The generic limits within Polygonaceae have long been very controversial. Two classifications were generally proposed in the last decades: Haraldson (1978) and Ronse Decraene & Akeroyd (1988). However, recent molecular phylogenetic studies (Galasso & al. 2009, Sanchez & al. 2011) suggest a narrower generic concept. Pending additional research it is followed here.
1. Tepals 6, 3 inner and 3 outer, often very dissimilar === 2
1. Tepals 5, all similar === 4
2. Outer tepals large, hard and spiny at maturity, enclosing the inner tepals. Annual === Emex
2. Outer tepals small, not hard and spiny, smaller than the inner tepals. Perennial (or very rarely annual or biennial) === 3
3. Leaves all basal, palmately veined. Stamens 9. Achenes winged, distinctly exceeding perianth. Inner tepals and outer tepals more or less similar, inner ones not markedly accrescent === Rheum
3. Cauline leaves usually present (if all basal, then pinnately veined). Stamens 6. Achenes not winged, not exserted. Inner and outer sepals very dissimilar, inner ones markedly accrescent at maturity === Rumex
4. Achene markedly exceeding perianth (at least 2x as long). Annual === Fagopyrum
4. Achene protected by perianth or only slightly exceeding. Perennial or annual === 5
5. Perianth succulent in fruit, white. Perennial climber with tiny leaves usually not exceeding 10 mm === Muehlenbeckia
5. Perianth never succulent and white in fruit. If perennial climber, then leaves much larger === 6
6. Outer tepals distinctly winged or keeled at maturity. Stem often twining or very robust === 7
6. Outer tepals not winged or keeled. Stem never twining, rarely robust === 9
7. Stems always twining. Stigmas capitate or peltate. Achenes without or with a very short beak === Fallopia
7. Stems erect, often woody-like. Stigmas fimbriate. Achenes, if produced, with distinct beak === 8
8. Stem relatively slender, usually less than 10 mm across, bowing over. Leaves acuminate at apex (very prominently so in young leaves!) === x Reyllopia
8. Stem robust, usually more than 10 mm across. Leaves acute-cuspidate at apex === Reynoutria
9. Most flowers axillary (subtended by a leaf or leaf-like bract). Inflorescences few-flowered (usually < 6 flowers per inflorescence) === Polygonum
9. Inflorescences at least in part in dense, terminal spikes, many-flowered (usually > 8 flowers per inflorescence) === 10
10. Inflorescence panicle-like and diffuse. Rhizomatous perennials === 11
10. Inflorescence spike-like and usually dense. Annuals or perennials === 12
11. Leaf blade cuneate to rounded at base. Lower leaf surface either glabrous or pubescent throughout. Achene (if produced) subequal to perianth or slightly protruding, 3-8 mm long. Flowering from (early) summer onwards === Aconogonon
11. Leaf blade truncate to cordate at base. Lower leaf surface strigose in distal part and on veins and margins only. Achene (if produced) included in perianth, 1,5-3 mm long. Not flowering before autumn === Rubrivena
12. Basal leaf-rosettes present === Bistorta
12. Basal leaf-rosettes absent === Persicaria
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.
Ghazalah Y. (2009) Taxonomic studies of two major genera Polygonum L. (complex) and Rumex L. of Polygonaceae from Pakistan. Department of Plant Sciences Quaid-i-Azam University, Islamabad (Pakistan). PhD thesis. [available online at: http://prr.hec.gov.pk/Thesis/757S.pdf]
Haraldson K. (1978) Anatomy and taxonomy in Polygonaceae subfam. Polygonoideae Meisn. emend. Jaretzky. Symb. Bot. Upsal. 22: 1-93.
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.
Sanchez A., Schuster T.M., Burke J.M. & Kron K.A. (2011) Taxonomy of Polygonoideae (Polygonaceae): a new tribal classification. Taxon 60: 151-160.