Berberis xottawensis C.K. Schneider (B. thunbergii x B. vulgaris) (Hort.) – A very rare escape from cultivation. Recorded in 1959 in woodland in St-Gilles (Brussel) but possibly not wild. From 2001 onwards seen in coastal dunes near Oostduinkerke (a single bush). Very persistent but not yet becoming naturalised so far. In 2010 furthermore seen in coastal dunes in Heist and Duinbergen (Willemspark and Park 58) and in De Panne (Houtsaegherduinen) (always young, solitary specimens, probably birdsown).
Berberis x ottawensis is not always easily identified and may well be less rare than usually admitted (see also Connolly & al. 2013). In fact, it is intermediate between its parents but most specimens rather look like vigorous plants of Berberis thunbergii. However, its leaves are minutely but obviously toothed. In most plants seen so far leaves are bronze-red. Such plants belong to cv ‘Auricoma’ or ‘Superba’, the most frequent taxon in cultivation.
Selected references:
Connolly B.A., Anderson G.J. & Brand M.H. (2013) Occurrence and Fertility of Feral Hybrid Barberry Berberis × ottawensis (Berberidaceae) in Connecticut and Massachusetts. Rhodora 115(962): 121-132. [available online at: http://www.bioone.org/doi/pdf/10.3119/11-05]
Hale I.L., Connolly B.A. & Bartaula R. (2015) The Occurrence of Hybrid Barberry, Berberis ×Ottawensis (Berberidaceae), in New Hampshire and Rhode Island. Rhodora 117(971): 384-387. [available online at: https://www.researchgate.net/publication/283567682_The_Occurrence_of_Hyb...
Van Splinter J.L., Burgess M.B., Spada D.M. & Werier D. (2017) Berberis ×Ottawensis (Berberidaceae): A New Addition To the Flora of New York. Rhodora 118(976): 412-414. [available online at: http://www.bioone.org/doi/pdf/10.3119/16-01]