3. Erigeron canadensis L. (syn.: Conyza canadensis (L.) Croncq.) (N-Am.) – A very commonly naturalised alien. First recorded in 1803. At present widespread, especially in Flanders (slightly less so in Wallonia, especially in the Ardennes more rare). Erigeron canadensis occupies a wide range of man-made habitats: waste land, road verges, canal banks, coalmine heaps, railway tracks, gardens or agricultural fields.
Erigeron canadensis is sometimes said to be rather variable and many infraspecific taxa have been described (see for instance Sell & Murrell 2006 for an overview of British varieties). These are mostly based on inflorescence dimensions and leaf shape and are probably mere ecotypes without taxonomical value. One particular variant with nearly glabrous stems and very narrow leaves is worth mentioning. Such plants have been ascribed to var. pusilla (Nutt.) Cronquist and is sometimes given species rank, for instance by Australian taxonomists (C. parva Cronquist). Such plants have been found recently in southern Spain (Márquez García & al. 2013) and are probably overlooked elsewhere.
Erigeron xhuelsenii Vatke (syn.: xConyzigeron huelsenii (Vatke) Rauschert), a hybrid with native E. acer (see Vatke 1871, Pliszko & Kostrakiewicz-Gieralt 2018), was recorded with its parents near a timber storage in 2008 in Heinsch (Stockem). Such plants are very variable and can resemble either parent. The few plants from Heinsch look like Erigeron canadensis but have much longer, lilac ligules and are probably (short-lived) perennials. Plants that are closer to Erigeron acer somehow look like E. bonariensis (Wurzell 1995; Sell & Murrell 2006).
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Selected literature:
Márquez García F., García Alonso D. & Vázquez Pardo F.M. (2013) 063.-Conyza canadensis (L.) Cronq. var. pusilla (Nutt.) Cronq. Fol.Bot.Extremadurensis 7: 60. [available online at: https://jolube.files.wordpress.com/2010/01/folia_botanica_extremadurensi...
Pliszko A. & Kostrakiewicz-Gieralt K. (2018) The morphological intermediacy of Erigeron x huelsenii (Asteraceae), a hybrid between E. acris and E. canadensis. Turk. J. Bot. 42: .... [available online at: https://www.researchgate.net/publication/324497012_The_morphological_int...
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Van Landuyt W. (2006) Conyza canadensis. 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: 304.
Vatke G. (1871) Erigeron huelsenii. Ein neuer Bastart aus der Posener Flora. Öster. Bot. Zeitschr. 21: 346-347.
Wein K. (1932) Die älteste Einführungs- und Einbürgerungsgeschichte des Erigeron canadensis. Bot. Arch. 34: 394-418.
Wurzell B. (1995) x Conyzigeron huelsenii in East London. BSBI News 68: 32-33 (+ frontcover).