4. Helianthus xlaetiflorus Pers. (= H. pauciflorus Nutt. x H. tuberosus; syn.: H. scaberrimus Elliott, H. severus E. Watson, H. superbus E. Watson) (Hort.) – A rather frequently naturalised escape from cultivation or garden throw-out. First recorded in 1923 on wasteland in Huy. Subsequently observed in numerous, very widely scattered localities and much increasing in the past decades. Most populations are concentrated around the larger cities, for instance Antwerpen and Gent (Verloove 2002). Helianthus xlaetiflorus is usually found on dumps, wasteland, along railway tracks and by road verges, in old gravel- or sand pits,… It often forms dense stands but only exceptionally in valuable habitats.
Helianthus xlaetiflorus only seldom produces viable seed in North America (Schilling 2006) and probably never in Belgium.
Helianthus xlaetiflorus is still frequently confused with H. tuberosus in Belgium. Many records of the latter are in fact Helianthus xlaetiflorus, without any doubt the most widespread in Belgium. It is readily distinguished by its much narrower leaves and tightly appressed, unequal involucral bracts. Helianthus xlaetiflorus also flowers much earlier than H. tuberosus (in Belgium from July onwards, whereas the latter rarely flowers before October, if at all).
The other parental species, Helianthus pauciflorus (syn.: H. rigidus (Cass.) Desf.), has often been claimed as an escape but these records require confirmation and possibly also belong with H. xlaetiflorus. Moreover, genuine Helianthus pauciflorus is, according to some authors, not in cultivation in Europe (Brown 2000; see however Balogh 2008, Jäger & al. 2008). Diacritic features for the distinction of Helianthus pauciflorus and H. xlaetiflorus are often contradictory. Plants with entirely reddish corolla lobes have been collected in Belgium and possibly belong to genuine Helianthus pauciflorus. However, and contrary to many sources, tubular florets in Helianthus xlaetiflorus may be reddish or brownish too, at least at tips (see Schilling 2006). This considerably blurs the boundaries between this hybrid and one of its parents. Such deviating populations should be critically examined.
Some populations of Helianthus xlaetiflorus-like plants are obviously aberrant (for instance some of the plants seen in a former sand pit in Flobecq in 2009). They have much wider leaves, longer petioles and phyllaries that are more or less equal but not tightly appressed. In fact, such plants much look like Helianthus tuberosus but they are less vigorous and start flowering in August. They possibly represent back-crosses with the latter and require further research.
Several additional, similar species have been reported as escapes in Europe (see for instance Balogh 2008 for an overview). See also under Helianthus 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.
Clevenger S. & Heiser C.B. (1963) Helianthus laetiflorus and Helianthus rigidus – hybrids or species? Rhodora 65: 121-133.
Heiser C.B. (1960) Notes on the origin of two ornamental sunflowers, Helianthus multiflorus L. and H. laetiflorus Pers. Baileya 8: 146-149.
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.
Spring O. & Schilling E.E. (1990) The origin of Helianthus x multiflorus and H. x laetiflorus (Asteraceae). Biochem. Syst. Ecol. 18(1): 19-23.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Helianthus laetiflorus. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 457.