Mazus pumilus (Burm.f.) Steenis (syn.: M. japonicus (Thunb.) Kuntze) (S and SE-As.) – An exceptional alien. Sometimes seen as a bonsai-weed in garden centers in Belgium. In 2014 observed for the first time in the wild: several plants grew on the gravelly banks of river Maas in the Negenoord-Kerkeweerd nature reserve (Lanaken). The origin of these plants remains obscure. Mazus pumilus had been recorded before in similar circumstances in the surroundings of Pavia in Italy (Gardini 1985, Desfayes 1997). More recently, still in Italy, it has also been observed as a pavement weed in the city of Verona (see: http://www.actaplantarum.org/floraitaliae/viewtopic.php?f=40&t=48519&sid=4164601c9356baa4f3a0cf1bc1d1b535).
The plants from Lanaken were initially ascribed to Lindernia spec. (L. dubia being known from the same area). The latter is readily distinguished by its smaller calyx ca. 3 mm with narrow lobes (c. 0.5 mm wide) that are free to base (vs. calyx campanulate, 3-8 mm, with ovate-triangular lobes almost as long as tube) and a slightly smaller corolla (c. 6.5 mm vs. 10 mm). Both these species may have been confused elsewhere in Europe.
Mazus pumilus itself is a very variable species (see Hong & al. 1998). The not enlarged calyx, not entire leaves and pedicel length (5-12 mm) suggest our plants to belong to var. pumilus.
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Selected literature:
Bacigalupo N.M. (1991) Mazus japonicus (Scrophulariaceae), adventicia en la Argentina. Bol. Soc. Argent. Bot. 27(1-2): 131-132.
Desfayes M. (1997) Mazus pumilus (Scrophulariaceae), adventice nouvelle pour l’Italie, et Lemna minuta (Lemnaceae) espèce nouvelle pour la province de Pavie. Saussurea 28: 65-66.
Gardini S.P. (1985) Mazus japonicus (Thunb.) Kuntze in Italia. Atti Ist. Bot. Univ. Pavia 2 serie 7: 95-98.
Hong D., Yang H., Jin C., Fischer M.A., Holmgren N.H. & Mill R.R. (1998) Scrophulariaceae. In: Wu Z.Y. & P.H. Raven (eds.), Flora of China, vol. 18. Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis: 1-212. [available online at: http://flora.huh.harvard.edu/china/PDF/PDF18/mazus.pdf]
Reddy C.S. & Raju V.S. (2000) On the occurrence of Mazus pumilis (Burm. f.) Steenis (Scrophulariaceae) in Kerala, India. J. Econ. Taxon. Bot. 24(2): 319.