Kerria japonica (L.) DC. (China, Japan) – A rare but probably increasing alien. Apparently first documented in the wild in Belgium in 1902 (Verloove 2006). In the past decades Kerria japonica has been recorded much more often but most records refer to relics of cultivation or more or less established garden throw-outs. In such instances it can slightly spread vegetatively. The species only rarely reproduces from seed. Apparently self-sown individuals have been recorded, for instance, on an old wall in Leuven in 2011 (http://waarnemingen.be). Kerria japonica is now known from widely scattered localities throughout the country; an up-to-date overview of observations is available here: http://waarnemingen.be/soort/view/129671.
The plant usually found in cultivation (and in the wild) has multiple petals and belongs to a cultivar named ‘Pleniflora’.
Selected literature:
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p. [available online at: http://alienplantsbelgium.be/sites/alienplantsbelgium.be/files/tabel_2.pdf]
Wadleigh J.S. (1975) The worthy Kerrias. Arnoldia 35(3): 160-161. [available online at: http://arnoldia.arboretum.harvard.edu/pdf/articles/1975-35-3-the-worthy-...