Flora of North America species comparison

Potentilla rivalis is most abundant in central and western North America; reports of occurrences in more eastern states (including those listed here) need confirming, as P. rivalis and P. norvegica often have been confused. Potentilla leucocarpa Rydberg was provided as a superfluous replacement name for P. millegrana; specimens annotated by Rydberg with this name include both P. biennis and P. rivalis.

Potentilla rivalis is sometimes divided into three species or varieties (for example, H. J. Scoggan 1978–1979). In a strict sense, var. rivalis has subpalmately compound (5-foliate) leaves. Variety milligrana, the most common phase, has 3-foliate leaves. Plants with both 3- and 5-foliolate leaves are var. pentandra, which also tends to have five stamens, though this latter character is not correlated with the leaf features. On the Great Plains, where var. pentandra is confined, all three expressions are often found in a single population (R. L. McGregor 1986b).

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

Even in the reduced circumscription used here, Potentilla pensylvanica is exceptionally widespread, occurring in North America from Alaska to Hudson Bay, in the Great Plains and the North American Cordillera to northern Mexico. The species also grows across Siberia (where it has sometimes been called P. sibirica) and as isolated occurrences in the southwestern Alps and Spain (J. Soják 1987c; A. Kurtto et al. in J. Jalas et al. 1972+, vol. 13). One place P. pensylvanica does not occur is in Pennsylvania; the epithet was applied by Linnaeus to cultivated plants originating from Canada. Collections from Quebec are probably introductions (J. Cayouette, pers. comm.). The Oregon citation of P. pensylvanica (D. Mansfield 2000) refers to P. jepsonii.

Significant variation occurs within Potentilla pensylvanica, especially regarding vestiture and plant stature. J. Soják (1987c) has described several varieties, all Eurasian except var. brevipila (long hairs 0.6–1 mm); diminutive specimens from the Brooks Range of northern Alaska, isolated from other known populations of P. pensylvanica, have been described as P. brooksensis. Although much of the ecogeographic variation may merit recognition, at least at varietal level, doing so is postponed pending further analysis and clarification of nomenclature. Among the extremes are plants from the Great Plains, which have abundant long silky hairs overlying and obscuring the velvety layer of short hairs (resembling sympatric P. litoralis), and glabrate (except for glands) populations in Alaska. E. Hultén (1968) erroneously referred to the latter as P. pensylvanica var. glabrata (Lehmann) S. Watson; that name is a synonym of P. ovina var. ovina (sect. Multijugae). Most populations in Alaska also have larger than average flowers (petals to 7 × 7 mm, anthers ca. 1 mm).

Although Potentilla pensylvanica var. strigosa Pursh has been one of the most commonly used varieties in North America, the name has been effectively lectotypified by J. Soják (1987c) on a collection from Siberia. Soják identified this collection as the hybrid between P. pensylvanica and the Asian P. sanguisorba D. F. K. Schlechtendal; var. strigosa accordingly does not occur in North America.

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