The tailcup lupine is the commonly encountered lupine at mid-season below the conifer zone.  It overlaps the range of our beloved long-spur lupine (L. arbustus) but generally blooms later and has more silvery leaves.  The flowers are also slightly smaller and more uniformly purple, with a shorter spur (i.e., nectar-bearing protrusion on the back of the calyx).  Intermediates are sometimes encountered, possibly hybrids, possibly some other explanation (these are, after all, lupines!)

The scientific name used here, Lupinus argenteus var. heteranthus, follows the recently published treatment of the genus in Flora of North America (Sholars & Riggins 2023).  This in turn follows the treatment in Intermountain Flora by Barneby (1989), who demoted several previously separate species to varieties of L. argenteus in  “an attempt to apply to Lupinus in our region standards of specific and subordinate rank comparable to other genera of Fabaceae”.  In contrast, both L. caudatus and L. holosericeus are still recognized as distinct species in Plants of the World Online, and hence iNaturalist (at least as of 30 July 2026).

As described in Intermountain Flora, L. argenteus var.  heteranthus is widespread in the western fourth of the Intermountain region, with the distribution in Idaho relatively vague (and even vaguer in Flora of North America).  Our plants nevertheless key out fairly well to this variety, including specimens that had been previously identified as L. argenteus var. utahensis or L. caudatus.  The distinction between var. heteranthus and var. utahensis is fairly subtle, with the former having “the more pronounced calyx spur and the more widely gaping flowers” (Sholars & Riggins 2023).  Both varieties are lumped together in L. caudatus in the second edition of Flora of the Pacific Northwest (Hitchcock & Cronquist 2018); alternatively, Plants of the World Online lists var. heteranthus as a synonym of L. caudatus subsp. caudatus while segregating var. utahensis as L. caudatus subsp. utahensis.

Barneby. R. C.  1989.  Fabales.  Intermountain Flora, vol. 3, part B.

Hitchcock, C. L. and A. Cronquist; edited by D.E. Giblin, B. S. Legler, P. F. ZIka, and R. G. Olmstead. 2018.  Flora of the Pacific Northwest: an illustrated manual. Second edition. Univ. Washington Press.

Sholars, T. and R. Riggins.  2023.  Lupinus.  Flora of North America north of Mexico 11.1: 193–249.