31a. Potentilla effusa Douglas ex Lehmann var. effusa
Potentilla effusa var. filicaulis Nuttall; P. hippiana Lehmann var. filicaulis (Nuttall) B. Boivin
Leaflets gray to white, teeth (1–)4–9 per side, abaxial surfaces moderately long-hairy especially on veins, moderately to densely cottony, adaxial moderately long-hairy and abundantly cottony. Flowers: epicalyx bractlets and sepals usually abundantly cottony at least proximally; anthers (0.6–)0.7–1 mm. 2n = 56.
Flowering summer. Dry rocky slopes, slightly moist meadows, grasslands, oak, aspen, and montane conifer woodlands (limestone pavement); 700–2600 m; Alta., Man., Sask.; Colo., Idaho, Minn., Mont., Nebr., N.Mex., N.Dak., S.Dak., Utah, Wyo.
Variety effusa occurs from southern Alberta to Manitoba southward on plains and in the mountains to central Colorado. Disjunct populations from northern foothills of the Uinta Mountains in Utah are included in var. effusa by B. C. Johnston (1980), but relegated to Potentilla hippiana by N. H. Holmgren (1997b). Verification is needed for single occurrences of the variety mapped by Johnston in eastern Idaho, northwestern Nebraska, and north-central New Mexico, without citation of specimens. A single collection from Valley County, Idaho, (E. A. Christenson 42, CONN) is almost certainly an introduction of unknown persistence.
The small, narrow, cottony-floccose epicalyx bractlets tend to be glabrescent at least distally, contrasting with the sharply acuminate sepals. Variety effusa, in its most distinctive form, has unevenly pinnate leaves with coarsely toothed gray leaflets that are equally cottony-floccose on both surfaces.
Collections of otherwise typical var. effusa from central Wyoming and adjacent Montana are uniformly smaller than average with only one to four teeth per side of the leaflets. These occur mainly on limestone outcrops (1300–2600 m elevation) and might qualify as a distinct variety.