Potentilla

This is a complex and difficult genus, including diploids and polyploids, complicated by hybridization and apomixis. We follow Ertter (2007) in recognizing Comarum, Dasiphora, Drymocallis, and Sibbaldiopsis segregated from Potentilla. Petals shrivel considerably if not carefully pressed.

1. Leaves (at least basal and lower cauline ones) pinnately compound (axis sometimes short).

2. Leaves all in basal tufts, the plant spreading by slender elongate stolons; flowers solitary on basal peduncles; leaflets mostly 13–21 (plus some alternating tiny ones), strongly whitened beneath with silvery-silky hairs.

P. anserina

2. Leaves all or mostly cauline, the plants without stolons; flowers in cymose inflorescences; leaflets 5–11, if whitened beneath then at least partly tomentose.

3. Lower surface of leaflets ± strongly whitened or silvery gray with mixture of dense tomentum and longer straight hairs.

4. Style at most glandular only at very base; calyx lobes without glands.

P. hippiana

4. Style prominently glandular-warty on at least its basal half; calyx lobes with ± dense sessile to short-stalked glands (except in the railroad waif P. bipinnatifida).

5. Calyx lobes and stipules with ± dense sessile to short-stalked glands (sometimes obscured by dense long hairs); plant of bluffs, rock outcrops, and open, sandy or gravelly sites near Lake Superior.

P. litoralis

5. Calyx lobes and stipules lacking glands; a railroad waif.

P. bipinnatifida

3. Lower surface of leaflets with green not obscured by pubescence, which is whitish to yellowish but not tomentose.

P. supina

1. Leaves all (except sometimes the uppermost bracts) palmately compound.

6. Principal leaves with only 3 leaflets.

7. Bractlets entire (very rarely 2-toothed at tip); receptacle not enlarging, dry.

P. norvegica

7. Bractlets of calyx with 3 (–5) teeth or lobes at summit; receptacle enlarging in fruit, becoming fleshy and red (as in Fragaria).

P. indica

6. Principal leaves (lower cauline and basal) with mostly 5 (–7) leaflets.

8. Flowers on solitary peduncles; stem at first erect, becoming arching (or stoloniferous) and rooting at tip, or plants prostrate and mat forming, with stems rooting at the nodes.

9. Plants with prostrate stems rooting at the nodes; petioles, stems, and peduncles with short (< 1 mm) ascending hairs and fine, very short curly, appressed hairs; rare lawn weed.

P. verna

9. Plants with tip-rooting stolons; petioles, stems, and peduncles silky pilose with ascending to spreading hairs up ca. 1–2 mm long; native plants of open forests, savannas, and successional habitats.

10. Well developed leaflets toothed more than halfway to the base; peduncle of the lowest flower arising in the axil of the second developed leaf on the stolon.

P. simplex

10. Well developed leaflets toothed halfway to the base or less; peduncle of the lowest flower arising in the axil of the first developed leaf on the stolon.

P. canadensis

8. Flowers in cymes; stems various, erect or prostrate to ascending, but without stolons and not rooting at tips or nodes.

11. Leaflets white-tomentose beneath (with any straight silky hairs also white and almost entirely restricted to the main veins); achenes smooth (or rarely with obscure pattern).

12. Stems usually prostrate to ascending; petals (2–) 3–4.5 (–5) mm long, scarcely if at all exceeding the sepals; leaflets sparsely but coarsely toothed to pinnatifid with (1–) 2–3 (–5) teeth per side.

P. argentea

12. Stems erect; petals ca. 5 (–10) mm long, distinctly exceeding the sepals; leaflets deeply pinnatifid with numerous segments.

P. gracilis

11. Leaflets not tomentose (though often glandular) or with fine to dense dull grayish tomentum besides long hairs beneath; achenes at maturity ± strongly rugose, with irregular curved ridges on surface.

13. Leaflets essentially green beneath; larger petals ca. 3–4.3 mm; longer anthers 0.3–0.6 mm long.

P. intermedia

13. Leaflets green or with gray tomentum beneath; petals (4–) 4.5–11 mm long (if leaflets green beneath then petals (6.5–) 8–11 mm long); larger anthers 0.7–1.4 mm long.

14. Leaflets with gray tomentum beneath in addition to straight hairs; larger petals (4–) 4.5–7 mm long, bright yellow; anthers ca. 0.7–0.9 (–1) mm long.

P. inclinata

14. Leaflets ± green beneath, with only ± straight hairs; petals (6.5–) 8–11 mm long, pale yellow; anthers (at least the largest) 1–1.4 mm long.

P. recta