2. Colutea x media Willd. (= C. arborescens x C. orientalis Mill.) (Hybr.) – A very rare escape or relic of cultivation but possibly overlooked. By far best known from coastal dunes. Discovered in Knokke (Oosthoek) in 1974 and more recently also known from Wenduine. At least in the latter location more or less naturalised.
Colutea x media always grows in shrubland on calcareous, dry and sunny substrates (often together with C. arborescens).
Belgian records of Colutea orientalis (see for instance D’hose & De Langhe 1976) require confirmation but are probably erroneous. Colutea orientalis much looks like C. x media but it has smaller leaves (none exceeding 15 mm) and a distinctly keeled beak. It is rarely cultivated and unlikely to occur as an escape.
In the British Isles Colutea xmedia is suggested to be as common as C. arborescens (Stace 2010) but this surely does not hold true for Belgium (and appears to be in conflict with Clement & Foster 1994).
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References
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
D’hose R. & De Langhe J.-E. (1976) Nieuwe groeiplaatsen van zeldzame planten in België. IV. Bull. Soc. Roy. Bot. Belg. 109: 29-41.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.