[en] Peculiarities of radionuclide concentration in the grass floor of forest biogeocenoses contaminated due to the Chernobyl accident are of consideration. Basic regularities of seasonal and long-term dynamics of ¹³⁷Cs concentration in the dominant herbaceous species in forest ecosystems of Ukrainian Polesie are shown. So-called species-concentrators and discriminators of ¹³⁷Cs are revealed. New approaches are suggested for radioecological ranging classification of herbaceous plants based on their weighted-average concentrations for entire vegetation period