1. Erigeron annuus (L.) Desf. (syn.: Stenactis annua (L.) Less., Phalacroloma annuum (L.) Dum.) (N-Am.) – A commonly naturalised and increasing escape from cultivation. Known at least since 1813. At present Erigeron annuus is most frequent in Flanders in the surroundings of some larger cities (Antwerpen, Brussel) and in the valley of river Maas (Verloove 2006). It is widely distributed in large parts of Wallonia as well (see for instance Frankard & Hauteclair 2009). It usually occupies ruderal, rather nithrophilous and often slightly damp habitats: disused railway yards, ruderal roadverges, waste land, railway slopes, canal and river banks,… (Verloove 2002). Erigeron annuus increasingly penetrates in nature-like habitats and might probably become a nuissance. Invasive behaviour has been met with on a coal mine heap in the surroundings of Liège (Frankard & Hauteclair l.c.).
Erigeron annuus is a rather variable species. Two additional species have been claimed from Belgium: Erigeron septentrionalis Fernald et Wiegand (syn.: E. annuus subsp. septentrionalis (Fernald et Wiegand) Wagenitz, E. strigosus Muhlenb. ex Willd. var. septentrionalis (Fernald et Wiegand) Fernald, E. ramosus (Walter) Britton, Sterns et Pogg. var. septentrionalis Fernald et Wiegand, Stenactis strigosa (Muhlenb. ex Willd.) DC. var. septentrionale (Fernald et Wiegand) J. Duvigneaud et Lambinon, Phalacroloma annuum subsp. septentrionale (Fernald et Wiegand) Adema) and E. strigosus subsp. strigosus (syn.: E. ramosus, E. annuus subsp. strigosus (Muhlenb. ex Willd.) Wagenitz, Stenactis strigosa (Muhlenb. ex Willd.) DC. var. strigosa, Phalacroloma annuum subsp. strigosum (Muhlenb. ex Willd.) Adema). Apparent intermediates between Erigeron annuus and E. septentrionalis are frequently encountered and they cannot be distinguished unambiguously in Belgium (Verloove 2002). Indeed, Frey & al. (2003) proved that they are in fact conspecific (despite Nesom 2006). Erigeron strigosus, on the contrary, was, still according to Frey & al. l.c. never confirmed from Europe, all early records being referable to E. annuus. However, a naturalized population of this taxon is known from near a military base in Bamberg (Germany) since several years (comm. R. Otto) and Sennikov & Kurtto (2019) demonstrated that this taxon is also naturalized in other parts of Europe (Fennoscandia but also in the Czech Republic, the Netherlands, Austria, Italy, Poland and Russia). It should be looked for in Belgium as well.
Erigeron annuus is remarkably often confused with Symphyotrichum lanceolatum in Belgian herbaria (both with numerous, very narrow white ligules). However, the former has much broader, coarsely dentate, more or less hirsute-hairy leaves.
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Selected literature:
Adema F. (1984) De madelief-fijnstraal, Phalacroloma annuum (L.) Dumort., in Nederland. Gorteria 12: 51-56.
Frankard P. & Hauteclair P. (2009) Inventaire et diversité de la flore vasculaire des terrils charbonniers liégeois – bilan de vingt-cinq années de prospections (1983-2007). Natura Mosana 62(2): 37-70.
Frey D., Baltisberger M. & Edwards P.J. (2003) Cytology of Erigeron annuus s.l. and its consequences in Europe. Bot. Helv. 113(1): 1-14.
Nesom G.L. (2006) Erigeron. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 256-348.
Sennikov A.N. & Kurtto A. (2019) The taxonomy and invasion status assessment of Erigeron annuus s.l. (Asteraceae) in East Fennoscandia. Memoranda Societatis pro Fauna et Flora Fennica 95: 40-59. [available online at: https://journal.fi/msff/article/view/79497]
Van Ooststroom S.J. & Reichgelt Th.J. (1964) Erigeron L. sectie Phalacroloma (Cass.) Cronquist in Nederland. Gorteria 2: 9-12.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Erigeron annuus. 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: 381.