Smooth mentzelia is a low annual, 5-12 cm tall. The dark stem is stout, erect, branching, and puberulent but not scabrous, at least when young, the hairs barbed at the tips. The leaves are crowded below, oblong-lanceolate or oblanceolate, obtuse, sessile or subsessile, and less puberulent than the stem. Leaf margins are entire, slightly wavy, or with one or two low teeth. Flowers are borne in small terminal clusters, nearly sessile and subtended by 2-3 reduced leaves. The calyx segments are broadly lanceolate and 0.4-0.6 cm long, the bright yellow petals narrowly ovate and 1.0-1.2 cm long. Stamens number 40-50 and are shorter than the petals, with filiform-subulate filaments. The style is much longer than the stamens, the stigma capitate. The capsule is 4-sided and about 2.5 cm long.
The entire floral bracts of smooth mentzelia and its lack of an obvious basal rosette help distinguish this species from the more common and widespread Mentzelia albicaulis, which overlaps its range. The rare M. packardiae is slender, upright, less profusely branched, has narrower leaves, and occurs slightly west of the range of smooth mentzelia.
When to survey
Surveys should be completed when this species is flowering, from May through mid-June.
Smooth mentzelia is restricted to green or gray montmorillonite substrates derived from the Sucker Creek formation, with extremely high levels of potassium. It occurs at elevations from 760-1350 m (2500-4420 ft). Associated plant species that share this barren habitat include Cleomella macbrideana, Phacelia lutea, Monolepis pusilla, Cleome platycarpa, and Camissonia claviformis.
Smooth mentzelia is an edaphic endemic known from Malheur County in southeastern Oregon, Owyhee County in adjacent Idaho, and from two occurrences in Humboldt County, Nevada.
Species of Concern
Habitat destruction due to mining activities is a major threat to smooth mentzelia. Trampling by livestock, ORV use, and exotic weed invasions also negatively impact this species. Smooth mentzelia populations appear to be extremely influenced by climatic fluctuation, with drastic reductions in population sizes observed during drought years.
Did you know?
This species was described by Morton Peck, who collected the type specimen near Rockville, Malheur County, Oregon in 1940.
Meinke, R.J. 1982. Threatened and endangered vascular plants of Oregon: An illustrated guide. Unpublished report for the U.S. Fish and Wildlife Service, Region 1, Portland, Oregon. Oregon Department of Agriculture, Salem, Oregon.
NNHP (Nevada Natural Heritage Program). 2001. Rare plant fact sheet: Metzelia mollis. Nevada Natural Heritage Program, Carson City, Nevada.
ORNHIC (Oregon Natural Heritage Information Center). 2007. Rare, threatened and endangered species of Oregon. Oregon Natural Heritage Information Center, Oregon State University, Portland, Oregon.
ORNHIC (Oregon Natural Heritage Information Center). 2010. ORNHIC element occurrence database. Portland, Oregon.
Peck, M. E. 1945. Mentzelia mollis. Leaflets of Western Botany. 4:183.
Peck, M. E. 1961. A manual of the higher plants of Oregon. Binfords and Mort, Portland, Oregon.
Smithman, L. C. 1989. Distribution and occurrence of Mentzelia mollis Peck. Unpublished report. Bureau of Land Management, Vale District, Vale, Oregon.