Barbara Ertter, James L. Reveal

Potentilla sect. Supinae (Lehmann) A. Nelson; Potentilla ser. Supinae Lehmann; Tridophyllum Necker ex Greene

Annuals, biennials, or short-lived perennials, 1-stemmed to ± tufted, not stoloniferous; taproots not fleshy-thickened; vestiture of long hairs, sometimes ± crisped hairs present, glands absent or sparse to abundant, not red (sometimes red-septate). Stems decumbent to erect, sometimes prostrate, not flagelliform, not rooting at nodes, from centers of ephemeral basal rosettes, (1–)2–6(–9) dm, lengths (1–)2–5(–12) times basal leaves. Leaves: basal not in ranks; cauline (0–)1–9(–14); basal and proximal cauline ternate or palmate to pinnate (with distal leaflets ± confluent), (2–)3–15(–25) cm; petiole: long hairs ascending to spreading, rarely appressed, weak to stiff, glands absent or sparse to abundant; leaflets 3–9, at tip to distal 2/3 of leaf axis, separate to overlapping, oblanceolate or elliptic to obovate, sometimes oval or nearly round, margins flat, distal 1/2 to ± whole length evenly to unevenly incised 1/4–1/2 to midvein, teeth (2–)3–8(–15) per side, surfaces ± similar, green (abaxial often paler), not glaucous, long hairs weak to stiff, cottony hairs absent. Inflorescences (5–)20–100+-flowered, ± cymose, sometimes racemiform, compact to open. Pedicels straight to ± recurved in fruit, 0.2–2(–3) cm, proximal often ± longer than distal. Flowers 5-merous; hypanthium (2–)3–6(–7) mm diam.; petals pale yellow to yellow, broadly oblanceolate to oblong or broadly obovate, (1–)1.5–5 mm, usually shorter than sepals, apex rounded to ± retuse; stamens (5–)10–20(–25); styles subapical, conic-tapered, slightly to strongly papillate-swollen ± whole length, 0.5–0.8 mm. Achenes smooth to strongly rugose.

Species 18–21 (4 in the flora): North America, Mexico, Central America, South America, Eurasia, Africa; introduced in Pacific Islands (New Zealand), Australia.

The North American representatives of sect. Rivales are morphologically distinctive in being annuals, biennials, or short-lived perennials (especially in cold regions) with very short, conic-tapered styles. Plants grow primarily in wet habitats, especially where periodically inundated. These distinctions led E. L. Greene (1906c) to adopt Tridophyllum. However, the gross morphological cohesiveness is not matched by molecular monophyly. Instead, chloroplast data scatter Potentilla norvegica, P. rivalis, and P. supina among the core Potentilla, while P. biennis is sister species to the monophyletic Ivesia-Horkelia-Horkeliella clade (C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011). Nuclear markers, on the other hand, place P. norvegica as sister to the Ivesia-Horkelia-Horkeliella clade (Töpel et al.), suggesting a possible hybrid origin and/or multiple origins of the biennial habit.

Existing herbarium annotations of Potentilla biennis, P. norvegica, and P. rivalis are not reliable, but the three species can be readily distinguished by vestiture of proximal petioles and stems. Only P. biennis has prominently septate gland-tipped trichomes; P. norvegica is characterized by relatively stiff, spreading, tubercle-based hairs to 3 mm, resembling those of P. recta. In addition, the tiny, smooth, pale achenes of P. biennis and P. rivalis contrast sharply with the larger, darker, strongly ridged ones of P. norvegica.

Basal leaves usually wither by anthesis, and cauline leaves (that is, those proximal to the first flowering and/or branching node) often wither by mid-anthesis as well. Unless otherwise indicated, leaves include both basal and cauline leaves. If leaves are ephemeral, and/or where stems branch at the proximal nodes, proximal inflorescence bracts can be used as leaf-equivalents in the keys and descriptions.