Solidago, as currently understood, counts ca. 100 species. It is distributed in the Americas and Eurasia but its center of diversity obviously lies in North America. Only one species, Solidago virgaurea L., is native in Belgium as well. Many species of Solidago are cultivated as ornamentals. Yeo (2000) and Jäger & al. (2008) provide a good overview of the genus in cultivation in Europe. Both list more than 20 species (incl. some that are now accommodated in Euthamia) but only a few are commonly grown. According to Jäger & al. l.c. plants in cultivation are now mostly hybrids of Solidago canadensis, S. rugosa and S. shortii Torr. et A. Gray. The latter is probably best distinguished in having fewer ligules (only 5-9 vs. 9-17) and shorter rhizomes. In general appearance, however, it is much reminiscent of Solidago canadensis and S. gigantea; it might have been overlooked in Belgium.
The generic boundaries of Solidago are now much better understood as a result of recent molecular research. Solidago graminifolia, for instance, is better placed in the segregate genus Euthamia. Solidago s.str. is distinguished by leaves that are not gland-dotted, cylindrical to pyramidal (not corymbose) inflorescences, shortly pedicellate (not sessile) heads and tubular florets that are more numerous than ligular florets.
Very useful additional information on the genus (with a lot of illustrations) is available at the Astereae Lab webpage (https://uwaterloo.ca/astereae-lab/research/goldenrods/classification-and... .
Species of Solidago (especially S. canadensis and S. gigantea) are reputed environmental weeds in many (temperate) parts of the world, including Belgium.
1. Cauline leaves triple-veined (a central vein with a pair of lateral parallel veins). Inflorescence always with divergent and recurved branches === 2
1. Cauline leaves reticulate veined (with pronounced lateral veins along the central vein). Inflorescence with or without divergent and recurved branches === 4
2. Stem glabrous (except in the inflorescence), often glaucous or suffused reddish === Solidago gigantea
2. Stem with short, crispate hairs, at least in the upper ½, not glaucous === 3
3. Stem hairy throughout. Involucre 3-4 mm long. Leaves thick and stiff with nearly entire margins. Stem tall, usually exceeding 150 cm in height. Flowering from end of October onwards === S. altissima
3. Stem hairy in the upper ½. Involucre 2-3 mm long. Leaves herbaceous, usually with sharply serrate margins. Stem rarely exceeding 150 cm in height. Flowering in late summer === S. canadensis
4. Inflorescence branches divergent and recurved. Involucre 2-4,5 mm long. Ligular florets 1,5-4 mm long. Leaves acuminate at apex === S. rugosa
4. Inflorescence branches ascending, not divergent. Involucre 4,5-8 mm long. Ligular florets 4-9 mm long. Leaves rounded to acute at apex (native) === S. virgaurea
Leaves of (left to right) S. canadensis, S. gigantea, S. rugosa
Hofmann T. & Schmidt C. (2016) Goldruten ganz handzahm. Solidago für den Garten (Teil 2). Gartenpraxis 10-2016: 8-15.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
McNeill J. (1976) Solidago. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 110-111.
Nesom G.L. (1994) Taxonomic infrastructure of Solidago and Oligoneuron (Asteraceae: Astereae) and observations on their phylogenetic position. Phytologia 75: 1-44.
Schmidt C. (2016) Wildes Gold. Solidago für den Garten (Teil 1). Gartenpraxis 09-2016: 8-15.
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Semple J.C. (2016) An intuitive phylogeny and summary of chromosome number variation in the goldenrod genus Solidago (Asteraceae: Astereae). Phytoneuron 2016-32: 1–8. [available online at: http://www.phytoneuron.net/2016Phytoneuron/32PhytoN-SolidagoPhylogeny.pdf]
Semple J.C. & Cook R.E. (2006) Solidago. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 107-166.
Semple J.C., Faheemuddin H., Sorour M. & Chong Y.A. (2017) A multivariate study of Solidago subsect. Triplinerviae in western North America: The Solidago lepida complex (Asteraceae: Astereae). Phytoneuron 2017-47: 1–43. [available online at: http://www.phytoneuron.net/2017Phytoneuron/47PhytoN-TriplinerviaeAnalysi...
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Szymura M. & Szymura T.H. (2015) Interactions between alien goldenrods (Solidago and Euthamia species) and comparison with native species in Central Europe. Flora 218: 51-61. [available online at: http://ac.els-cdn.com/S0367253015001486/1-s2.0-S0367253015001486-main.pd...
Yeo P.F. (2000) Solidago. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 566-570.
Zhang J.J. (1996) A molecular biosystematic study on North American Solidago and related genera (Asteraceae: Astereae) based on chloroplast DNA RFLP analysis (phylogenetics). Ph.D. dissertation. University of Waterloo.