Cytisus Desf.
(incl. Chamaecytisus Link, Sarothamnus Wimmer)
As currently circumscribed (Mabberley 2008) Cytisus is a genus of about 60 species, native to Europe, North Africa, West Asia and Macaronesia. Only one species, Cytisus scoparius (L.) Link, is native in Belgium. Several more are cultivated as ornamentals in gardens and parks (see de Vries 1995 for an overview of the genus in cultivation in Europe). Some species are widely planted for erosion control (see for instance Dupont & al. 1984) and readily escape.
Numerous cultivars of native Cytisus scoparius are cultivated and sometimes occur as escapes. They mostly differ in flower colour (varying from deep-brown to whitish) and flowering period (see also de Vries 1995). Forma andreanus (A. Puissant) Zabel was recently recorded at the junction of river Schelde and Ringvaart near Melle (2010), see photo.
1. Corolla always whitish. Calyx pubescent, 2,5-3,5 (-4) mm long === 1. Cytisus multiflorus
1. Corolla yellow (rarely whitish in cultivars but then calyx always glabrous). Calyx glabrous or pubescent, (4-) 5-7 mm long === 2
2. Calyx pubescent. Pod slightly inflated, 17-30 x 7,5-12 mm, densely pubescent throughout === 2. C. striatus
2. Calyx glabrous. Pod flattened, 20-50 x 7-12 mm, with glabrous faces (native) === C. scoparius
Additional aliens: Cytisus decumbens (Durande) Spach (syn.: Genista pedunculata L’Hérit. subsp. decumbens (Durande) Gams, G. prostrata Lam.) (SC-Eur., garden escape?), C. hirsutus L. (syn.: C. supinus L., Chamaecytisus hirsutus (L.) Link, C. supinus (L.) Link) (C and E-Eur., garden escape?) and C. sessilifolius L. (SW-Eur., garden escape). Out of these, Cytisus hirsutus was abundantly naturalised along a railway track in Heverlee (Park), at least between 1876 and 1888.
References
Cristofolini C.& Troìa A. (2006) A reassessment of the sections of the genus Cytisus Desf. (Cytiseae, Leguminosae). Taxon 55(3): 733-746.
Cubas P., Pardo C. & Tahiri H. (2002) Molecular approach to the phylogeny and systematics of Cytisus (Leguminosae) and related genera based on nucleotide sequences of nrDNA (ITS region) and cpDNA (trnL-trnF intergenic spacer). Pl. Syst. Evol. 233(3-4): 223-242.
De Vries F.T. (1995) Cytisus. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 538-544.
Dupont P., Godeau M. & Rivière G. (1984) Remarques sur des espèces ibériques d’ajoncs et de genêts semées au long des routes du Morbihan, de Loire-Atlantique et des territoires voisins. Bull. Soc. Sc. Nat. Ouest de la France N.S. 6(3): 125-129.
González Andrés F. & Ortiz J.M. (1996) Morphometrical characterization of Cytisus and allies (Genisteae: Leguminosae) as an aid in taxonomic discrimination. Israel J. Plant Sci. 44(2-3): 95-114.
Heywood V.H. (1968) A synopsis of the European species of Cytisus and allied genera. Feddes Repert. 79: 20-23.
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. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Terrisse A. (1992) Note sur la présence de Cytisus striatus en Ariège, suivie de remarques sur les problèmes rencontrés par le botaniste amateur. Bull. Soc. Bot. Centre-Ouest n.s. 23: 39-44.
Van de Laar H.J. (1971) Cytisus en Genista. Dendroflora 8: 3-18.
Zieliński J. (1975) Rodzaj Cytisus L. s.l. w Polsce. Arb. Kornickie Roczn 20: 47-111.