This is the great bulk of the former inclusive genus Aster in Michigan, and remains a large and difficult genus. It includes all the native species with large, paniculate, many-headed inflorescences.
Symphyotrichum prenanthoides (Willd.) G.L. Nesom is represented by a collection by Farwell (754) from Keweenaw Co., in 1890. It is so far out of range as to suggest a labeling error, and is not mapped or treated. It somewhat resembles S. laeve, but with zigzag stems with fine pubescence in lines. Similarly, a sheet of S. patens, with no data but "Lansing" is surely a label error.
Hybridization seems more frequent than in most genera (although perhaps not as common as sometimes supposed) and morphological variation is great within species (sometimes depending upon habitat). Ample specimens in flower and with lower leaves and basal parts are vital for determination. Rays that are white when fresh may turn pink or blue when dried; others are colored from the beginning. Disk corollas often turn from yellow to rose or violet as they mature. The depths of lobing in relation to the whole limb (the expanded, not tubular, portion of the corolla) of disk corollas is needed in the key, being deep in S. lateriflorum and S. ontarionis. The large lobes of these species are best seen on mature (rosy) corollas with the lobes spreading. In measuring length of rays, it is of course necessary to find mature, flat (well pressed, if dry) ones; look for the largest ones on a specimen.
1. Blades of at least the lower and/or basal leaves with distinct petioles and cordate to broadly rounded or truncate bases.
2. Margins of leaves at middle of stem entire or at most with a few irregular teeth.
3. Leaves at middle of stem sessile and clasping, smooth and glabrous, ± glaucous.
S. laeve (in part)
3. Leaves at middle of stem petioled or tapered to base, scabrous to the touch (at least above), not glaucous.
4. Upper part of stem essentially glabrous (or unevenly pubescent, e.g., in lines or stripes) and undersides of leaves scabrous to sparsely pubescent (with some longer, softer hairs especially along the midrib); phyllaries glabrous, at least the widest usually (0.9–) 1 mm broad, with the diamond-shaped green tip often only 1–2 times as long as wide.
4. Upper part of stem and undersides of leaves evenly rough-hispidulous with tiny widely spreading hairs; phyllaries strigose on back, all distinctly less than 1 mm wide, with the diamond-shaped green tip mostly at least twice as long as wide.
2. Margins of leaves at middle of stem toothed along all or most of the blade.
5. Lower and basal leaves with blades deeply cordate, the sinus 5–15 (–30) mm deep, and prominently toothed, some teeth (1.5–) 2–5 mm long on forward margin; petioles, especially on mid-cauline leaves, wingless or narrowly winged (1 mm or less on each side); phyllaries merely acute, the diamond-shaped green area usually ca. 3 times as long as broad or shorter (at least on outer phyllaries); inflorescence open, paniculiform, the heads with rays pale blue to purplish (occasionally white).
5. Lower and basal leaves with blades truncate, broadly rounded, or subcordate at base, the sinus rarely over 5 mm deep, and usually shallowly toothed; petioles, especially on mid-cauline leaves, often winged 1–3.5 (–6) mm on each side (especially near the blade); phyllaries acuminate or attenuate, the diamond-shaped green area prolonged or (especially in S. urophyllum) obscure; inflorescence and rays various.
6. Inflorescence open-paniculiform, with spreading branches and heads on pedicels of very uneven lengths; colonial from long-creeping rhizomes; rays (6.5–) 7–10.5 (–12) mm long, blue to purple.
6. Inflorescence racemiform, with ascending branches and all (or most) pedicels about equally short; plants with a short, thick perennial base, elongate rhizomes lacking; rays white, less often blue or pinkish, 3.5–10 mm long.
7. Rays white (rarely pale pinkish or lavender), 3.5–6 (–7) mm long; widespread.
7. Rays bright blue, 7–10 mm long; southern Michigan.
1. Blades of leaves not cordate or if somewhat so, the leaves sessile, not distinctly petioled.
8. Leaves and phyllaries silvery-silky with long appressed hairs on both surfaces.
8. Leaves and phyllaries with quite different sorts of pubescence or glabrous.
9. Upper cauline leaves sessile and ± clasping (auriculate) or the phyllaries with ± dense stalked glands (or both conditions present).
10. Leaves, stems, involucres, and often pedicels glabrous, eglandular, and glaucous; outer phyllaries about half as long as the inner ones or even shorter.
