Clethra alnifolia L. (syn.: C. paniculata Ait.) (N-Am.) – A very rare, more or less locally naturalised alien. Most of the time a mere relic of cultivation in or near former estates (now often public parks), slightly extending by clonal growth but probably not reproducing sexually. The presence of Clethra alnifolia in Belgium is poorly documented. Discovered in a roadverge-wood margin in 1984 in Kapellen (road towards Essen). Furthermore known since at least 40 years from the Inslag in Brasschaat and from Mastenbos in Kapellen (comm. Dirk Leyssens, 2010). In 2011 also discovered in woodland in Gooreind (Wuustwezel), along with Rhododendron luteum. Possibly neglected in other old parks and private estates in this area. Outside the Kempen probably exceptional: in 2012 found in woodland (Merkenveld) in Loppem, near Brugge.
Clethra alnifolia preferably grows in moist, sandy and acidic soil, most often in shady or half-shady places. It is a rather rare escape elsewhere in western Europe. Clement & Foster (1994) report about similar records in the British Isles and van der Meijden (2005) does the same for the Netherlands.
Wann (1997), De Koning & al. (2000) and Roloff & Bärtels (2006) (and doubtlessly others as well) are rather imprecise in their identification keys. Possibly as a result of this Belgian populations have also been (erroneously) ascribed to the Asian Clethra barbinervis Sieb. et Zucc., a species with larger leaves with more veins, longer petioles, etc. The latter also has leaf margins that are very sharply serrate throughout (versus less sharply serrate and more or less entire in the lower 1/3 in Clethra alnifolia). Moreover, inflorescences in Clethra barbinervis are usually compound and merely racemose in C. alnifolia. Finally, petioles are never as long as indicated by Tucker & Jones (2009); these authors obviously intermixed centimeters and millimeters! Belgian populations have single racemes, small capsules and petals that are not emarginate. All these features are characteristic for genuine Clethra alnifolia. However, leaves are often larger with more than 10 veins and slightly hairy on the veins beneath. Such plants probably belong to cultivars that resulted from hybridisation which possibly also involve Clethra barbinervis.
Selected literature
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 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.
Reed S.M. (2006) Reproductive biology of Clethra alnifolia. HortScience 41(3): 567-570.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Sleumer H. (1967) Monographia Clethracearum. Bot. Jahrb. Syst. 87: 36-175.
Tucker G.C. & Jones S.C. (2009) Clethra. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 8. Oxford University Press, New York-Oxford: 364-366.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.