Clethra is a genus of ca. 75-85 species, mainly distributed in tropical America and parts of Asia (a single species in North America and Madeira). Eight species are cultivated as ornamentals in Europe (Wann 1997) but only Clethra alnifolia is more or less widespread in cultivation.
Literature
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.
Hoffman M.H.A. (2007) Clethra – sortimentsonderzoek en keuringsrapport. Dendroflora 44: 43-54.
Hu S. (1960) A revision of the genus Clethra in China. Journ. Arnold Arbor. 41: 164-190.
Preston F.G. (1949) The genus Clethra. Journ. Roy. Hort. Soc. 74: 245-247.
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.
Wann J.D. (1997) Clethra. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 415-416.