Reynoutria Houtt.
As currently circumscribed (Galasso & al. 2009) Reynoutria is a genus of 3 to 7 species, all native to Asia. Some species (especially Reynoutria japonica and R. sachalinensis) are often cultivated as ornamentals, as a soil binder and formerly also for fodder. Reynoutria is sometimes included in Fallopia but the latter genus is apparently restricted to climbing or sprawling, fibrous rooted annuals or perennials. This was recently confirmed by molecular studies (Galasso & al. 2009). However, the existence of an intergeneric hybrid (Fallopia baldschuanica x Reynoutria japonica; see below) confirms their close relationship.
The identification of the large, strongly rhizomatous perennials related to Reynoutria japonica is sometimes problematic. As a result of introgression specific boundaries have become obscure. Leaf characters are best measured on well-developed side branches (basal leaves from the main stem are often atypical).
All representatives of the genus Reynoutria in Belgium are undesirable invaders, especially R. japonica (and perhaps also the lesser known R. xbohemica).
1 Largest cauline leaves up to 18 cm long, often smaller, truncate at base and cuspidate at apex. Leaves virtually glabrous on lower surface (at most minutely scabrous on the veins) === 2
Cauline leaves usually at least 20 cm long, deeply cordate to almost truncate at base. Leaves minutely hairy on the veins on lower surface === 3
2 Plant usually less than 70 cm tall. Leaves rarely longer than 6 cm, often with crisped margins. Flowers reddish-brown === 2. Reynoutria compacta
Plant usually more than 70 cm tall. Leaves usually longer than 6 cm with flat margins. Flowers greenish white === 3. R. japonica
3 Hairs on the veins on lower leaf surface long and slender, up to 0,6 mm long. Leaf base deeply cordate. Leaves usually ca. 25-45 cm long === 4. R. sachalinensis
Hairs on the veins on lower leaf surface minute, of variable length, usually conical at base. Leaf base shallowly cordate to almost truncate. Leaves ca. 20-30 cm long === 1. R. xbohemica
Literature:
Anjen L. & Park C.W. (2003b) Reynoutria. In: Wu Z.Y. & al. (eds.), Flora of China, vol. 5. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis: 319.
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.
Kim S.T., Kim M.H. & Park C.W. (2000) A systematic study on Fallopia section Fallopia (Polygonaceae). Korean J. Pl. Taxon. 30: 35-54.
Lamberti-Raverot B., Piola F., Thiébaut M., Guillard L., Vallier F. & Puijalon S. (2017) Water dispersal of the invasive complex Fallopia: The role of achene morphology. Flora 234: 150-157. [available online at: http://www.sciencedirect.com/science/article/pii/S0367253017332693]
Tiébré M.-S., Vanderhoeven S., Saad L. & Mahy G. (2007) Hybridization and sexual reproduction in the invasive alien Fallopia (Polygonaceae) complex in Belgium. Annals of Botany 99: 193-203. [available online at: http://bictel-fusagx.ulg.ac.be/ETD-db/collection/submitted/FUSAGxetd-112...