Persicaria (L.) Mill.
The generic limits of Persicaria have long been very controversial. Ronse Decraene & Akeroyd (1988) included, among others, Aconogonum, Bistorta and Rubrivena in Persicaria. However, recent molecular phylogenetic research strongly recommends their exclusion from the latter (Galasso & al. 2009). As currently circumscribed, Persicaria counts ca. 100 species that are mostly confined to the temperate regions of the northern hemisphere (with some species in the West Indies and tropical South America). Six species are native in Belgium (see Lambinon & al. 2004).
Persicaria lapathifolia (L.) Delarbre is an exceedingly variable native species in which numerous infraspecific taxa have been described (see for instance Danser 1924 for an overview in the Netherlands). Unusual, probably non-native forms are sometimes found near grain mills in port areas. As they never produce ripe achenes, their exact identity remains uncertain. Such plants may represent Persicaria lapathifolia s.l. or possibly belong to closely related (sub-) tropical weedy species (see for instance Wilson 1990). At least Persicaria glabra (Willd.) Gómez de la Mata (syn.: Polygonum glabrum Willd.) and P. senegalensis (Meissner) Sojàk (syn.: Polygonum senegalense Meissner) have been recorded in the British Isles (Clement & Foster 1994). Yet another species, Persicaria extremiorientalis (Vorosch.) Tzvelev, is very reminiscent of native P. maculosa and has long been overlooked in the United States (Atha & al. 2010). It should be looked for in western Europe as well.
1. Inflorescence capitate. Petioles winged === 2
1. Inflorescence spicate. Petioles not winged === 3
2. Densely glandular-hairy perennial. Inflorescence not subtended by leaf-like bracts. Abaxial leaf surface not glandular punctate. Achene more or less trigonous === 1. Persicaria capitata
2. Glabrous to sparsely hairy annual. Inflorescence subtended by one or two leaf-like bracts. Abaxial leaf surface glandular punctate. Achene biconvex === 2. P. nepalensis
3. Rhizomatous perennial (native) === P. amphibia
3. Annual === 4
4. Plant densely hairy throughout. Ochreas usually green and leaf-like in upper part === 3. P. orientalis
4. Plant glabrous or very sparsely hairy. Ochreas entirely membranaceous === 5
5. Glands present on peduncles and perianth === 6
5. Glands absent (native) === P. maculosa, P. minor and P. mitis
6. Inflorescence dense. Leaves without peppery taste. Ochrea glabrous or shortly ciliate at apex === 7
6. Inflorescence lax. Leaves with distinct peppery taste. Ochrea long-ciliate at apex (native) === P. hydropiper
7. Glands on peduncles stalked. Flowers bright pink === 4. P. pensylvanica
7. Glands on peduncles sessile. Flowers whitish, greenish or pale pink (native) === P. lapathifolia
Literature:
Akeroyd J.R. (1989) Polygonum. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 125-128.
Atha D.E., Nee M.H. & Naczi R.F.C. (2010) Persicaria extremiorientalis (Polygonaceae) is established in the flora of the eastern United States of America. Journal of the Torrey Botanical Society 137(4): 333-338. [available online at: http://www.bioone.org/doi/pdf/10.3159/10-RA-033.1]
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
Danser B.H. (1924) Determineertabel voor de in Nederland in het wild gevonden Polygonaceeën. Nederl. Kruidk. Arch. 1923: 271-306.
Galasso G., Banfi E., De Mattia F., Grassi F., Sgorbati S. & Labra M. (2009) Molecular phylogeny of Polygonum L. s.l. (Polygonoideae, Polygonaceae), focusing on European taxa: preliminary results and systematic considerations based on rbcL plastidial sequence data. Atti Soc. It. Sci. Nat. Museo Civ. Stor. Nat. Milano 150(1): 113-148. [available online at: http://www.zooplantlab.btbs.unimib.it/attachments/211_070_Polygonum.pdf]
Hinds H.R. & Freeman C.C. (2005) Persicaria. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 5. Oxford University Press, New York-Oxford: 574-594.
Karlsson T. (2000) Polygonaceae. In: Jonsell B. (ed.), Flora Nordica, vol. 1. The Bergius Foundation, Stockholm: 235-318.
Kim S.-T. & Donoghue M.J. (2008)Molecular phylogeny of Persicaria (Persicarieae, Polygonaceae). Systematic Botany 33(1): 77-86. [available online at: http://phylodiversity.net/donoghue/publications/MJD_papers/2008/170_Kim_SysBot08.pdf]
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.
Ronse Decraene L.-P. & Akeroyd J.R. (1988) Generic limits in Polygonum and related genera (Polygonaceae) on the basis of floral characters. Bot. J. Linn. Soc. 98: 321-371.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Wilson K.L. (1990) Some widespread species of Persicaria (Polygonaceae) and their allies. Kew Bull. 45(4): 621-636.