Pittonia 3: 99. 1896.
Plants biennial, bushlike. Stems solitary, decumbent to erect, straight; branches along entire stem, distal or proximal longest, antrorse, straight or upcurved, proximal decumbent; hairy. Leaves: blade 32–86 × 5.8–15 mm, widest intersinus distance 1.7–5.2 (–7.1) mm; proximal oblanceolate to elliptic, margins dentate to pinnate, teeth or lobes 6–12, perpendicular to leaf axis, 1.2–6.3 mm; distal elliptic to lanceolate, base not clasping, margins dentate to pinnate, teeth or lobes 4–10, perpendicular to leaf axis, 1.8–5.2 mm; abaxial surface with simple grappling-hook, complex grappling-hook, and occasionally needlelike trichomes, largest trichomes without pearly white bases, adaxial surface with simple grappling-hook and needlelike trichomes. Bracts: margins entire or toothed. Flowers: petals golden yellow, 14.2–19.8 × 3.8–6.5 mm, apex acute to rounded, glabrous abaxially; stamens golden yellow, 5 outermost petaloid, filaments narrowly spatulate, slightly clawed, 12–18.1 × 2.7–4.8 mm, with or without anthers, second whorl with anthers; anthers straight after dehiscence, epidermis smooth; styles 8.3–11.1 mm. Capsules cylindric, 12.2–18 × 4.3–8.3 mm, base tapering, not longitudinally ridged. Seeds: coat anticlinal cell-walls wavy, papillae 7–17 per cell. 2n = 20.
Phenology: Flowering Jun–Aug.
Habitat: Moist canyon walls, lower talus slopes, gravelly and sandy soils.
Elevation: 1600–2400 m.
Mentzelia densa is known only from Chaffee and Fremont counties. It is in the Center for Plant Conservation’s National Collection of Endangered Plants.