3. Helianthus decapetalus L. (syn.: H. trachelifolius Mill.) (N-Am.) – A very rare escape from cultivation or throw-out but possibly under-recorded. Discovered in 2011 on rough ground adjacent to a garden in Eksel and in 2015 on ground heaps in Brugge.
Helianthus decapetalus might have been overlooked so far in Belgium. It superficially resembles Helianthus xlaetiflorus and H. tuberosus but is, on closer examination, readily distinguished. It shares the general habit of the former (stature, etc.) but differs by the much longer, very loosely appressed and long-acuminate involucral bracts. From Helianthus tuberosus it is distinguished by the nearly glabrous stems, the absence of tubers, the (much) smaller stature, the smaller flowers, etc. The involucral bracts of both species are similar but those of Helianthus decapetalus are still longer (especially when compared with the disc diameter), narrower and often reflexed at maturity.
In cultivation Helianthus decapetalus is often replaced by a hybrid with H. annuus; such plants are referred to as H. xmultiflorus (syn.: H. decapetalus var. multiflorus (L.) A. Gray) (see for instance Stace 2010) and often have “double” flowers (flore pleno), more flexible stems and inflorescences with more than a single capitula. They are surely encountered in gardens in Belgium but not (yet) in the wild. It is, according to Clement & Foster (1994), not rare in the British Isles.
Other similar species might also have been overlooked so far, especially Helianthus strumosus L. It shares the glabrous stems and loose, long-acuminate involucral bracts of Helianthus decapetalus but has narrower leaves (at least three times as long as wide) with a very rough-hairy upper side and its involucral bracts are shorter (not distinctly longer than the disc diameter). It should be looked for.
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.