Species are generally easily recognized when seen in their appropriate habitat and with typical inflorescences and leaves present. Hybrids occur, but are rare and usually recognizable as clear intermediates. In a few species the inflorescences are axillary, subtended by ordinary-sized foliage leaves. Such a habit grades into a strictly terminal inflorescence, of which there are three principal shapes in our species. A few have flat-topped or corymbiform inflorescences (as also in Euthamia), with a somewhat dome-like (convex) or sunken (concave) outline across the top, the outer (lower) branches being longer than the central (upper) branches. Some species have an elongate ± cylindrical inflorescence, a terminal “wand” or “rod.” A number of conspicuous species have a pyramidal inflorescence broadest at or just above the base and tapering to the apex, which may nod a little; the lower branches in some species are ± recurved with the heads one-sided (i.e., oriented on top of the branches).

In many species the inflorescence varies considerably. Plants growing in stressful conditions may have inflorescences abnormally small or condensed. The change, or lack of it, in leaf size from the base of the plant upwards is often very helpful for identification and complete specimens should always be collected. In some species, separate basal rosettes are formed, with leaves similar to (or even larger than) the lowest cauline leaves (which sometimes wither by flowering time). Species with the mid-cauline leaves no smaller than the lower ones do not produce separate rosettes. Incomplete specimens and extreme growth forms are not likely to work well in the key.

In determining length of an involucre, measure from its base to the tip of the longest phyllaries in mature heads. Likewise, look for mature achenes to determine their pubescence, for unlike the heads of some people, some achenes seem to become more hairy as they age. On the other hand, stem pubescence may abrade, especially on sand dunes, so that only scars may be visible on older stems.

Solidago macrophylla Pursh comes as far south in Ontario as Batchawana Bay, not far north of Sault Ste. Marie. It has axillary inflorescences and large leaves, resembling those of S. flexicaulis, but also very large involucres (ca. 7.5–10 mm long) and glabrous achenes. Solidago lepida DC. is also well known from the north shore of Lake Superior, and several northern Michigan specimens have been so named. These specimens, however, do not look much alike, and are here included with S. altissima. Nevertheless, S. lepida should be sought in northern Michigan. See Melville & Morton (1982) for more details.

The species with heads in a terminal ± flat-topped corymbiform inflorescence keying out under the first lead of couplet 1 are sometimes separated into the genus Oligoneuron.

1. Heads in a terminal ± flat-topped (somewhat domed to convex) corymbiform inflorescence.

2. Blades of middle and upper cauline leaves ovate to elliptic (less than 3 times as long as broad), densely pubescent on both surfaces.

S. rigida

2. Blades of middle and upper cauline leaves linear to lanceolate or oblanceolate (over 10 times as long as broad), glabrous or nearly so.

3. Rays 12–18, white, 4.5–8 mm long; pappus hairs slightly but clearly thickened (slenderly clavate) toward tip; upper cauline leaves slightly oblanceolate (broadest beyond the middle).

S. ptarmicoides

3. Rays 10 or (usually) fewer, yellow, not over 4.5 (–7) mm long; pappus hairs not thickened (or some thickening scarcely visible in S. houghtonii); upper cauline leaves broadest at or below the middle.

4. Rays 1.5–3 mm long and involucre ca. 3.5–5.5 (–6.5) mm long; pedicels smooth and glabrous or rough-hispidulous.

5. Pedicels smooth and glabrous or nearly so; leaf blades with one longitudinal vein (but often some principal lateral veins), flat.

S. ohioensis

5. Pedicels and inflorescence branches densely rough-hispidulous; leaf blades with 3 or more longitudinal veins at the base, all or mostly folded inwards longitudinally.

S. riddellii

4. Rays 3–4.5 (–7) mm long and involucre ca. 5–9 mm long; pedicels scabrous-hispidulous.

6. Larger involucres 5–7 (–8) mm long; larger plants mostly 30–60 cm tall; basal leaves entire; plants occurring on or near the Great Lakes shores, centered on the Straits of Mackinac.

S. houghtonii

6. Larger involucres 7–9 mm long; larger plants mostly 50–80 cm tall; basal leaves sparsely serrulate; inland in swales among Pinus banksiana.

S. vossii

1. Heads in an elongate or pyramidal inflorescence or in axillary clusters.

7. Inflorescence terminal, often ± pyramidal (broadest toward base, about equally long, slightly nodding at top) but sometimes grading into axillary branches, and with curving, one-sided branches (the heads mostly directed upwards on well-developed branches).

8. Cauline leaves (at least the main ones) “triple-nerved,” i.e., with a pair of elongate veins arising below the middle of the midrib and distinctly stronger than other lateral veins.

9. Leaves entire, succulent; saline habitats (e.g., edges of heavily salted highways).

S. sempervirens

9. Leaves with at least tiny and/or irregular teeth, of normal herbaceous texture; various habitats.

10. Axis, pedicels, and branches of inflorescence glabrous; prairie and dry prairie-like habitats, blooming late in the season; lower and rosette leaves linear-lanceolate.

S. missouriensis

10. Axis, pedicels, and branches of inflorescence at least sparsely but distinctly pubescent [10–20×]; or if glabrous (S. juncea), the lower and rosette leaves much larger than the mid-cauline leaves, ± elliptic, and the plant blooming early in the season in dry habitats.

11. Stem glabrous all of its length below the inflorescence, rarely with a few scattered, spreading, short hairs. [Note: Occasionally a plant of S. uliginosa may have obscurely triple-nerved leaves, but that species can usually be readily identified by the clasping base of the lowest leaves, which encircle half or more of the stem].

