The research was carried out from April to August 2018 in the lowland willow-poplar forests on wettest alluviums of bigger lowland rivers on the forest areas of Dolný háj and Veľký les (SW Slovakia), which include European Union habitats. We have produced 30 phytosociological reléves of 124 taxa, containing 15 invasive alien plant species (Aster novi-belgii agg., Solidago gigantea, Asclepias syriaca, Negundo aceroides e.g.) and 8 interesting species for protection (Epipactis helleborine agg., E. voethii, Platanthera bifolia e.g.). Using the Czech expert system for automatic identification of vegetation units we found out evaluation of diagnostic, constant and dominant species of the plant communities (Fraxino pannonicae-Ulmetum glabrae and invaded plant communities Rudbeckio laciniatae-Solidaginetum canadensis, Sambucetum ebuli, Chelidonio majoris-Robinietum pseudoacaciae e.g.). We report also higher occurrence of Aster novi-belgii agg. and Impatiens parviflora, but all invasive species need to be mapped and removed as they are located nearby and some of them directly in nature reserve or Natura 2000 habitats. Negative impact of invasive plants on the native floodplain forest ecosystems is generally known, especially in protected areas.