Mem. New York Bot. Gard. 1: 206. 1900.
Shrubs, 1–10+ dm diam. Stems prostrate or decumbent, loosely intertwined, 1–4+ cm, and erect or ascending, tightly coalesced, 0.2–0.5 cm, internodes 0.1–1 cm. Leaves: blade spatulate, 0.4–1.8 × 0.2–0.4 cm, venation rarely visible except on long-shoot leaves, 1(–3)-veined, apex acute, abaxial surface pilose to sericeous, rarely sparsely strigose on lamina and veins. Panicles widely branched to compact and racemose, [0.5–]1–15[–20] × 0.5–6 cm, sericeous; bracts subulate to narrowly obtrullate, 3–8 mm, pilose to sericeous. Pedicels 0.5–2.5 mm; bracteoles 1, extending from middle to well beyond apex of sepals. Flowers 3–6 mm diam.; hypanthium 1 mm, densely sericeous; sepals erect, ovate-lanceolate, 1–1.5 mm, margins sericeous, abaxial surface sericeous to tomentose; petals narrowly oblanceolate, 1–2.5 mm, apex obtuse to slightly cleft or acute to acuminate; stamens 20, lengths 1.5–2(–2.5) times petals (1.5–3 times sepals); carpels (3–)5(–6), adaxially connate. Follicles 2 mm. 2n = 18.
Ariz., Calif., Colo., Idaho, Mont., N.Mex., Nev., S.Dak., Tex., Utah, Wash., Wyo., ne Mexico.
Subspecies 2 (2 in the flora).
Petrophytum caespitosum primarily inhabits arid rocky outcrops and talus slopes at high elevations in mountain ranges of the western United States and northeastern Mexico.