Plants 0.3-0.8(-1) × 0.2-0.8 dm. Leaf blades (1-)1.5-4(-6) × 0.2-0.4(-0.7) cm. Peduncles 0.5-1 mm. Flowers: perianth 1.2-1.8(-2) mm; filaments 0.6-1 mm; anthers 0.4-0.5 mm. Achenes 1.7-2 mm. 2n = ca. 34. Flowering May-Jun. Sandy to gravelly flats and slopes, chaparral communities, pine-oak woodlands; of conservation concern; 200-700 m; Calif. Dodecahema leptoceras is known only from the foothills of the San Gabriel Mountains of Los Angeles County, the San Bernardino Mountains of San Bernardino County, and the San Jacinto Mountains of western Riverside County. The urbanization of the greater Los Angeles area has significantly reduced the number of known northern populations. Slender-horned spinyherb is federally listed as endangered.
Dodecahema leptoceras is in the Center for Plant Conservation's National Collection of Endangered Plants.