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 === 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 === H. annuus
3. Involucral bracts lanceolate (up to 4 mm wide), gradually narrowed at apex. Leaf blade truncate or rounded at base === H. debilis
4. Leaves narrow, linear to lanceolate, 2-30 mm wide, (sub-) entire at margin === 5
4. Leaves much wider, ovate and serrate at margin === 6
5. Disc corolla lobes reddish. Leaves very narrow, linear, 2-12 mm wide. Stem glabrous === H. salicifolius
5. Disc corolla lobes yellow. Leaves wider, lanceolate. Stem scabrous to hirsute === H. maximilianii
6. Involucral bracts subequal, very loosely appressed, ca. 10-20 mm long, long-acuminate at apex === 7
7. 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 === H. tuberosus
7. 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 === H. decapetalus
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