1. Rheum xrhabarbarum L. (syn.: R. xhybridum Murray, R. undulatum L.) (Origin obscure) – A frequent garden plant but probably never escaping. Rheum xrhabarbarum is sometimes seen as a relic of cultivation in old or derelict gardens or merely as a garden throw-out. It is exceptionally observed outside gardens, for instance under a crash barrier of the A19 motorway near Ieper since 2007. It often persists for some time but never becomes truly naturalised in Belgium.