S. laeve (in part)
10. Leaves, stems, involucres, and/or pedicels pubescent, in some species glandular, and not glaucous; outer phyllaries about equaling the inner ones or even longer.
11. Phyllaries glabrous (except sometimes for marginal cilia), eglandular; leaves (largest ones) often at least sparsely toothed.
12. Plants with long-creeping rhizomes, colonial; stems glabrous or pubescent in lines, green or sometimes purple; cauline leaves mostly glabrous beneath; ray florets white to pale lavender.
12. Plants with a short, usually ascending perennial base, solitary or cespitose, not colonial from long-creeping rhizomes; stems bristly pubescent, usually purplish; cauline leaves conspicuously pubescent along the midvein on the underside; ray florets blue to purple.
11. Phyllaries (and pedicels) partly to densely glandular; leaves entire.
9. Upper cauline leaves without auriculate bases (may, if sessile, be slightly sheathing) and the phyllaries without glands.
13. Rays none or minute (less than 2 mm long, scarcely exceeding the involucre); annual, with small taproots.
14. Heads rayless; leaf margins scabrous-ciliate with stout but tiny prickles; broadest leaves less than 4 mm wide.
14. Heads with tiny (blue-pink) rays; leaf margin essentially smooth; broadest leaves (except in the most depauperate individuals) (3.5–) 6.5–10 (–11) mm wide.
13. Rays conspicuous; plants perennial, usually rhizomatous.
15. Phyllaries pubescent (at least slightly strigose) on back.
16. Tips of phyllaries (or most of them) and bracts in the inflorescence with a tiny white or colorless spinule.
16. Tips of phyllaries and bracts at most with a colored callus (not spinule).
S. ontarionis (in part)
15. Phyllaries glabrous on back (may be ciliate).
17. Tips of phyllaries (or most of them) with prolonged, ± translucent or somewhat colored, spine-like; involucre slightly urn-shaped (constricted just above the middle).
17. Tips of phyllaries merely acute or callused, not spinulose or prolonged; involucres top- or cup-shaped, not constricted above the middle.
18. Leaves beneath with distinct regular reticulate pattern formed by dark veinlets around paler green areoles ca. 0.6 mm in diameter; rays blue (very rarely white); southern Lower Peninsula.
18. Leaves beneath without distinct reticulation or the areoles clearly irregular or elongate; rays white (occasionally pinkish; blue in one species and one variety, both northern); ranges various.
19. Lobes of disk florets about half or more the total length of the limb (expanded corolla, not the slender tube); leaves normally pubescent on at least the midrib beneath.
20. Leaves pubescent primarily on the midrib beneath (rarely glabrous); stems often clumped, from a sturdy perennial base, lacking long-creeping rhizomes.
20. Leaves on upper stem evenly short-pubescent all across the lower surface (moderately to densely so, rarely nearly glabrous); stems colonial from slender rhizomes.
S. ontarionis (in part)
19. Lobes of disk florets about a third or less of the total length of the corolla limb; leaves glabrous or at most scabrous beneath [if pubescent, cf. S. oolentangiense].
21. Heads on pedicels (or branchlets) mostly (0.7–) 1–5 cm long with many of the bractlets less than 5 (–7) mm long; heads less than 1.5 cm broad when fresh or well pressed.
21. Heads on often shorter pedicels with the bractlets few (or none) and mostly not so reduced; heads 1.5–2.5 (–3) cm broad, except in some forms of S. lanceolatum.
22. Cauline leaves ± clasping (most bases, especially lower on the stem, circling more than half the circumference of the stem); phyllaries nearly equal in length (or the outer ones over half as long as the inner ones); rays bright blue-violet; Upper Peninsula and northern Lower Peninsula.
22. Cauline leaves not clasping (all bases circling half or less the circumference of the stem); phyllaries of different lengths, imbricate; rays usually white; throughout Michigan.
23. Rhizomes and stems less than 2 (–2.2) mm in diameter; inflorescence usually with divaricate or spreading branches (if any) and 1–12 (–20) heads; leaves less than 6 mm broad, the margins often revolute; wet peatlands, occasionally along shores.
23. Rhizomes and stems stouter (at least 2.5 mm); inflorescence with ± ascending branches and (except in the most depauperate specimens) more than 20 heads; leaves often 6–18 (–33) mm broad (though sometimes narrower), with flat margins; generally of moist habitats.