Urtica

Urtica L.

Urtica is a subcosmopolitan genus and counts, as currently accepted, 63 species (Grosse-Veldmann & al. 2016). It is most diverse in the northern hemisphere (Mabberley 2008). In Europe 11 indigenous species occur, two of which are native in Belgium as well: U. dioica L. and U. urens L. (Lambinon & Verloove 2012).
Urtica is a taxonomically complex genus. For instance, the variability of U. dioica is poorly studied in Belgium. Late flowering plants with few stinging hairs may belong to subsp. galeopsifolia (Wierzb. ex Opiz) Chrtek (syn.: U. galeopsifolia Wierzb. ex Opiz), a taxon that is claimed from most neighbouring territories (e.g. Stace 2010, Tison & de Foucault 2014), although according to other authors it is restricted to southeastern Europe (e.g. E+M Plantbase, as subsp. pubescens (Ledeb.) Domin). For the same taxon the name subsp. subinermis (R. Uechtr.) Weigend has also been applied (e.g. Frank 2008). The taxonomic status of this plant and its status in western Europe (native versus introduced) remains unclear and requires further study. Current studies by Grosse-Veldmann and colleagues might elucidate the subject (e.g. Henning & al. 2014, Grosse-Veldmann & Weigend 2015, Grosse-Veldmann & al. 2016).
Molecular studies by these authors have also shown previously unsuspected relationships, implying that classically used taxonomic characters (e.g. leaf morphology, gender distribution and growth habit) are highly homoplasious. Species delimitation is problematical, and several virtually indistinguishable accessions (mostly subspecies of Urtica dioica) were retrieved in widely different places in the phylogeny (Grosse-Veldmann & al. 2016).

1 Female flower heads globose === Urtica pilulifera
All flowers in spike-like racemes === 2
2 Dioecious perennial. Racemes usually branched, at least in part (native) === U. dioica
Monoecious annual. Racemes simple === 3
3 Each node with two stipules (these sometimes bifid). Male and female flowers in different racemes. Inflorescence axis inflated === U. membranacea
Each node with four entire stipules. Male and female flowers in mixed racemes. Inflorescence axis not inflated (native) === U. urens

Additional alien: Urtica incisa Poiret

Literature:

Ball P.W. (1964) Urtica. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 1. Cambridge University Press, Cambridge: 67-68.
Corsi G., Garbari F. & Maffei F. (1999) The genus Urtica L. (Urticaceae) in Italy: a biosystematic revision. Webbia 53(2): 193-239.
Frank D. (2008) Man sieht nur, was man kennt. Nicht beachtete indigene Taxa der Gattungen Pteridium und Urtica. Mitt. florist. Kart. Sachsen-Anhalt (Halle 2008) 13: 29-40. [available online at: https://core.ac.uk/download/files/542/14527148.pdf]
Grosse-Veldmann B., Nürk N.M., Smissen R., Breitwieser I., Quandt D. & Weigend M. (2016) Pulling the sting out of nettle systematics – A comprehensive phylogeny of the genus Urtica L. (Urticaceae). Molecular Phylogenetics and Evolution 102: 9-19. [available online at: https://www.researchgate.net/publication/303411763_Pulling_the_Sting_out...
Grosse-Veldmann B. & Weigend M. (2015) Weeding the nettles III: Named nonsense versus named morphotypes in European Urtica dioica L. (Urticaceae). Phytotaxa 208(4): 239-260.
Henning T., Quandt D., Grosse-Veldmann B., Monro A. & Weigend M. (2014) Weeding the Nettles II: a delimitation of “Urtica dioica L.”(Urticaceae) based on morphological and molecular data, including a rehabilitation of Urtica gracilis Ait. Phytotaxa 162(2): 61-83. [available online at: https://www.researchgate.net/publication/260815095_Weeding_the_Nettles_I...
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Tison J.-M. & de Foucault B. (coord.) 2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.
Weddell H.A. (1856-57) Monographie de la Famille des Urticées. 591 pp. +19 plates. [available online at: https://catalog.hathitrust.org/Record/005228400]