Rorippa x armoracioides  

Rorippa x armoracioides (Tausch) Fuss (R. austriaca x R. sylvestris) (Hybr.) – An exceptional alien. A small population was discovered in a grassy roadside in 2016 in Boom.
This is a rhizomatous, sterile hybrid that is able to spread in the absence of both parents. In Boom no other species of Rorippa were noticed in the vicinity of the population.
Rorippa x armoracioides is quite variable. Some populations are morphologically closer to one parent, others to the other parent. Especially hybrids that closely resemble R. sylvestris (Bleeker 2003) may have been overlooked.
The author citation of Rorippa x armoracioides is often erroneously assigned to either Fries or Fuss, e.g. in IPNI, Tropicos, etc. Tausch (1840) apparently was the first to publish the name Nasturtium armoracioides. Fuss (1866) made the corresponding combination in Rorippa. Fries’ binomial, Nasturtium armoracia (L.) Fr., based on Cochlearia armoracia L., refers to a white-flowered plant and obviously corresponds with Armoracia rusticana.

Selected literature

Bleeker W. (2003) Hybridization and Rorippa austriaca (Brassicaceae) invasion in Germany. Mol. Ecol. 12(7): 1831-1841.
Fuss J.M. (1866) Flora Transsilvaniae Excursoria.
German D.A. (1999) Rorippa armoracioides (Tausch) Fuss (Cruciferae Juss.) is the new for Siberian flora species (in Russian). Turczaninowia 2(4): 33-36. [available online at: http://ssbg.asu.ru/turcz/p33-36.pdf]
Jarolímová V. (2005) Experimental hybridization between species of the genus Rorippa. Preslia 77: 277-296. [available online at: http://www.preslia.cz/P053CJar.pdf]
Jonsell B. (1968) Studies in the north-western European species of Rorippa s.str. Symb. Bot. Upsal. 19(2): 222 pp + 11 plates.
Jonsell B. (1975) Hybridization in yellow-flowered European Rorippa species. European Floristic and Taxonomic Studies: 101-110.

Lansdown R.V. (2009) A Field Guide to the Riverine Plants of Britain and Ireland, Ardeola Environmental Services, UK.
Radics F. (1977) The identification of Rorippa species and hybrids (Cruciferae) based on external morphological features of their seeds. Studia Bot. Hung. 12: 55-70. [available online at: http://publication.nhmus.hu/pdf/Studia/StudiaBotHung_1977_Vol_12_55.pdf]
Tausch J.F. (1840) Ueber die in Böhmen vorkommenden, zu Brachylobos gehörigen Arten von Nasturtium. Flora 23: 706-718. [available online at: http://www.biodiversitylibrary.org/item/935#page/323/mode/1up]
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Zijlstra O.G. (1992) Geelbloeiende kruisbloemigen in Twente (1). Nieuwsbrief FLORON-FWT 7: 1-5. [available online at: http://www.fwtwente.nl/n07_geelbloeiende_kruisbloemigen_in_twente_(1).pdf]