12. Basal leaves none; cauline leaves narrowly (rarely broadly) elliptic and the lowest withered by flowering time; middle and upper cauline leaves crowded (numerous), about the same size as the lowest leaves or larger, and distinctly 3-nerved; plants blooming late (starting August–September); branches of inflorescence ± densely pubescent.

S. gigantea

12. Basal (including rosette) and lower cauline leaves with oblanceolate to elliptic blades and long petioles, persistent; middle and upper cauline leaves remote (relatively few), distinctly smaller than basal leaves, and only weakly 3-nerved; plants blooming early (starting in July); branches of inflorescence glabrous or occasionally sparsely spreading-pubescent.

S. juncea (in part)

11. Stem pubescent all or most of its length. [Note: Occasionally a plant of S. nemoralis may have obscurely triple-nerved leaves, but can be distinguished by the dense, ashy-gray puberulence of the stem and leaves].

13. Involucres all or mostly 3.1–4.6 (–5) mm long.

S. altissima

13. Involucres all or nearly all 2–3 mm long.

S. canadensis

8. Cauline leaves with distinct midrib but the other (weaker) veins ± pinnate.

14. Stems ± pubescent, at least on the upper half of the plant.

15. Cauline leaves entire or obscurely crenate-toothed; leaves and stems uniformly and densely puberulent throughout; lower and basal (including rosette) leaves oblanceolate, tapered into a winged petiole and larger than mid-cauline leaves; sandy or rocky, open and usually very dry soil.

S. nemoralis

15. Cauline leaves sharply toothed; leaves beneath (at least on main veins) and stem with mostly spreading, longer hairs (over 0.5 mm); lower and basal leaves (none in rosettes) no larger than mid-cauline leaves (but usually absent at flowering time), all of them elliptic-lanceolate; moist or shaded ground.

S. rugosa

14. Stems glabrous (except sometimes just below and in the inflorescence).

16. Lowest cauline leaves with tapering base clasping stem (encircling it for at least half its circumference); wet habitats, with leaves nearly smooth above.

S. uliginosa (in part)

16. Lowest cauline leaves not clasping stem; dry habitats or, if wet, the leaves very scabrous above.

17. Stem with strongly raised angles or ribs; upper leaf surface very scabrous, with dense, tiny, stiff conical projections (feeling like a cat’s tongue and resisting rubbing toward the base with a finger); swamps and other wet habitats.

S. patula

17. Stem terete (may be many-ridged); upper leaf surface smooth to slightly scabrous; ± dry open or forested habitats.

18. Basal (including rosette) and lower cauline leaves much larger than mid-cauline leaves, persistent (blades often 7–20 cm long on petioles half or more as long); branches of inflorescence glabrous or occasionally sparsely spreading-pubescent; leaves often tending to have prominent longitudinal veins, usually glabrous beneath but occasionally with some hairs on midrib; throughout Michigan, beginning to bloom in July (before other goldenrods).

S. juncea (in part)

18. Basal and lower leaves often withered by flowering time or, if present, not much larger than mid-cauline leaves; branches of inflorescence rather densely pubescent; leaves clearly pinnate-veined, with midrib and principal veins beneath spreading-pubescent (as in S. rugosa); southern Lower Peninsula, blooming late.

S. ulmifolia

7. Inflorescence axillary or terminal, but even if pyramidal the branches not one-sided and the top not nodding.

19. Leaves decreasing in size from middle of stem to the base, the mid- to upper cauline leaves sharply toothed, much exceeding the distinctly axillary inflorescences (not necessarily any branches) they subtend; stems glabrous (except rarely on upper internodes), the lowest leaves usually withered by flowering time; achenes ± densely pubescent.

20. Leaf blades narrowly elliptic, sessile; stem terete, glaucous when fresh, not (or scarcely) zigzag; leaves glabrous (except for short-ciliate margin); cespitose.

S. caesia

20. Leaf blades broadly ovate-elliptic, abruptly contracted to a winged petiole; stem ribbed or angled throughout, ± zigzag from node to node; leaves (at least the midrib beneath and petiole margins) ± sparsely pubescent; colonial from creeping rhizomes.

S. flexicaulis

19. Leaves increasing in size from middle of stem to the base, the mid-cauline leaves usually entire to crenate-toothed and usually not subtending inflorescences (these more clearly terminal); stems glabrous or pubescent, the lowest leaves usually persistent; achenes glabrous or glabrate (except in S. simplex).

21. Stem ± densely pubescent its entire length and leaves pubescent on both sides.

22. Rays white or cream when fresh; involucres ca. 3–4 (–4.5) mm long.

S. bicolor

22. Rays yellow; involucres 4–7 mm long.

S. hispida (in part)

21. Stem glabrous or nearly so, at least below the middle and leaves glabrous or essentially so.

23. Lower cauline leaves ca. 6–18 times as long as broad, the petiole clasping the stem for half or more of its circumference; wet habitats (including rock crevices on Lake Superior).

S. uliginosa (in part)

23. Lower cauline leaves ca. 3–8 times as long as broad, not clasping (leaves of basal rosettes sometimes as much as 11 times as long as broad); mostly of dry habitats.

24. Achenes ± densely antrorse-strigose; involucres and leaves resinous (easier determined when fresh, but appearing varnished, shiny, or glandular when dry).

S. simplex

24. Achenes glabrous or nearly so; involucres and leaves not resinous.

25. Cauline leaves ca. (11–) 15–30 below inflorescence; margins of lower and middle leaves entire to sparsely toothed in the upper half; plant of prairies, jack pine plains, and oak barrens, sandy fields and rock outcrops (inland).

S. speciosa

25. Cauline leaves (3–) 5–15 (–17) below inflorescence; margins of lower and middle leaves crenulate; rock outcrops and dunes on Lake Superior and northern lake Huron.

S. hispida (in part)