Acer palmatum  

Acer palmatum Thunb. (Japan, Taiwan) – A rare but increasing escape from cultivation. At least at present it probably always is an ephemeral. First observed in 2013 in Overpelt and since then seen most years in scattered localities, usually in the vicinity of planted individuals.
Acer palmatum is a very variable species. Van Gelderen (2014-2015) distinguished not less than 17 cultivar groups (see also Roloff & Bärtels 2006). Saplings observed in Belgium of the Palmatum group of Acer are also variable and may belong to more than two species (A. japonicum and A. palmatum). Also, records of the former often require confirmation, confirmed records only being available from Bois de Bon-Secours.
Acer palmatum has been reported in similar circumstances from other European countries, for instance from the London area in the British Isles (Wurzell 1993, Clement & Foster 1994). In some areas outside its native range it has been able to naturalize and is sometimes even considered an invasive species, for instance in parts of Australia (e.g. https://www.invasiveplantatlas.org/subject.html?sub=3001).

Selected references

Chang C.S. (1990) A reconsideration of the Acer palmatum complex in China, Taiwan and Korea. J. Arnold Arbor. 71(4): 553-565. [available online at: https://www.biodiversitylibrary.org/item/33634#page/573/mode/1up]

Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.

Mukherjee A. & Namhata D. (1987) A note on Acer palmatum Thunb.: a new plant in India. Geobios New Rep. 6(2): 189-190.

Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.

Van Gelderen D.M. (2014-2015) Acer palmatum en verwanten – voorstel tot nieuwe indeling in cultivargroepen. Dendroflora 51: 8-23.

Wurzell B. (1993) Foreigners in our midst. BSBI News 64: 38-42. [available online at: http://archive.bsbi.org.uk/BSBINews64.pdf]