Species of the genus Reynoutria Hook. were originally intentionally introduced from East Asia and in the twentieth century, three of them were successfully naturalized in Europe: R. sachalinensis (F. Schmidt) Nakai, R. japonica Houtt. and their cultigenous hybrid R. × bohemica Chrtek & Chrtková, which arose in the secondary distributional range of the parent species. As the taxa of the genus Reynoutria bloom very late, in late September-October, the characteristics of their leaves – shape, and type of pubescence – are mainly used as diagnostic ones. Identification of the species is often difficult, so this paper aims to identify diagnostic characters of their leaves in the secondary distribution range. Spontaneous invasive populations in Luxembourg, the Czech Republic, and Russia have been investigated. Comparisons have been made with plants growing in the natural range in Japan. Morphological characters were studied using a Keyence VHX-1000E digital microscope and an LEO 1430 VP scanning microscope. R. sachalinensis was found to be well distinguished from other species by its larger leaves with a heart-shaped base and a non-retracted apex. On the underside of the leaf, there are long uniseriate filiform trichomes along the lateral veins or outside the veins and peltate glands with 4-, 6- or 8-cell heads. For R. × bohemica, the presence of unicellular conical trichomes on the midrib was found to be a diagnostic feature in plants growing in Middle Russia (but not always) and is not applicable to plants from Czech populations, from which this species was actually described! In the taxonomic description of R. japonica, it has been suggested that the maximum height of the shoots should be significantly reduced (from 3 m to 1 m), as in the natural range this taxon has prostrate shoots. Plants of the secondary range with tall shoots are likely to be really R. × bohemica hybrids. It has been suggested that in Eastern Europe R. japonica grows generally very rare and predominantly in culture, and the information about most plants from invasive populations described in the literature as R. japonica should in fact be attributed to the hybrid complex R. × bohemica.
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