Plants silvery to whitish, densely sericeous to somewhat lanuginose. Stems (1-)2-10, ascending to ± prostrate; proximal internode lengths mostly 0.5-1.5(-2) times leaf lengths. Capitular leaves ± erect, appressed to heads, linear to narrowly oblanceolate, widest in distal 2/3, longest 12-20 mm, lengths mostly 6-12 times widths, (3-)3.5-5 times head heights. Heads ± spheric, largest 4-6 mm. Receptacles unlobed. Pistillate paleae individually visible through indument, longest mostly 1.5-2.7 mm. Staminate corollas 0.7-1.4 mm, lobes mostly 4. Cypselae narrowly ± cylindric, terete, 0.6-1.2 mm. Flowering and fruiting late Mar-mid Aug. Seasonally inundated or flooded clay soils (vernal pool margins, drainages, moist rocky slopes); 10-1800(-2400) m; Calif., Idaho, Nev., Oreg., Wash. Psilocarphusoregonus occurs from west-central California through most of Oregon to southeastern Washington, western Idaho, and northern Nevada. Relatively narrow-leaved, montane forms of P. tenellus account for reports of P. oregonus from the southern Sierra Nevada to Baja California; further study may show these to be intermediates between the two taxa.
A malformed plant collected in Merced County, California, appears to have been a sterile hybrid between P. oregonus and Hesperevax caulescens (J. D. Morefield 1992c).