Myriophyllum rubricaule sp. nov. a M. aquaticum (Vell.) Verdc. look-alike only known in cultivation.. Myriophyllum rubricaule sp. nov.

Plants belonging to this taxon have been in trade in the Netherlands/north-western Europe since quite some time as Myriophyllum brasiliensis or M. brasiliense. In the work by Moody and Les (2010) mention is made of non-descript material in horticulture as Myriophyllum resp. ‘red 1’ and ‘red 2’. Material belonging to this taxon was included in our previous study as M. ‘brasiliense’ trade material and erroneously referred to as M. robustum Hook.f. (Ghahramanzadeh et al. 2013). Once having received true M. robustum from New Zealand and after observed flowering in 2013 (Valkenburg 3739)this diagnosis of the trade material was adapted and labelling subsequently altered in Q-bank plants to Myriophyllum sp. ‘trade name ‘brasiliensis’. In the absence of any link to occurrences in the wild we still refrained from formally describing this taxon. In the meantime plants have proven to be hardy in outdoor mesocosms at Wageningen since 2007. Following inclusion of Myriophyllum aquaticum (Vell.) Verdc. on the list of invasive alien species of Union concern (‘the Union list’) pursuant to EU regulation no. 1143/2014 since August 2016 it has fully replaced M. aquaticum in horticultural trade in Europe. As might have been expected we now see an increasing number of sightings in urban waters as well as in more natural habitats of plants belonging to this taxon. Obviously as a result of inappropriate garden waste disposal or as deliberate plantings in urban waterways/residential areas. The ongoing confusion with respect to the epithet brasiliensis has prompted us to formally describe this taxon only known in cultivation (as far as we could unearth)