2. Poteridium occidentale (Nuttall) Rydberg in N. L. Britton et al., N. Amer. Fl. 22: 388. 1908.
Sanguisorba occidentalis Nuttall in J. Torrey and A. Gray, Fl. N. Amer. 1: 429. 1840
Plants winter-annual, glabrous; taproots 3–10 dm. Stamens 2(or 4). Fruits: hypanthia ridges rounded, thickened; sepals not thickened proximally.
Flowering May–Jul; fruiting Jun–Aug. Prairies, sandy open ground, sagebrush flats, vernal pools, drawdown shorelines of streams and lakes, grassy clearings, roadsides, particularly with surficial or subterranean moisture; 10–2100 m; B.C.; Ariz., Calif., Idaho, Mont., Nev., N.Mex., Oreg., Wash., Wyo.
The taxonomic status of Poteridium occidentale has been controversial. Most floristicians of the twentieth century accepted it as distinct from the more eastern P. annuum (usually treating the two in Sanguisorba); but in recent decades a trend developed to merge the two taxa. While superficially similar, they seem to represent independent evolutionary lineages, as indicated by morphologic distinctions and allopatric distributions.