Artemisia princeps Pamp. (syn.: A. indica Willd. var. maximowiczii (Nakai) H. Hara) (China, Japan, Korea) – A rare but much increasing, aggressive alien for which the vector of introduction is obscure. This species was first recognized in 2011 in the Antwerp port area, on rough ground and railway sidings close to Ekerse Dijk and Bospolder nature reserve. The species grew in a massive, monospecific population in the vicinity of known localities of A. verlotiorum (and A. vulgaris). Based on its intermediate morphology and phenology it was initially thought to be their hybrid, A. xwurzellii C.M. James & Stace (James & al. 2000). On closer examination, however, plants were shown to be fully fertile and turned out to belong to A. princeps, a vigorous Far Eastern species that is sometimes grown as a vegetable (mochi). In the intervening years several new populations were discovered, at first – in 2013 – alongside E17 motorway between Lokeren and Gent, later also in the port of Zeebrugge (talus slope and roadsides) and alongside A12 motorway in Wilrijk. Probably the largest populations, however, are known from the Antwerp port area between Lillo and Berendrecht. In this area Artemisia princeps grows in nearly monospecific stands on rough ground, by railway sides and fences for several kilometers. It is known from there since at least 2010 (probably much longer) but was confused with A. verlotiorum for many years. Similar records are known from the Netherlands, mainly from the Rotterdam area. A paper about A. princeps in western Europe is in preparation (Verloove & al. 2017).
It is unknown how Artemisia princeps was introduced in western Europe. The oldest known populations are from port areas (Antwerp, Zeebrugge) and could point at an inadvertent introduction with Asian goods. Another possibility is that this species was introduced on purpose by Asian immigrants in their gardens. Since it is extremely vigorous (as a result of its long rhizomes) it is probably soon discarded and ends up on dumps, etc. From there its rhizomes are easily further dispersed with ground for talus slopes, etc.
Artemisia princeps is, at least in western Europe, a much more aggressive species than A. verlotiorum. The latter probably is a more thermophilous species that thrives better in southern Europe where it is a known invader.
James C.M., Wurzell B.S. & Stace C.A. (2000) A new hybrid between a European and a Chinese species of Artemisia (Asteraceae). Watsonia 23: 139-147.
Verloove F., Andeweg R. & Zonneveld B. (2017) Artemisia princeps (Asteraceae), an overlooked potentially invasive species in western Europe.