Cact. Succ. J. (Los Angeles) 2: 466, fig. 1931.
Plants tiny, usually unbranched in the wild, nearly covered by soil (often having only 2–5 areoles exposed). Stems erect, spheric, (1–) 2–3 × 1–2 (–2.5) cm; ribs 6–9, crests poorly defined, undulate; areoles of largest plants 6–7 (–10) mm apart. Spines 8–14 (–15) per areole, in relatively old plants curving irregularly in random directions, gray or white, some or all tipped black, dark reddish-brown, or dull reddish purple; radial spines 8–14 per areole, ± pectinately arranged, (5–) 10–15 (–19) mm; central spines 0 (–1) per areole, terete, 10–12 mm, closely resembling largest radial spines. Flowers 1.5–2 × 1.5–2 (–2.7) cm; flower tube 5–8 × 5–10 mm; flower tube hairs very short, inconspicuous; inner tepals bright yellow-green, often with maroon midstripes, 9–18 × 2.7–3.8 mm, tips relatively thin and delicate; anthers yellow; nectar chamber 1 mm. Fruits purplish brown or remaining green when ripe, 5.5–9 (–11) mm, pulp white. 2n = 22.
Phenology: Flowering Feb–Mar; fruiting 1 1/2-2 months after flowering.
Habitat: Semidesert grasslands, novaculite outcrops
Elevation: 1200-1300 m
The relatively heavy, brown-and-white, often slightly contorted radial spines of Echinocereus davisii help to distinguish these tiny plants from seedlings of their larger congeners; remains of flowers and fruits, such as scattered flower tube spine-clusters, provide proof of sexual maturity. Echinocereus davisii is easily distinguished from E. viridiflorus; it is parapatric with E. viridiflorus without evidence of introgression. Echinocereus davisii is less compatible with E. viridiflorus than any varieties of E. viridiflorus are with one another (E. E. Leuck 1980).
Echinocereus davisii is in the Center for Plant Conservation’s National Collection of Endangered Plants.
"thin" is not a number.