Herbs, perennial, 25-205 cm tall; rhizomes thick. Stems erect, simple or sometimes branched, reddish striate, glabrous or villosulous to strigillose, sometimes glabrescent, eglandular or minutely stipitate glandular. Leaves cauline; middle cauline leaves usually largest, abaxially pale green, adaxially green, sometimes shiny, thinly or thickly papery to subleathery, abaxially villosulous, sparsely scabridulous, or glabrous, minutely stipitate glandular or eglandular, veins densely villosulous to sparsely strigose or glabrate, adaxially ± densely scabridulous, eglandular or sometimes ± minutely stipitate glandular, resinous, margin serrate to coarsely serrate, sometimes serrulate, ciliate, often prominently abaxially 3-veined at base or above (triplinerved), apex obtuse to acute or acuminate, mucronate; lowest leaves withered by anthesis, long narrowly winged petiolate (petiole to 6 cm); blade spatulate or ovate to narrowly lanceolate, 0.5-15(-17) × 0.3-5.5 cm, base attenuate; lower to upper cauline leaves shortly petiolate, ovate- or lanceolate-pandurate to lanceolate, 4.5-15 × 1.2-5 cm, base attenuate to rounded; synflorescence leaves shortly petiolate, ovate to linear-lanceolate, 1-8 × 0.2-3 cm, base cuneate to ± rounded. Capitula 7-100 or more florets in terminal paniculate-corymbiform to corymbiform synflorescences, 1.5-2 cm in diam.; peduncles 5-30 mm, strigillose, minutely stipitate glandular; bracts linear to lanceolate, entire. Involucres campanulate, 3-7 × 4-15 mm; phyllaries 3- or 4-seriate, unequal, apically green, sometimes ± purplish, abaxially glabrate to villosulous (on green part), apically sometimes minutely stipitate glandular, base hardened, margin scarious, erose, sometimes purplish, ciliate, midvein more pronounced distally, ± translucent, apex rounded to acute, erose, ciliate; outer phyllaries ovate to lanceolate, 2-2.5 × 0.5-1 mm, leaflike or distally leaflike; inner phyllaries oblong, 3-6 mm. Ray florets 6-15, purple, lavender, reddish, or white, tube ca. 2 mm, lamina 7-11 × 1.5-3 mm, glabrous, eglandular; disk florets yellow, 4-6.5 mm, tube base flared, limb campanulate, 2.2-4 mm, base sparsely strigillose, lobes spreading, narrowly triangular, 1.3-2 mm, glabrous, sparsely to moderately minutely stipitate glandular. Achenes gray-brown, oblanceolate to obovoid, slightly compressed, 2-2.5 mm, sparsely to moderately strigillose, minutely stipitate glandular or eglandular, 2- or 3-ribbed. Pappus 3-seriate, whitish or ± reddish, of slender, barbellate bristles; outermost bristles few, 0.2-0.3 mm; inner bristles 3-5 mm, tapering; innermost bristles 4.5-6 mm, clavate. Fl. and fr. Jul-Dec.

Y. Ling and Y. L. Chen (in FRPS 74: 159-167. 1985) segregated Aster trinervius from A. ageratoides (see key to subspecies below). Grierson (Notes Roy. Bot. Gard. Edinburgh 26: 67-163. 1964), however, mentioning that there are intermediate forms between the two taxa, treated A. ageratoides as a subspecies of A. trinervius. Although the name of the former has been used in E Asia for a long time and there are many described varieties, differences between the two species do not appear to warrant species-level segregation at the present time.

Aster trinervius subsp. ageratoides is widely distributed and polymorphic. Kitamura (Mem. Coll. Sci. Kyoto Imp. Univ., Ser. B, Biol. 13: 327-377. 1937) and Handel-Mazzetti (Acta Horti Gothob. 12: 203-226. 1938) divided it into many varieties or subspecies (of A. ageratoides). The indumentum, leaf shape, capitula, and shape and size of the involucre are variable and often overlapping, however, and differences between some varieties are not clear. A thorough biosystematic and molecular phylogenetic investigation of the whole complex in Asia would be needed to identify entities that are worth recognition, as was done for Japanese members of the complex (e.g., Ito & Soejima in Iwatsuki et al., Fl. Japan 3b: 59-73. 1995), particularly in view of the possibility of hybridization among entities and with species of A. sect. Pseudocalimeris. Therefore, varieties usually recognized under A. ageratoides are here listed under A. trinervius subsp. ageratoides, with their main characteristics. It should be noted that some taxa listed here as varieties are recognized as species by Ito and Soejima (loc. cit.).

Forests, forest margins, thickets, wet places in valleys; 100-3400 m. Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Ningxia, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xizang, Yunnan, Zhejiang [Bhutan, NE India, Japan, Korea, N Myanmar, Nepal, E Russia, N Thailand, Vietnam].