Helianthus is a New World genus of ca. 50 species, all endemic to North America and Mexico. One, Helianthus annuus, is an economically important species. It is widely cultivated as an oilseed crop. At least 15 species are cultivated as ornamentals in Europe (Brown 2000, Jäger & al. 2008) but rather few are more or less commonly encountered in cultivation.
The identification of Helianthus is rarely straightforward. It is important to note whether or not a species is perennial. In the absence of roots an accurate identification is often much complicated. Perennial species usually grow in dense colonies (as a result of the development of strong rhizomes) while annual species always grow solitary. Some of the annuals treated below might perennate in their area of origin (see Schilling 2006) but they are always cultivated as annuals in Europe.
The taxonomy of the genus Helianthus is very complex of its own and further complicated as a result of artificial hybridisation and selection in horticulture. Plants currently found in gardens (and in the wild) in Europe often do not longer correspond with their putative ancesters in the New World. The most comprehensive and up-to-date account for Helianthus in the wild in Central and Eastern Europe is provided by Balogh (2008). A closer examination of the Belgian populations (especially of the perennial species) will probably yield additional taxa. With the key that is presented beneath a large majority of the Belgian plants can be reliably identified. However, some populations are aberrant and require further investigation (see also under Helianthus xlaetiflorus).
Several perennial species of Helianthus are regarded as invasive environmental weeds in parts of Europe, especially along river courses (see Balogh l.c.).
1. Annual === 2
1. Perennial === 4
2. Central receptacular scales with a conspicuous tuft of long white hairs at apex === 5. Helianthus petiolaris
2. Central receptacular scales without a tuft of long white hairs at apex (but scales often with some scattered, short white hairs towards apex) === 3
3. Involucral bracts usually broadly ovate (5-8 mm wide), abruptly narrowed at apex. Leaf blade often rounded to slightly cordate at base, sometimes truncate === 1. H. annuus
3. Involucral bracts lanceolate (up to 4 mm wide), gradually narrowed at apex. Leaf blade truncate or rounded at base === 2. H. debilis
4. Leaves very narrow, linear-lanceolate, 2-12 mm wide, (sub-) entire at margin. Stem glabrous === 6. H. salicifolius
4. Leaves much wider, ovate and serrate at margin. Stem scabrous to hirsute === 5
5. Involucral bracts subequal, very loosely appressed, ca. 10-20 mm long, long-acuminate at apex === 6
6. Stem rough hairy throughout, up to 3 m tall. Involucral bracts usually not surpassing disc diameter. Laminae of ray florets 25-40 mm long. Rhizomes usually with tubers (but often hardly produced outside of cultivation!). Flowering in late autumn (from October onwards) or not at all === 7. H. tuberosus
6. Stem glabrous, at least in the lower half, rarely exceeding 1,5 m. Involucral bracts surpassing disc diameter. Laminae of ray florets 20-25 mm long. Rhizomes never with tubers. Flowering from early August onwards === 3. H. decapetalus
Balogh L. (2008) Sunflower species (Helianthus spp.). In: Botta-Dukát Z. & Balogh L. (eds.), The most important invasive plants in Hungary. Institute of Ecology and Botany, Hungarian Academy of Sciences, Vácrátót, Hungary: 227-255.
Brown N. (2000) Helianthus. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 652-656.
Filep R., Balogh L. & Csergo A.-M. (2010) Perennial Helianthus taxa in Târgu-Mures city and its surroundings. J. Plant Develop. 17: 69-74 [available online at: http://www.plant-journal.uaic.ro/docs/2010/8.pdf].
Gudžinskas Z. & Petrulaitis L. (2014) Helianthus grosseserratus, A New Alien Plant Species in Lithuania. Botanica Lithuanica 20(2): 173-176. [available online at: http://www.degruyter.com/view/j/botlit.2014.20.issue-2/botlit-2014-0018/botlit-2014-0018.xml?format=INT]
Heiser C.B. (1976) The sunflower. University of Oklahoma Press: 198 p.
Heiser C.B., Smith D.M., Clevenger S. & Martin W.C. (1969) The North American sunflowers (Helianthus). Mem. Torrey Bot. Club 22(3): 1-218.
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.
Kirschner J. & Šída O. (2004) Helianthus. In: Slavík B. & Štĕpánková (eds.), Kvĕtena České Republiky, vol. 7. Academia, Praha: 322-331.
Kress C. (2014) Beeindruckende Staudensonnenblumen. Gartenpraxis 04.2014: 12-17.
Rehorek V. (1997) Pestované a zplanelé vytrvalé druhy rodu Helianthus v Evrope. Preslia 69: 59-70.
Rogers C.E., Thompson T.E. & Seiler G.J. (1982) Sunflower species of the United States. National Sunflower Association, Bismarck: 75 p.
Schilling E.E. (2006) Helianthus. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 21. Oxford University Press, New York-Oxford: 141-169.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Thellung A. (1913) Die in Mitteleuropa kultivierten und verwilderten Aster- und Helianthusarten nebst einem Schlüssel zur Bestimmung derselben. Allg. Bot. Zeitschr. 19: 87-89, 101-112, 132-140.
Watson E.E. (1929) Contributions to a monograph of the genus Helianthus. Pap. Michigan Acad. Sc. 9: 305-475.