Jacobaea Mill.
Jacobaea is a genus of ca. 35 species, all native to Eurasia. It is a segregate of the polyphyletic genus Senecio and recent molecular investigations confirm its distinctness (Pelser & al. 2002; Mabberley 2008). Generic status is furthermore supported by hybridisation behaviour: there are several hybrids within Jacobaea while there are none between Jacobaea and Senecio. However, morphologically both genera are poorly distinguished and according to Stace (2010) both genera are best keyed out together. In Jacobaea the number of inner involucral bracts ca. equals the number of ligules, whereas in Senecio s.str. the number of inner involucral bracts is ca. 1,5-2 times the number of ligules. Moreover, in Jacobaea the surface of the achenes is finely papillose while achenes are usually smooth in Senecio s.str. Representatives of the genus Jacobaea are mostly biennial or perennial.
Four species of Jacobaea are native in Belgium: J. aquatica (Hill) P. Gaertn., B. Mey. et Schreb. (syn.: Senecio aquaticus Hill), J. erucifolia (L.) P. Gaertn., B. Mey. et Schreb. (syn.: Senecio erucifolius L.), J. paludosa (L.) P. Gaertn., B. Mey. et Schreb. (syn.: Senecio paludosus L.) and J. vulgaris P. Gaertn. (syn.: Senecio jacobaea L.). Only the latter is more or less common throughout. Jacobaea erucifolia is sometimes recorded outside its native range (introduced intentionally on coalmine heaps, newly sown roadsides,…). Non-native relatives might be involved and such plants therefore possibly require further examination. A related taxon from southeastern Europe, Jacobaea erucifolia subsp. arenaria (Soó) B. Nord. et Greuter (syn.: Senecio velenovskyi Borbás, S. erucifolius subsp. arenarius Soó), is known as an alien in Germany (Vollrath & al. 2005) and might have been overlooked. The infraspecific variability of native Jacobaea aquatica probably also requires further examination. Var. erratica (Bertol.) Pelser et Meijden (syn.: Senecio aquaticus subsp. erraticus (Bertol.) Matthews; S. aquaticus subsp. barbareifolius (Wimm. et Grab.) Walters) has been recorded in Belgium but is possibly not native (see van Rompaey & Delvosalle 1978; Lambinon & al. 2004 for more details).
Some species of Jacobaea with tomentose leaves are cultivated as ornamentals in Europe. Their identity is often obscure. In addition to the single species treated in this account, a few others might be involved (see Jäger & al. 2008).
1. Leaves serrate (native) === Jacobaea paludosa
1. Leaves pinnately divided === 2
2. Entire plant tomentose (especially stem and leaves beneath) === 1. Jacobaea maritima
2. Plant glabrous to variably pubescent but never entirely tomentose (native) === J. aquatica, J. erucifolia and J. vulgaris
Literature:
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Mabberley D.J. (1983) The dusty miller's tale or Senecio cineraria DC. restored. Watsonia 14(3): 279-280.
Nordenstam B. (2006) Additions to the genus Jacobaea Mill. (Asteraceae - Senecioneae). Compositae Newsletter 44: 12-13. [available online at: http://www.biodiversitylibrary.org/item/48846#page/1/mode/1up].
Pelser P.B., Gravendeel B. & van der Meijden R. (2002) Tackling speciose genera: species composition and phylogenetic position of Senecio sect. Jacobaea (Asteraceae) based on plastid and nrDNA sequences. Am. J. Bot. 89(6): 929-939.
Pelser P.B., Veldkamp J.-F. & van der Meijden R. (2006) New combinations in Jacobaea Mill. (Asteraceae - Senecioneae). Compositae Newsletter 44: 1-11. [available online at: http://www.biodiversitylibrary.org/item/48846#page/1/mode/1up].
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Van Rompaey E. & Delvosalle L. (1978) Atlas de la flore belge et luxembourgeoise. Ptéridophytes et Spermatophytes. Commentaires. Jardin Botanique National de Belgique, Meise: 116 p.
Vollrath H., Gerstberger P. & Lauerer M. (2005) Senecio velenovskyi Borbás als Neubürger in Mitteleuropa. Flor. Rundbr. 39: 1-6.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.