Symphytum caucasicum Bieb. (Caucasus) – A very rare, probably locally naturalized escape from cultivation (or merely a throw-out). Apparently first seen in 2009 in Limburg: on rough ground in Hasselt and on a shady off-ramp between Diepenbeek and Rapertingen. At least in the latter locality Symphytum caucasicum occurs in relative abundance and looks perfectly established. In the past years occasionally seen elsewhere, for instance in Kaprijke (2011) and in a woody roadside in Kortrijk (2013). See http://waarnemingen.be/soort/view/122736 for an up-to-date overview for Flanders.
Symphytum caucasicum superficially resembles S. asperum and these species have been confused. Both are erect, non-stoloniferous perennials with sky-blue corollas. In the former the calyx is only shallowly divided at flowering (always less than ½ way to base) whereas in the latter the calyx is always divided more than ½ way to base). This character is much less obvious in the fruiting stage. In addition, nutlets are minutely tuberculate in Symphytum caucasicum and more or less papillose with angular markings in S. asperum. Finally, in ‘pure’ Symphytum asperum leaves are not decurrent at all (the upper often shortly stalked) while in S. caucasicum leaves are always shortly decurrent along stem. A putative hybrid between these species has been recorded in the British Isles (Learmonth & Perring 1995).
Symphytum caucasicum, Kortrijk (Wolvendreef), shady roadside, May 2013, F. Verloove
Bomble W. & Schmitz B.G.A. (2012) Kaukasischer Beinwell (Symphytum caucasicum M. Bieb.) und Hidcote-Beinwell (Symphytum ×hidcotense P. D. Sell) im Aachener Raum. Veröff. Bochumer Bot. Ver. 4(6): 50-54. [available online at: http://www.botanik-bochum.de/]
Learmonth R.W.C. & Perring F.H. (1995) A Symphytum (Comfrey) hybrid new to Britain. B.S.B.I. News 69: 74. [available online at: http://archive.bsbi.org.uk/BSBINews69.pdf]
Synge P.M. (1981) Notes from Fellows: an unusual comfrey and two magnolias. Garden (London) 106(11): 474-475.