The narrowly endemic vascular plant species Eriogonum calcareum (S. Stokes) Grady & Reveal (Polygonaceae) is restricted to barren outcrops composed of Miocene to Pleistocene silicic volcanic ashes scattered throughout southwestern Idaho and southeastern Oregon. We hypothesize that varieties calcareum and sceptrum are morphologically, geographically, and ecologically differentiated. To test this hypothesis, we measured morphologic and edaphic parameters among 27 populations of E. calcareum across the range of the species. Eriogonum calcareum var. calcareum ranges from eastern Malheur County, Oregon, to southwestern Washington and Payette counties, Idaho, has sessile involucres with hairs sparsely to moderately covering the tube, shorter scape lengths, and narrower leaves. Eriogonum calcareum var. sceptrum (Reveal) Reveal & Mansfield ranges from Canyon County, Idaho, southeast through Owyhee, Twin Falls, and Elmore counties, has pedunculate involucres that are glabrous to partially floccose at the distal end of the tube, longer scape lengths, and wider leaves. Although some populations have not fully differentiated, plants in the northwestern end of the range (var. calcareum) occupy soils significantly higher in both Mg (9.78 cmol/kg; P < 0.001) and K (2.84 cmol/kg; P = 0.018), and lower in pH (5.15; P = 0.002) and Ca/Mg (3.03; P = 0.016) than plants in the southeastern end of the range (var. sceptrum: Mg: 2.81 cmol/kg; K: 0.82 cmol/kg; pH: 7.82; Ca/Mg: 29.64). The morphological differentiation of the populations justifies a separation of var. calcareum and var. sceptrum despite the existence of populations with ambiguous morphologies. Population differentiation is likely driven by both geographic isolation and edaphic selection.