as subsp. hirsuta for the stiff hairs on the stems, petioles, and pedicels of most plants in the flora area. This feature and other purported differences between the two races, in addition to being relatively subtle and inconstant, can be found in both America and Eurasia, though undoubtedly at least in part as introductions. Some populations in eastern Canada with glabrous stems have been distinguished as var. labradorica (for example, M. L. Fernald 1950), but such plants typically are intermixed with hairy individuals. The Löves (Á Löve 1954; Á Löve and D. Löve 1966) have argued that all three variants should be treated as distinct species due to chromosomal differences (hirsuta 2n = 56; labradorica 2n = 42; norvegica in the narrow sense 2n = 70) and obligate apomixis. Further research is needed to determine the taxonomic validity and rank of these expressions. If treated as species, P. flexuosa antedates P. labradorica (as noted by J. Soják 1969), while P. monspeliensis, although commonly used for the American race, is based on a European type (as summarized by A. Kurtto et al. in J. Jalas et al. 1972+, vol. 13).

An even greater challenge is determining native versus introduced ranges, especially given the likelihood that both native and Eurasian populations are widespread in North America. Achenes are produced prolifically and easily dispersed, to the extent that Potentilla norvegica is a contaminant in clover and hay fields and considered a weed at least in Canada (P. A. Werner and J. D. Soule 1976).

(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.)