Saxifraga xurbium D.A. Webb (Hybr.) (S. umbrosa L. x S. spathularis Brot.) – A rare escape from cultivation, possibly persisting locally, at least for some time. Reliably recorded in recent times in Waimes (Ovifat: Reinhardstein), Crupet, Furfooz, Lille and Brugge. It is usually seen in damp, shady (woody) places, on rocks or ruins, abandoned gravel pits, etc.
This hybrid is, according to Gornall (1995), one of the commonest saxifrages in gardens these days, although it was only relatively recently described (Webb 1963). This apparently corroborates with recent Belgian observations (see also Stace 2010 for the British Isles). It is quite similar to the much less frequent Saxifraga xgeum and both are often intermingled. Compared with the latter it has leaves with a much shorter petiole that is scarcely distinguished from the blade.
Selected literature:
Gornall R.J. (1995) Saxifraga. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 251-280.
Gudžinskas Z. (2005) New alien Saxifragaceae in Lithuania.Botanica Lithuanica 11(1): 17-21. [available online at: https://www.researchgate.net/publication/269993276_New_alien_Saxifragace...
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Webb D.A. (1963) Flora Notulae Systematicae No. 2. Saxifragaceae. Feddes Repert. Spec. Nov. Regni Veg. 68: 198-210.