6. Rumex longifolius DC. (syn.: R. domesticus Hartm.) (Eur.) – A very rare and ephemeral alien but possibly overlooked. In the 19th century repeatedly recorded (but without further details). In 2001 rediscovered in an urban environment in the city of Gent (wasteland); not confirmed afterwards. In 2012 also seen (a single specimen) by a roadside near grain mills in the port of Gent. In the past years several new records came to light (Verloove 2013) and in some localities Rumex longifolius seems firmly established.
Rumex longifolius is very reminiscent of R. patientia and is probably overlooked in Belgium. Both share the tall stature and large, nearly orbicular valves with entire margins. However, Rumex longifolius has all valves without tubercles (or one with at most a slightly thickened vein) and it usually is slightly smaller (60-150 cm tall). Moreover, its stems are usually greenish. Rumex patientia, in turn, has at least one valve with a distinct tubercle, usually exceeds 150 cm and its stems are always reddish (subsp. patientia). Despite their resemblance, both species are not related (Rechinger 1933). Small plants of Rumex longifolius may be overlooked for R. crispus (see also Adler 1992). In the latter all valves usually have fully developed tubercles.
The exact original distribution area of Rumex longifolius is very obscure. It has a very disjunct distribution in large parts of Europe and is accepted as native in several neighbouring countries (France, Great Britain). In Germany it is a naturalized xenophyte. It recently also re-occurred in the Netherlands along river Maas in Limburg (van der Meijden 2005). In the Alps Rumex longifolius was confused for quit a long time with R. crispus (Adler 1992).
The infraspecific variability of Rumex longifolius in Belgium has not been studied yet. Plants with wider leaves and nearly uncrisped margins have been ascribed to subsp. sourekii K. Kubat (Sonnberger 2017).
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Literature:
Adler W. (1992) Ist Rumex longifolius (Polygonaceae) in den Alpen doch heimisch? Verh. Zool.-Bot. Ges. Österr. 129: 153-158 [available online at: http://www.landesmuseum.at/pdf_frei_remote/VZBG_129_0153-0158.pdf].
Fasel P. (1995) Neufund des Nordischen Ampfers Rumex longifolius D.C. (= Rumex domesticus Hartm.) in Nordrhein-Westfalen. Flor. Rundbr. 29(1): 42-43.
Holm C. & Korpelainen H. (1999) Geographical differentiation of Rumex longifolius. Nordic J. Bot. 19(1): 15-22.
Jalas J. & Lindholm K. (1975) Biosystematics of Rumex longifolius DC. of Fennoscandia and the Pyrenees. Anales Inst. Bot. A.J. Cavanilles 32(2): 197-202.
Jansen P. & Wachter W.H. (1913) Floristische aanteekeningen VII. Rumex. Nederl. Kruidk. Arch. 1913: 91-140.
Kubinova D. & Krahulec F. (1998) Phenology of Rumex longifolius: a key factor for the success of an invasive species? Preslia 70(4): 339-348.
Ludwig W. (1995) Kurze Hinweise auf neue Rumex longifolius Funde. Hess. Flor. Briefe 44(3): 40-41.
Rechinger K.H. (1933) Vorarbeiten zu einer Monographie der Gattung Rumex II. Die Arten der Subsektion Patientiae. Repert. Spec. Nov. 31: 225-283.
Rechinger K.H. (1947-48) Beiträge zur Kenntnis von Rumex IX. Candollea 11: 229-241.
Rechinger K.H. (1990) Zwei kritische Rumex- Arten, R. longifolius DC. und R. aquitanicus Rech. f., und ihre Verbreitung. Bot. Jahrb. 111(3): 347-364.
Sonnberger B. (2015) Rumex longifolius DC. - Ein bemerkenswerter Neophyt Mitteleuropas. Ber. Bayer. Bot. Gesell. 85: 79-86. [available online at: https://www.researchgate.net/publication/287912251_Rumex_longifolius_DC_...
Sonnberger B. (2017) Zwei Sippen von Rumex longifolius DC. im bayerisch-böhmischen Grenzgebirge. Ber. Bayer. Bot. Gesell. 87: 187-190.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Verloove F. (2013) Verder onderzoek binnen het genus Rumex (Polygonaceae) in België. Dumortiera 102: 3-9. [available online at: http://www.br.fgov.be/DUMORTIERA/DUM_102/Dum_102_3-9_Rumex_Verloove.pdf]