Polygonum L.
Polygonum is here treated in a narrow sense following Haraldson (1978), Ronse Decraene & Akeroyd (1988) and Galasso & al. (2009). It formerly included many species that now are accommodated in Fallopia, Persicaria, etc… As here circumscribed, Polygonum counts about 75 species, two of which are native in Belgium: Polygonum aviculare L. and P. oxyspermum C.A. Mey. et Bunge ex Ledeb. subsp. raii (Bab.) D.A. Webb et Chater.
Several non-native species of Polygonum much resemble P. aviculare and might pass undetected. Their identification is seldom straightforward.
1. Uppermost leaves (bracts) much reduced, mostly shorter than the flowers. Stem often erect === 2
1. Uppermost leaves not markedly reduced, mostly longer than the flowers. Stem rarely erect === 4
2. Outer tepals petaloid (conspicuously broadened at apex), divergent with a broad pink (to white) margin. Stem procumbent to ascending. Achene roughened, dull === 2. Polygonum arenarium subsp. pulchellum
2. Outer tepals erect to incurved, not broadened at apex, with a narrow white (to pink) margin. Stem erect. Achene shiny or dull === 3
3. Achene smooth, shiny. Perianth ca. 1,8-2,4 mm long. Tepals usually with narrow pink margin at flowering === 3. P. argyrocoleon
3. Achene roughened by small papillae, rather dull. Perianth ca 3,5-4,3 mm long. Tepals usually with narrow white margin at flowering === 5. P. bellardii
4. Margin of tepals usually yellow or green. Tepals often +/- keeled. Plant light green or yellowish === 1. P. achoreum
4. Margin of tepals usually white or pink. Tepals not keeled. Plant green or more or less glaucous === 5
5. Achene 1,5-3,5 mm long, not or only slightly longer than perianth, rather dull. Tepals 1,5-2,5 mm long with narrow scarious margin === 4. P. aviculare s.l.
5. Achene 3,5-5,5 mm long, slightly to much longer than perianth, shiny. Tepals ca. 3 mm long with broad scarious margin (native) === P. oxyspermum subsp. raii
Additional aliens: Polygonum patulum Bieb. (SE-Eur., wool alien) and P. plebejum R. Brown (E-Afr., S-As., Aus., wool alien).
Literature:
Costea M., Tardiff F.J. & Hinds H.R. (2005) Polygonum. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 5. Oxford University Press, New York-Oxford: 547-571.
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.
Haraldson K. (1978) Anatomy and taxonomy in Polygonaceae subfam. Polygonoideae Meisn. emend. Jaretzky. Symb. Bot. Upsal. 22: 1-93.
Karlsson T. (2000) Polygonaceae. In: Jonsell B. (ed.), Flora Nordica, vol. 1. The Bergius Foundation, Stockholm: 235-318.
Raffaelli M. (1982) Contributi alla conoscenza del genere Polygonum L. 4. Le species italiene della sect. Polygonum. Webbia 35: 361-406.
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.
Webb D.A. & Chater A.O. (1964) Polygonum. In: Tutin T.G. & al. (eds.), Flora Europaea, volume 1. Cambridge University Press, Cambridge: 76-80.
Wolf S.J. & McNeill J. (1986) Synopsis and achene morphology of Polygonum section Polygonum (Polygonaceae) in Canada. Rhodora 88: 457-479.