Ber. Deutsch. Bot. Ges. 10: 263. 1892.
Plants biennial or perennial, bushlike, perennials with ground-level caudices. Stems solitary, erect, straight; branches distal or along entire stem, proximal or distal longest, antrorse, straight to upcurved; hairy. Leaves: blade 72–295 × 14–45 mm, widest intersinus distance 10.1–23.3 mm; proximal oblanceolate or elliptic, margins serrate to pinnate, teeth or lobes 16–26, slightly antrorse, 1–16.5 mm; distal elliptic to lanceolate, base clasping or not, margins serrate to pinnate, teeth or lobes 9–20, slightly antrorse, 5.3–13.7 mm; surfaces with needlelike trichomes. Bracts: margins pinnate. Flowers: petals white, 47–75 × 13–22.7 mm, apex acute to attenuate, glabrous abaxially; stamens white to yellow, 5 outermost petaloid, filaments spatulate, strongly clawed, 48–75 × 12–23 mm, without anthers, second whorl with anthers; anthers straight after dehiscence, epidermis smooth; styles 36–53 mm. Capsules cylindric, 30–43 × 12–17 mm, base tapering, not longitudinally ridged. Seeds 2.3–4 mm; coat anticlinal cell-walls straight, papillae 4–10 per cell. 2n = 22.
Phenology: Flowering Jun–Aug(–Oct).
Habitat: Rock outcrops, slopes, dry short-grass prairies, riverbanks, roadsides, loam, limestone, sandy, silty, clayey, and gravelly soils.
Elevation: 300–2400 m.
Alta., Man., Sask., Colo., Ill., Iowa, Kans., Minn., Mont., Nebr., N.Mex., Okla., S.Dak., Tex., Utah, Wyo.
Mentzelia decapetala is introduced in Grundy County, Illinois. It appears to be native throughout the rest of its distribution.