5. Potentilla rivalis Nutt. (brook cinquefoil)

P. rivalis var. millegrana (Engelm. ex Lehm.) S. Watson

P. rivalis var. pentandra (Engelm.) S. Watson

P. millegrana Engelm. ex Lehm.

P. pentandra Engelm.

Pl. 535 g; Map 2468

Plants annual or biennial. Stems 5–60 cm long, ascending to loosely ascending from a spreading base, moderately to densely pubescent with longer, straight, spreading to loosely appressed hairs and shorter, crinkly hairs. Leaves with the petioles mostly 1–6 cm long (those toward the stem tips usually sessile), when present these hairy. Stipules 5–15 mm long, narrowly ovate to ovate, sharply pointed at the tip, the margins entire or toothed and often hairy, the surfaces hairy. Leaf blades 2–6 cm long, palmately compound or the largest leaves sometimes with a short rachis, with 3 or 5 leaflets, these 0.5–6.0 cm long (the central or terminal leaflet the largest), oblanceolate to obovate or oval, rounded to bluntly pointed at the tips, the margins coarsely toothed or scalloped, the upper surface green, glabrous or sparsely to moderately hairy, the undersurface lighter green, moderately hairy. Inflorescences terminal panicles, sometimes reduced to racemes, mostly with numerous flowers. Bractlets 3–6 mm long, narrowly elliptic, moderately hairy. Hypanthia 4–5 mm in diameter, shallowly cup-shaped. Sepals 3–7 mm long, broadly ovate, sharply pointed at the tips. Petals 1.5–3.0 mm long, oblanceolate to obovate, yellow to pale yellow. Stamens (5–)10(–15). Ovaries with the styles attached near the tips. Fruits 0.7–0.9 mm long, the surface smooth or occasionally slightly wrinkled, brown, unwinged. 2n=14, 70. May–October.

Scattered in counties adjacent to the Mississippi, Missouri, and Des Moines Rivers (western U.S. east to Wisconsin and Arkansas; sporadically east to Maine and Virginia; Canada, Mexico). Banks of streams and rivers, openings of bottomland forests and swamps, and rarely ledges of bluffs; also pastures and open, disturbed areas.

Opinions have varied on whether to segregate three varieties or species from the P. rivalis complex, based on whether some of the leaves are palmately rather than pinnately compound and whether all of the leaves have three leaflets or the larger ones have five. They seem best-accounted for informally.