Horkelia tenuiloba

(Torrey) A. Gray

Proc. Amer. Acad. Arts 6: 529. 1865.

Common names: Thin-lobed Santa Rosa or Sonoma horkelia
Conservation concernEndemic
Basionym: Horkelia fusca var. tenuiloba Torrey in War Department [U.S.], Pacif. Railr. Rep. 4(5): 84. 1857
Synonyms: Potentilla micheneri Greene P. stenoloba Greene
Treatment appears in FNA Volume 9. Treatment on page 252. Mentioned on page 249, 251, 254.

Plants loosely matted, green. Stems ascending to erect, 1–4 dm, hairs ± spreading. Basal leaves weakly planar to ± cylindric, 5–15(–20) × 0.5–1.5 cm; stipules entire; leaflets 8–16(–20) per side, ± overlapping especially distally, cuneate to flabellate, 3–10 × 2–10 mm, 1/2 to nearly as wide as long, divided 1/2–3/4+ to midrib into 3–8 linear to narrowly oblanceolate or narrowly elliptic lobes, sparsely villous at least marginally, often with a tuft of hairs apically. Cauline leaves 2–5. Inflorescences open to congested, flowers arranged individually and in glomerules, these sometimes subcapitate. Pedicels 1–6 mm. Flowers 10 mm diam.; epicalyx bractlets narrowly to broadly lanceolate, 1–3 × 0.5–1 mm, slightly shorter than sepals, entire; hypanthium 1–1.2 × 2.5–4.5 mm, less than 1/2 as deep as wide, interior pilose; sepals spreading to reflexed, lanceolate, 3–5 mm; petals oblanceolate, 2.5–4.5 × 1.5 mm, apex emarginate; filaments (1–)1.5–2 × 0.5 mm, anthers 0.4–0.6 mm; carpels 10–25; styles 1.8–2.2 mm. Achenes light brown, 1.5 mm, smooth or slightly rugose. 2n = 28.

Phenology: Flowering summer.
Habitat: Sandy soil, openings, in chaparral, oak woodlands
Elevation: 50–500 m


Of conservation concern.

Horkelia tenuiloba occurs on the western edges of the northern Coast Ranges in Marin, Mendocino, and Sonoma counties. Populations from San Luis Obispo formerly included in this species now are part of H. yadonii. A specimen (M. K. C[urran], July 5, 1885, UC) unequivocally of H. tenuiloba purportedly from San Luis Obispo is in all likelihood mislabeled with respect to locality.

Horkelia tenuiloba is commonly associated with seral openings in chaparral and woodlands and might be dependent on periodic disturbance by fire.

W. L. Jepson (1909–1943, vol. 2) used Potentilla stenoloba (1895) for the species encompassing the types of Horkelia tenuiloba and P. micheneri. The epithet micheneri (1893) has priority at species rank within Potentilla, since P. tenuiloba (Torrey) Greene is a later homonym of P. tenuiloba Jordan.

Selected References


Lower Taxa

... more about "Horkelia tenuiloba"
Barbara Ertter +  and James L. Reveal +
(Torrey) A. Gray +
Horkelia fusca var. tenuiloba +
Thin-lobed +, Santa Rosa +  and or Sonoma horkelia +
50–500 m +
Sandy soil, openings, in chaparral, oak woodlands +
Flowering summer. +
Proc. Amer. Acad. Arts +
Conservation concern +  and Endemic +
Potentilla micheneri +  and P. stenoloba +
Horkelia tenuiloba +
Horkelia sect. Horkelia +
species +