Rhododendron luteum Sweet (syn.: R. flavum G. Don, Azalea pontica L.) (E-Eur., W-As.) – A rare but possibly increasing escape from or relic of cultivation. First recorded in 1963 near an old estate in ‘s Gravenwezel (abundantly). Subsequently collected near Beernem (Lindeveld), Malle (‘s Heerenbos), Pulle, Rumst, Tilff, Wuustwezel and doubtlessly elsewhere. In most cases Rhododendron luteum is a mere relic of cultivation but becomes naturalised as a result of extensively suckering. It is chiefly confined to peaty woodland on sandy, more or less acidic soils and is by far most widespread in the Kempen.
A similar species with yellow flowers and pubescent leaves has also been claimed as an escape from cultivation in Belgium, Rhododendron molle (Blume) D. Don (see Lambinon & al. 2004). Non-confirmed records are available from the surroundings of Kapellen and Pulle. Both species are indeed closely related and belong to the same, taxonomically complex section Pentanthera G. Don (syn.: Azalea series Luteum). However, Rhododendron molle has a broadly funnel-shaped corolla with stamens hardly exserted whereas R. luteum has a narrowly funnel-shaped corolla with well-exserted stamens. The few Belgian records of Rhododendron molle proved to be in error for R. luteum.
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References:
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.
Wróbel D. (2013) Nowe stanowisko Rhododendron luteum (Ericaceae) w Kotlinie Sandomierskiej (południowo-wschodnia Polska) [New locality of Rhododendron luteum (Ericaceae) in Sandomierz Basin]. Fragm. Flor. et Geobot. Pol. 20(1): 136-139.