Horkelia [unranked] Tridentatae Rydberg, Monogr. N. Amer. Potentilleae, 120, 122. 1898; Horkelia [unranked] Ambiguae A. Gray; Horkelia [unranked] Hirsutae Rydberg; Horkelia sect. Hirsutae (Rydberg) O. Stevens; Horkelia [unranked] Sericatae Rydberg; Horkelia sect. Sericatae (Rydberg) O. Stevens

Plants forming rosettes or tufts, sometimes loose mats, green to grayish or silvery, inconspicuously to moderately glandular, not resinously aromatic. Stems decumbent to erect, 0.5–5 dm. Basal leaves planar to ± cylindric; stipules entire, forked, or pinnately divided into linear to filiform lobes; leaflets 2–20 per side, separate to ± overlapping, divided either 1/2–3/4+ to midrib into (0–)2–15 ultimate lobes or teeth not restricted to apex, or 1/10–1/4 or less to midrib into (0–)3(–5) teeth restricted to apex. Inflorescences open to ± congested, flowers arranged individually, in corymbiform clusters, and/or in capitate or non-capitate glomerules. Pedicels remaining straight, outermost sometimes ± reflexed in congested inflorescences, 1–10(–20) mm. Flowers: epicalyx bractlets linear to lanceolate or narrowly elliptic, 0.2–0.5(–0.8) mm wide, entire; hypanthium interior usually glabrous, sometimes pilose; sepals obtuse to acute to slightly acuminate; petals white to pink or cream, sometimes drying yellowish, linear or cuneate to obovate or obcordate, apex usually acute to rounded to emarginate, sometimes truncate, rarely mucronate; filaments white, glabrous, anthers longer or shorter than wide; carpels 2–15(–20). Achenes 1.5–3 mm, smooth or rugose.

Species 5 (5 in the flora): w United States.

P. A. Rydberg (1908c) assigned the species here composing this section to three of his unranked groups: Hirsutae, Sericatae (also including Horkelia hispidula), and Tridentatae, validated inadvertently as sections by O. A. Stevens in the index to volume 22 of North American Flora. In addition to lacking the characteristic Horkelia odor, plants are often rosette-forming from simple caudices; stipules are sometimes highly divided; and leaflets are either few-toothed or deeply lobed. The section is centered in the Siskiyou-Klamath region of northwestern California and southwestern Oregon, with some affinity for ultramafic substrates. Horkelia tridentata extends south in the Sierra Nevada; H. congesta is a component of the Willamette Valley flora. Much of the variation in the section is highly localized, with a full series of intergrading morphologies, indicative of a relatively recent and active radiation. More segregates are recognized here than in some recent floras (M. E. Peck 1941; P. A. Munz 1959; B. Ertter 1993d), and additional variants are likely to warrant taxonomic recognition.