Urticaceae is a large family and counts 54 genera and ca. 2500 species (Wu & al. 2013). They are mostly distributed in the tropics with rather few species extending to more temperate regions (Mabberley 2008). In Belgium only one genus is certainly native (Urtica L.) (Lambinon & Verloove 2012). A second, Parietaria L., is usually also considered native but this is less obvious. Both Belgian representatives of this genus (P. judaica L. and P. officialis L.) are mostly found in artificial habitats, the former on old walls, while the latter has long been cultivated as a medicinal plant.
Several species of the genus Pilea Lindl. have been recorded as weeds in greenhouses. These include Pilea hyalina Fenzl and P. microphylla (L.) Liebm. None of these has been recorded out of doors so far. Another species of the latter genus, Pilea peperomioides Diels, was claimed from an old wall in the city of Brugge (Robbrecht 1987) but this record has never been confirmed.
Molecular studies have shown Urticaceae (incl. Cecropiaceae) to be monophyletic (Wu & al. 2013). The family is close to Moraceae and both were once united but this is not confirmed by molecular data.
1. Stinging hairs present. Leaves with dentate margins === 2
1. Stinging hairs absent. Leaves with entire margins === 3
2. Leaves opposite. Style absent === Urtica
2. Leaves alternate. Style present, persistent in fruit === Laportea
3. Stem procumbent, rooting at nodes. Leaves suborbicular, usually less than 8 mm across. Flowers solitary === Soleirolia
3. Stem decumbent to erect, never rooting at nodes. Leaves ovate to elliptic, much larger. Flowers clustered (native) === Parietaria
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.
Robbrecht E. (1987) Pilea peperomioides in België? IFBL-Driemaandelijks contactblad 5: 5.
Wu Z.-Y., Monro A.K., Milne R.I., Wang H., Yi T.-S., Liu J. & Li D.-Z. (2013) Molecular phylogeny of the nettle family (Urticaceae) inferred from multiple loci of three genomes and extensive generic sampling. Molecular Phylogenetics and Evolution 69: 814-827. [available online at: https://www.researchgate.net/profile/Zeng-Yuan_Wu/publication/249319195_...(Urticaceae)_inferred_from_multiple_loci_of_three_genomes_and_extensive_generic_sampling/links/0c960524eee3d0ebca000000.pdf]