21. Potentilla thurberi A. Gray, Pl. Nov. Thurb. 318. 1854.
Scarlet or Thurber’s cinquefoil
Stems (2–)3–7(–10) dm. Basal leaves palmate, (2–)4–15(–30) cm; petiole (2–)3–10(–25) cm, long hairs sparse to common, spreading, 1–3.5 mm, weak, glands sparse to abundant, sometimes absent; leaflets 5(–7), at tip of leaf axis, central one (1–)2–6(–8) × (0.5–)1–2.5(–4) cm, petiolules 0(–10) mm, distal 3/4 to whole margin incised 1/5–1/4 to midvein, teeth (4–)7–15(–19) per side (sometimes secondarily toothed), (1–)2–3(–5) mm, teeth apex acute to ± obtuse, surfaces slightly to strongly dissimilar, abaxial pale green to white, straight hairs sparse to abundant, often dense on veins, 0.5–3 mm, cottony hairs absent or sparse to dense, glands ± sparse or absent, sometimes obscured, adaxial green, straight hairs sparse to abundant, 0.2–1 mm, cottony hairs absent, glands ± sparse or absent. Cauline leaves: stipules usually toothed, sometimes entire. Inflorescences (4–)10–35-flowered. Pedicels 0.5–4 cm. Flowers: epicalyx bractlets narrowly to broadly lanceolate-elliptic (rarely toothed or lobed), 4–9(–14) × (1–)1.5–2 mm; sepals 4–10(–15) mm, apex acute to acuminate; petals ± dark reddish throughout, (3.5–)6–10 × 6–10 mm; filaments 1.5–3 mm, anthers 0.8–1 mm; carpels 30–70, styles 2.5–3.5 mm. Achenes 1.5 mm, ± rugose.
Varieties 2 (2 in the flora): sw United States, nw Mexico.
The abaxially white-cottony leaflets of var. atrorubens usually make it easily distinguished from var. thurberi, which lacks cottony hairs and is usually less hairy in general. The two varieties sometimes occur together and intermediates between the extremes are common.
The degree of glandularity and the lengths of the central leaflets are greater in the southern populations of both varieties. The glands are less widely distributed on the plants northward and are fewer in northern Arizona and New Mexico. Leaflets are shorter and broader north of Graham County, Arizona, and Otero County, New Mexico.