Taxon 31: 359. 1982.
Perennials, (10–) 20–120 cm, colonial or cespitose; usually long-rhizomatous, sometimes with branched caudices. Stems 1–3+, erect (straight to ± flexuous), glabrate to sparsely hirsute or strigillose, especially distally. Leaves thin, margins coarsely, sharply serrate to crenate-serrate or serrulate, ciliate to scabrous, apices acute or acuminate, mucronulate, abaxial faces glabrate to sparsely hirsute, midveins usually densely hirsute, sometimes glabrous, adaxial glabrous or glabrate to scabrellous; basal usually withering by flowering (sometimes persistent on small plants), long-petiolate (petioles slightly winged, sheathing, ciliate), blades ovate, (24–) 40–120 (–270) × 15–70 mm, bases usually shallowly cordate, sometimes rounded; proximal cauline often withering by flowering, winged-petiolate, blades ovate to oblanceolate or lanceolate, (24–) 60–150 × (10–) 20–60 mm, reduced distally, bases subcordate to cuneate; distal sessile or sometimes subsessile (petioles widely-winged), blades lanceovate to linear-lanceolate or linear, 18–75 × (2–) 5–25 mm, bases cuneate, sometimes ± clasping, margins serrulate or entire. Heads [(6–) 13–50 (–100+)] in open, paniculiform arrays, branches ascending. Peduncles 0.2–1.5+ cm, unequal, reduced distally, ± hirsutulous, bracts 0–4, subulate or linear. Involucres campanulate, (4–) 5–6.5 mm. Phyllaries in 4–5 series, narrowly oblong-lanceolate to oblong-oblanceolate or linear-oblanceolate (innermost), ± unequal, bases indurate 1/3–2/3, margins narrowly scarious, erose, hyaline or infrequently purplish, sparsely ciliolate, green zones lanceolate to linear-lanceolate or linear, apices acuminate to long-acuminate, faces glabrous. Ray-florets (12–) 14–20; corollas pale to deep blue or bluish purple, laminae (8.3–) 10–15 × 1–2.3 mm. Disc-florets 14–25 (–29); corollas yellow becoming reddish purple, 4.3–6.4 mm, tubes shorter than funnelform throats, lobes narrowly triangular, 0.5–1.1 mm. Cypselae yellowish white, obovoid, compressed, 1–2 mm, 5–6-nerved, faces glabrate to sparsely strigillose; pappi white to pinkish, 3–6 mm. 2n = 48.
Phenology: Flowering late Jul–Oct.
Habitat: Rich, open, often ± calcareous, boreal deciduous forests (aspen or aspen-birch-fir-spruce), edges of woods, clearings, aspen or bur oak thickets, sometimes open pine forests, streambanks, trails, roadsides
Elevation: 0–2000+ m
Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., Que., Sask., Yukon, Ill., Maine, Mass., Mich., Minn., Mont., N.H., N.Y., N.Dak., S.Dak., Vt., Wis., Wyo.
Symphyotrichum ciliolatum sporadically hybridizes with S. laeve var. laeve or var. geyeri in their areas of overlap, notably across the Canadian prairies and in south-central Ontario (Manitoulin Island, Bruce Peninsula). It also hybridizes with S. novi-belgii var. novi-belgii in the Gulf of St. Lawrence area [Symphyotrichum ×subgeminatum (Fernald) G. L. Nesom; syn. Aster subgeminatus (Fernald) B. Boivin]. Aster ciliolatus forma comatus Fernald is densely ciliolate along the midnerves and represents an extreme of the variation found in the species.
"thin" is not a number.