Arecaceae
Arecaceae (syn. Palmae) is a family confined to the tropics and warm-temperate regions of the world and counts about 189 genera and 2400 species (Mabberley 2008). It is particularly well represented in New Guinea (Mabberley l.c.) and, evidently, not native in Belgium.
Numerous species are of importance, e.g. economically (coconut, date palm, etc.) and horticulturally. A useful account for the palm family in horticulture in Europe is provided by Walters & al. (1984).
Only two genera (and species) have been reliably recorded in the wild in Belgium so far, Phoenix L. and Trachycarpus H. Wendl. Since both have only been recorded at the juvenile stage, often as mere seedlings, their identification in the key here under is solely based on early development character states (Henderson 2006). At maturity both genera are quite different: Phoenix is pinnate leaved whereas Trachycarpus is palmate leaved.
A young, self-sown individual from yet another genus (most likely Washingtonia H. Wendl. or a similar genus, for instance Sabal Adans.) was recorded in Brussels in 2015 at the foot of a wall (see photo). Its origin remained uncertain. Nearby florist shops offer dried palm inflorescences for ornament which may explain its appearance. It was removed soon after its discovery.
1 Eophyll (seedling leaves) lanceolate, not rigid and often more or less curved. Seed kidney shaped === Trachycarpus
Eophyll very narrow, straight and rigid. Seed elongate === Phoenix
Literature:
There is a very extensive literature on palms. Useful additional references are provided by Walters & al. (1984) and Mabberley (2008). See also:
PalmWeb – Palms of the World Online at: http://www.palmweb.org/
EUNOPS – European Network of Palm Specialists at: http://eunops.org/
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
Dransfield J., Uhl N.W., Asmussen C.B., Baker W.J., Harley M.M. & Lewis C.E. (2005). A new phylogenetic classification of the palm family, Arecaceae. Kew Bulletin 60: 559-569.
Henderson F. (2006) Morphology and Anatomy of Palm Seedlings. The Botanical Review 72: 273-329. [available online at: https://www.researchgate.net/publication/232687103_Morphology_and_Anatom...
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Walters S.M., Brady A., Brickell C.D., Cullen J., Green P.S., Lewis J., Matthews V.A., Webb D.A., Yeo P.F. & Alexander J.C.M. (eds.) (1984) The European Garden Flora, vol. 2 (part II). Cambridge University Press: 318 p.
Walther GR. (2000) Laurophyllisation in Switzerland. Dissertation, Swiss Federal Institute of Technology (ETH) Zurich. [available online at: http://e-collection.library.ethz.ch/view/eth:23429]
Walther G.R., Gritti E.S., Berger S., Hickler T., Tang Z. & Sykes M.T. (2007) Palms tracking climate change. Global Ecol. Biogeogr. 16:801-809. [available online at: https://emmanuelsgritti.files.wordpress.com/2013/03/walther2007.pdf]