Smooth White American Aster

Symphyotrichum porteri is the only western member of subsect. Porteriana; it grows in the foothills of the Rocky Mountains.

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.

Aster porteri A. Gray, Notes Compositae, 99. 1880; A. ericoides Linnaeus var. strictus Porter

Symphyotrichum porteri (formerly Aster porteri) is a species of flowering plant in the family Asteraceae endemic to the foothills of the Rocky Mountains in the U.S. states of Wyoming, Colorado, and New Mexico. Commonly known as Porter's aster, it is a perennial, herbaceous plant that may reach 10 to 50 centimeters (4 to 20 inches) tall. Its flowers have white, rarely pinkish, ray florets and yellow, becoming pink then brown, disk florets. S. porteri grows at elevations of 1,800–2,900 meters (5,900–9,500 feet). Its limited range makes it a NatureServe Vulnerable (G3) species, and it is classified Critically Imperiled (S1) in Wyoming.

S. porteri herbarium specimen