This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial erect herbs, sometimes woody at the base, simple, or little branched, with alternate simple toothed or entire leaves, and small heads of both tubular and radiate, yellow or rarely white flowers, in terminal or axillary panicles, thyrsi, or cymose-corymbose or capitate clusters. Involucre oblong or narrowly campanulate, its bracts imbricated in several series, the outer successively shorter. Receptacle small, flat, or somewhat convex, generally alveolate. Ray-flowers in one series, pistillate. Disk-flowers mostly all perfect, their corollas tubular or narrowly campanulate, 5-cleft or 5-lobed. Anthers obtuse and entire at the base. Style-branches flattened, their appendages lanceolate. Achenes terete or angled, usually ribbed. Pappus of numerous capillary rough nearly equal bristles in 1 or 2 series. [Greek. to make whole.] Golden-rod; also called yellow-top, yellow-weed or flower-of-gold.
About 125 species, mostly of North America, 2 or 3 in Europe, a few in Mexico and South America. Type species: Solidago Virgaurea L.
A. Tips of the involucral bracts, or some of them spreading or recurved.
Leaves glabrous or glabrate, 4'-10' long.
t. S. squarrosa.
Leaves rough-cihate, 1 -2 long.
2. S. petiolaris.
B. Tips of the involucral bracts all erect and appressed.
* Heads in axillary clusters, or also in a terminal spike-like, sometimes branched thyrsus.
1. Heads 2"-3" high, chiefly in axillary clusters; achenes pubescent. Stem and branches terete: leaves lanceolate to oblong.
3. S . caesia.
Stem and branches grooved or angled.
Leaves broadly oval, contracted into margined petioles.
Leaves lanceolate, sessile.
5. S. Curtisu.
2. Heads 2"-3" high, chiefly in a terminal spike-like thyrsus; achenes glabrous, or nearly so. Rays white; stem pubescent.
6. S. bicolor.
Rays yellow; stem densely pubescent.
7. S. hispida.
Rays yellow; stem glabrous, or sparingly pubescent.
Leaves thick, not acuminate, dentate, or the upper entire.
8. S. ereca.
Leaves thin, acuminate, sharply serrate.
9. S . monticola.
3. Heads s"-6" high; bracts elongated, acute; leaves ovate.
10. 5. macrophylla.
** Heads in a terminal simple or branched thyrsus, not at all secund on its branches, or scarcely so. Plant rough-pubescent or scabrous; leaves sessile, entire; bracts acute.
II. S. Lindheimeriana.
Plants glabrous, puberulent, or sparingly pubescent above. Low arctic-alpine species, seldom over 10' high. Heads with 30 flowers or more.
12. S. Culteri Heads with fewer than 30 flowers.
13. S. multiradiata.
Taller species, not arctic-alpine.
Bracts of the involucre linear-subulate. verv acute: stem puberulent.
14. S. puberula.
Bracts of the involucre obtuse or merely acutish; stem glabrous, or sparingly pubescent above. Glabrous throughout; upper leaves very small, appressed. At least the involucre or peduncles pubescent or puberulent.
15. S. stricta.
Bog species; inflorescence wand-like.
16. S. uliginosa.
Upland species, the inflorescence various. Heads very short-peduncled.
Leaves thick, firm in texture, little toothed or entire; tall species. Lower leaves ovate to broadly oval, serrate.
17. S. speciosa.
Leaves lanceolate to oblong-lanceolate, entire.
Leaves thin in texture, at least the lower serrate; low species.
19. S. Randii.
Heads distinctly slender-peduncled.
Basal leaves narrowly oblanceolate. 4" wide or less.
20. S. racemosa.
Basal leaves broadly oblanceolate or obovate, 6"-18" wide. Heads 4" high; basal leaves sharply serrate.
21. S. Gillmani.
Heads 3 high; leaves nearly entire.
22. S. sciaphila.
*** Heads in a terminal, usually large panicle, secund on its spreading or recurved branches. X Plant maritime; leaves thick, fleshy, entire.
23. S. sempervirens.
Plants not maritime; leaves not fleshy.
1. Leaves pinnately-veined, not triple-nerved.
(a) Leaves all entire, thin and glabrous.
24. S. odora.
(b) Leaves, at least the lower, more or less dentate or serrate, Leaves linear or linear-oblong, 1'-2' long, scabrous.
25. S. tortifolia.
†† Leaves broader, lanceolate, oblong, or ovate, 2'-10' long. X Stem densely pubescent; leaves more or less so. Leaves not rugose, sparingly dentate or entire.
26. S. fistulosa.
Leaves rugose-veiny beneath, sharply serrate.
27. S. rugosa.
XX Stem glabrous, or merely puberulent above. Leaves very rough on the upper surface, serrulate.
28. S. patulu.
Leaves smooth, or minutely roughened on the upper surface. Racemes few, widely divergent, very slender. Lower leaves oblong, coarsely serrate, thin.
29. S. ulmifolia.
Lower leaves ovate or lanceolate, rather finely serrate, firm.
30. S. Boottii.
Racemes numerous, spreading, recurved or ascending. Leaves all oblong or oblong-lanceolate, sessile.
At least the lower leaves petioled, lanceolate or ovate-lanceolate.
Leaves firm, ovate-lanceolate or oblong-lanceolate; heads about 2" high; racemes short; rays several.
32. S. neglecta.
Leaves firm, narrowly lanceolate; heads about 2" high; racemes few, short; rays 1-5.
33. S. uniligulata.
Leaves firm, lanceolate or oval-lanceolate; heads 1 1/2"-2 1/2" high; racemes numerous, slender.
34. S. juncea.
Leaves thin, the lower broadly ovate, short-acuminate; heads 2 1/2" -31/2" high; racemes numerous.
35. S. arguta.
2. Leaves triple-nerved, i. e., with a pair of lateral veins much stronger than the others. Heads small, the involucre only 1 1/4" high or less; stem glabrous, or pubescent.
36. S. canadensis.
Heads larger, the involucre 11/2"-3" high. Stem glabrous.
Leaves, and involucral bracts thin.
37. S. serotina.
Leaves, and involucral bracts firm, somewhat rigid. Leaves linear-lanceolate; achenes glabrous.
38. S. glaberrima.
Leaves oblong-lanceolate; achenes silky-pubescent.
39. S. Shortii.
Basal leaves oblanceolate, upper bract-like.
40. S. Gattingeri.
Stem pubescent or scabrous.
Leaves lanceolate, sharply serrate or entire, rough above.
41. S. altissima.
Leaves oblanceolate, spatulate, oblong, or ovate, the lower crenate.
Minutely rough-pubescent, grayish; lower leaves oblanceolate; heads 2"-3" high.
42. S. nemoralis.
Canescent and pale; leaves oblong or ovate; heads 3" high.
43. S. mollis.
Very scabrous, green, not grayish, nor canescent.
44. S. radula.
Leaves broadly ovate-oval, sharply serrate, finely pubescent.
45. S. Drummondit.
♦*** Heads in a terminal, corymbiform, sometimes thyrsoid cyme, forming a flat-topped inflorescence.
(Genus Oligoneuron Small.) Leaves ovate, oblong, or oval, mostly rough on both sides.
46. S. rigida.
Leaves lanceolate, linear, oblong, or oblanceolate, glabrous or nearly so Lower leaves oblong-lanceolate, serrulate.
47. S. ohioensis.
Leaves all lanceolate or linear, entire.
Stout; leaves lanceolate, the basal 8'-12' long.
Slender; leaves linear, the basal 4 -5 long.
49. S. Houghtoni.