Rumex crispus L. (Euras.) – A very common and exceedingly variable native species (see also Danser 1921 for an extensive account of its variability). The variability of Rumex crispus is insufficiently studied in Belgium. Lawalrée (1953) distinguished var. unicallosus Peterm. with valves with a single tubercle. It is probably widely distributed and not rare. Lambinon & al. (2004) no longer mention this variety but draw the attention to a particular coastal race with larger valves, all with a well-developed tubercle (var. littoreus Hardy). See also Stace (2010).
Raycheva (2009), mainly following Pestova (1998), distinguished several infraspecific taxa (two subspecies and several varieties) in the Bulgarian flora. One subspecies, subsp. robustus, was recently recorded in Belgium, probably as an introduction. Both subspecies are opposed in the following couplet:
1. Valves ca. 5-5,5 mm wide, slightly cordate at base, ca. as wide as long, with a single inconspicuous tubercle, ca. 1,5 mm long. Lateral inflorescence branches slightly spreading, not tightly appressed to the main axis. Leaves lanceolate to broadly so, with weakly undulate margins. Plant tall, much exceeding 100 cm === subsp. robustus
1. Valves ca. 3,5-5 mm wide (longer than wide), truncate at base, with 1-3 tubercles, the larger ca. 2 mm long. Lateral inflorescence branches often tightly appressed to the main axis. Leaves narrower and more distinctly undulate at margin. Plant smaller, rarely exceeding 100 cm (native) === subsp. crispus
Subsp. robustus (Rech.) Pestova (syn.: var. robustus Rech.) (Euras.) – An exceptional record but possibly overlooked. A single plant was recorded on rough ground in Ronse in 2012 and several individuals were observed at a coal mining spoil heap in Hornu in 2015. In general appearance this plant seems perfectly intermediate between native Rumex crispus and alien R. patientia. It shares the tall stature, wider and less crispate leaves and larger valves with the latter but leaves are distinctly tapered at base like in R. crispus (deeply cordate in R. patientia). Rumex xconfusus, the hybrid between R. crispus and R. patientia, is even more reminiscent of subsp. robustus (as already noticed by Rechinger 1892) but is largely sterile, has leaves with a shallowly cordate to truncate base, valves (when developed) with often 3 tubercles, etc.
Although fairly distinct, this taxon is very poorly known. It was initially described by Rechinger (1892) from Austria. Pestova (1998) reported it from Ukraine and Raycheva (2009) furthermore from Bulgaria, Finland (doubtfully native?) and Iran. Danser (1921) saw similar plants from the Netherlands.
Pestova (1998) raised this taxon to subspecies rank, probably correctly so. Subsp. robustus is strikingly distinct from subsp. crispus and, in some way, more closely resembles Rumex patientia.
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References
Danser B.H. (1921) Bijdrage tot de kennis der Nederlandsche Rumices. Nederl. Kruidk. Arch. 1921: 167-228.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Lawalrée A. (1953) – Rumex. In Robyns W. (ed.), Flore générale de Belgique. Vol. 1, fasc. 2: 172-206. Bruxelles, Jardin Botanique de l’Etat.
Pestova I.O. (1998) Genus Rumex L. (Polygonaceae) in the flora of Ukraine. Ukr. Bot. Zhurn. 55(1): 26-29 (in Ukrainian).
Raycheva T. (2009) Critical reassessment of the distribution of some taxa of Rumex subgenus Rumex (Polygonaceae) in Bulgaria – 2. Phytol. Balcan. 15(2): 155-169. [available online at: http://www.bio.bas.bg/~phytolbalcan/PDF/15_2/15_2_02_Raycheva.pdf]
Rechinger K. (1892) Beitrag zur Kenntnis der Gattung Rumex. Öster. Bot. Zeitschr. 42: 17-20.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.