Knotweeds, Reynoutria japonica (RJ) and R. sachalinensis (RS) are invasive species that strongly interfere with the soil environment and disrupt the biogeochemical cycles of many chemical elements. This paper analyses the content of C, N, P, K, Na, Mg, Ca, Al, Fe, Mn, Zn, Ni, Cu, Cd, Cr and Pb in the above-ground biomass of RJ and RS and in the soil (0-15 cm) in order to assess the accumulation properties of knotweed. Studies conducted in northern Poland showed statistically significant (p<0.05) differences in the content of Na in the soil of the studied knotweed. The elemental composition of the leaves and stems showed a good supply of macronutrients and increased concentrations of some trace elements. The leaves of RJ and RS were shown to be good bioaccumulators of N, K, Na, Mg, Ca, P, Mn, Zn, Ni, Cu and Cd, and the stems of N, K, Na, Ca, Ni, Cd, Cr and Pb. Based on the values of bioconcentration factors (BCF), the similarity between the studied knotweeds concerning Mn, Cu and Cr in the leaf/soil relation and Al, Fe, Cu, Cd, Cr and Pb in the stem/soil relation was demonstrated. The highest mobility from stems to leaves expressed by the translocation factor (TF) was exhibited by Mn and Mg, and the lowest by Cr. Despite the low content in the soil, RJ and RS leaves and stems accumulated significant amounts of trace elements, which indicates their phytoextractive properties.