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Keyed in multiple places:

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Key to Symphyotrichum, Key A: perennial asters with petiolate, cordate-bladed lower leaves
[of subgenus Symphyotrichum, sections Heterophylli and Concinni]

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1 Phyllary tips squarrose, or spreading and foliaceous; heads larger: involucre 5-10 mm high; disc flowers 33-50 (or more); ray flowers (13-) 20-45.
  2 Phyllaries linear, in 4-7 series; disc flowers 4.0-5.5 mm long; [dry, rocky soils of open woodlands and prairies; IL and n. MO south through MO and se. KS to s. AR and se. OK]
  2 Phyllaries broadly lanceolate, foliaceous, in 3-5 series; disc flowers 5.8-6.8 mm long; [moist forests of the higher-elevation Southern Appalachians; w. NC and e. TN south to nw. SC and n. GA]
1 Phyllary tips appressed or ascending; heads smaller: involucres 3.8-7(-8) mm high; disc flowers (8-) 10-25 (-30); ray flowers 8-20 (-25).
    3 Leaves glaucous, slightly fleshy, glabrous on both faces; stems and array branches nearly glabrous; [series Concinni]
    3 Leaves not glaucous, herbaceous in texture; stems and array branches densely pubescent to glabrate.
      4 Leaves entire to weakly undulate or crenate-serrate, typically narrowly lanceolate or linear-lanceolate at midstem; phyllaries with a short diamond-shaped green blaze (1-1.5× as long as wide).
        5 Stem leaves nearly all distinctly petiolate, petioles scarcely winged, blade bases cordate to subcordate; leaves glabrous adaxially except on veins.
        5 Stem leaves usually sessile above, cuneate and gradually narrowed onto petiole and/or petiole wings dilated at bases; leaves usually scabrous or pubescent adaxially.
          6 At least some cauline leaves with petiole wings dilated to cordate-clasping bases (often panduriform); cauline leaf blades ovate to lanceolate, bases cordate to broadly cuneate, upper sessile and clasping; upper stems and often adaxial leaf surfaces densely hirtellous with whitish hairs
          6 Petioles not notably dilated at the base; mid and upper cauline blades narrowly lanceolate to linear, bases cuneate to attenuate, gradually narrowed onto the petiole or sessile; upper stems minutely strigillose or pilosulous, rarely glabrous; [series Concinni].
             7 Cauline leaves narrowly lanceolate; basal leaves cordate; inflorescence denser, rays blue or purple, sometimes pale; [MN, NY, and ON south to TN and AR]
             7 Cauline leaves (except lowermost) linear or linear-lanceolate; basal leaves often rounded or cuneate; inflorescence sparse, heads often long-pedunculate with white rays; [West Gulf Coastal Plain, apparently disjunct eastwards in MS and AL]
      4 Leaves regularly serrate or crenate-serrate (uppermost sometimes entire), mostly broader; phyllary green blazes lanceolate to linear (> 2× as long as wide; except S. lowrieanum and S. schistosum).
               8 Rays white (rarely pale lilac); phyllary green zones not broadened towards tip, linear-lanceolate to linear (> 4× as long as wide), apices long-acuminate and ascending; inflorescence usually densely racemose, the branches stiffly erect; stems glabrate or sparsely pilose distally
               8 Rays bluish or purplish (rarely white); phyllary green zones diamond-shaped to lanceolate (< 4× as long as wide; the inner may be linear-lanceolate in S. drummondii), apices various; inflorescence usually diffusely paniculiform, branches spreading to ascending.
                 9 Stems densely pubescent distally and in arrays; phyllary green zones lanceolate to linear-lanceolate.
                   10 Cauline leaves cordate-clasping, either sessile or with petioles conspicuously dilated to a cordate-clasping base
                   10 Cauline leaves not clasping.
                     11 Plants 4-12 dm tall; cypselas glabrous
                     11 Plants 3-8 dm tall; cypselas strigillose
                 9 Stems glabrate or distally sparsely pubescent; phyllary green zones diamond-shaped to lanceolate (1-3× as long as wide).
                       12 Basal and proximal leaves deeply cordate; petioles narrowly winged; leaf blades usually pilose abaxially
                       12 Basal and proximal leaves shallowly cordate to rounded; petioles broadly winged, wings expanded near blade; leaf blades usually glabrous, sometimes thinly glaucous.
                          13 Plants solitary or several-stemmed; [more widespread]
                          13 Plants strongly rhizomatous-colonial; [shale in VA]

Key to Symphyotrichum, Key E: perennial asters with clasping or sheathing leaves
[of subgenus Symphyotrichum, sections Symphyotrichum and Concinni]

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1 Mid-stem leaves narrowed above the clasping base to an elongate petiolar portion (1/2-1/8 of the blade length) which flares abruptly into an ovate to lanceolate upper portion 10-55 mm wide; stem slightly to strongly zigzag in the upper portion; [series Punicei]
1 Mid-stem leaves linear to lanceolate; stem usually straight (lacking bends alternating at the nodes).
  2 Phyllaries appressed, apices acute to acuminate (also keyed in Key F); plants 1.5-4 (-6) dm; [ultramafic outcrop barrens, Clay co., NC]; [series Punicei]
  2 Phyllaries spreading; plants (1.7-) 6-25+ dm; [other habitats, collectively widespread].
    3 Phyllaries mostly obtuse to acute, sometimes acuminate, green zones foliaceous; mid and upper stem leaves mostly lanceolate or oblanceolate to linear, faces glabrous, bases slightly auriculate-clasping to short-petiolate and attenuate; stems sparsely hairy in lines above, otherwise glabrous; [series Symphyotrichum].
      4 Lower stem leaves distinctly petiolate, blades often with rounded bases; stems often flexuous; [moist forests; w. NC and e. TN south to n. GA].
      4 Lower stem leaves attenuate, sessile to winged-petiolate; leaves thick-textured, often slightly fleshy; stems erect; [marshes, pine savannahs; Atlantic Coastal Plain of GA northwards, disjunct in w. NC]
        5 Leaves linear, > 10× as long as wide; leaf bases only subclasping
        5 Leaves lanceolate, 6-10× as long as wide; leaf bases clasping
    3 Phyllaries mostly long-acuminate to caudate; mid and upper stem leaves mostly ovate, obovate, lanceolate, or oblanceolate, scabrous adaxially (except S. firmum), bases strongly auriculate-clasping (except S. elliottii); stems densely hispid to sparsely hairy in lines; [series Punicei].
          6 Stem leaves cuneate or rounded at base; leaves thick, firm
          6 Stem leaves strongly auriculate-clasping; leaves membranaceous.
             7 Stems glabrous, occasionally hispidulous in lines; abaxial cauline leaf midvein glabrous; capitulescence dense, leafy; heads with white to pale lavender ray florets; shoots arising singly from elongate rhizomes
             7 Stems densely pubescent, usually purplish; abaxial cauline leaf midvein moderately to densely pubescent; capitulescence widely spreading and heads with lavender to purple ray florets; shoots often found in clumps of 2-6 arising from a persistent stout caudex
               8 Upper and lower leaf surfaces nearly the same color; upper stem leaves smaller in size than the midstem leaves
               8 Upper and lower leaf surfaces distinctly different in color (the lower pale green, the upper dark green); upper stem leaves (on branches leading to heads) equal or nearly so in size to mid-stem leaves
                 9 Plants to 6 dm tall; {other characters}; [nw. NC]
                 9 Plants 10-30 dm tall; {other characters}; [widespread]
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