Luc Brouillet, John C. Semple, Geraldine A. Allen, Kenton L. Chambers, Scott D. Sundberg+ in Flora of North America (vol. 20)
Perennials, 10-50 cm, cespitose; usually with branched caudices, sometimes long-rhizomatous. Stems 1-4+, ascending to erect (straight), glabrous. Leaves thin, margins usually entire, sometimes very sparsely serrulate distally, scabrous, apices acute, mucronulate, faces glabrous; basal usually deciduous at flowering (new winter rosettes developed by flowering), petiolate (petioles narrow or ± winged, then appearing sessile, slightly sheathing, ciliate), blades oblanceolate to obovate or spatulate, 5-45+ × 2-6 mm, cuneate, apices obtuse; proximal cauline usually withering by flowering, petiolate or sessile (petioles winged, slightly clasping, ciliate), blades oblanceolate or lanceolate to linear-oblanceolate or -lanceolate, 40-80(-90) × 2-5(-6) mm, bases cuneate; distal sessile, blades linear-lanceolate to linear, ± falcate, 8-60 × 0.5-3 mm, reduced distally, bases cuneate to rounded-cuneate. Heads in paniculiform arrays, branches ascending, ± densely leafy with narrow branch leaves. Peduncles 0.2-2.8 cm, bracteate, glabrous, bracts 1-3, linear to linear-lanceolate. Involucres campanulate to cylindro-campanulate, 3.6-5(-6) mm. Phyllaries in 4-6 series, linear-lanceolate to -oblanceolate (outer) or oblong-lanceolate (inner), unequal, bases indurate 1 / 4 - 3 / 4 (sometimes outermost foliaceous), margins proximally narrowly scarious, erose, hyaline, distally ciliolate, green zones lanceolate, apices acute, ± spreading, involute, mucronulate, faces glabrous. Ray florets (12-)20-34(-42); corollas usually white, seldom pinkish, laminae 6.8-12 × 0.7-1.8 mm. Disc florets (30-)40-75(-103); corollas pale yellow becoming pink and later brown, (2.8-)3.1-3.9(-4.9) mm, tubes much shorter than narrowly funnelform throats, lobes triangular, (0.3-)0.5-0.7(-0.8) mm. Cypselae purple to stramineous, obovoid to oblanceoloid, compressed, 1-2 mm, 4-5-nerved, faces sparsely, finely strigillose; pappi white, 1.2-3.5 mm. 2n = 16. Flowering late Jul-Sep. Lower montane; 1800-2900 m; Colo., N.Mex., Wyo. Symphyotrichum porteri is the only western member of subsect. Porteriana; it grows in the foothills of the Rocky Mountains.