Juncus ensifolius Wikstr. (syn.: J. oligocephalus Satake et Ohwi) (N-Am.) – A very rare and mostly ephemeral alien, of uncertain origin. Initially probably deliberately introduced as an ornamental but rather weedy and readily escaping (or further spreading by waterfowl?). Possibly also introduced with garden peat. First found in the wild in 2006 in Hergenrath in the valley of river Gueulle (clearing in wood). Probably ephemeral (only one specimen found). Also discovered after infrastructural works in nature reserve Gulke Putten in Wingene in 2009 (a single specimen). A large population with ca. 100 plants was observed in 2012 in a cattle pool in nature reserve Maskobos near Jabbeke (here possibly more or less naturalised). Apparently increasing lately: in 2013 also seen in Lier and Turnhout. Likely to occur elsewhere, especially on bare, damp ground.
Juncus ensifolius has been increasingly reported in several neighbouring countries as well (the British Isles, Luxemburg, the Netherlands, Germany). It belongs with the very distinct subgenus Ensifolii (Snogerup) Snogerup, a predominantly American complex of ca. 7 closely related species (Brooks & Clemants 2000). Members of this subgenus are characterised by flattened ensiform leaves and confusion with native as well as with the non-native taxa treated here is impossible.
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Selected literature
Adema F. & Mennema J. (1976) Wederom een nieuwe Juncus voor Nederland. Gorteria 8: 77-80.
Anttoine S., Bour A., Mahévas T., Pax N. & Tinguy H. (2019) Juncus ensifolius Wikstr. var. ensifolius en Lorraine. Les Nouvelles Archives de la Flore jurassienne et du nord-est de la France 17: 95-101.
Brooks R.E. & Clemants S.E. (2000) Juncus. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 211-255.
Kiffe K. (1988) Juncus ensifolius Wikström 1823, eine sich einbürgernde Adventivpflanze. Flor. Rundbr. 21: 86-88.
Poland J. (2005) Juncus ensifolius Wikström: a 2nd British record. BSBI News 98: 47-48.
Snogerup S. (2010) Juncaceae i Norden 3. Perenna Juncus-arter. Sektionerna Stygiopsis, Iridifolii och Juncus (Nordic species of Juncus sectt. Stygiopsis, Iridifolii and Juncus). Svensk Bot Tidskr. 104: 339-350. [available online at: http://sbf.c.se/www/pdf/104%285%29/snogerup.pdf]