Metal contamination of soil is a major environmental issue, as it can cause contamination of food and various health problems for humans. Metal-contaminated soils require specific management approaches, such as chemical remediation with various inorganic materials (conditioners) that reduce the mobility and bioavailability of metals. The study was performed to investigate the impact of lower (1% m/m) and higher (4% m/m) doses of flyash from a biomass cogeneration plant facility in contaminated soils on: i) the chemistry of metals as well as ii) vegetation parameters and iii) mineral composition of the test crop (Cannabis sativa, L.) under controlled conditions. The assumption was that the addition of flyash to Cd-contaminated soil will significantly change the chemical mechanisms in the soil and thus the growth and development conditions of the test crop due to a strong alkaline reaction. The application of a lower bioash dose had no significant influence on certain vegetation parameters (stem height, number of leaves, fresh leaf mass, stem mass), while the application of a higher dose of bioash had a negative influence on the mentioned parameters compared to the control variant. The application of fly ash in the roots of the test crop reduced the accumulation of Cd by 54%, Cu by 28% and Zn by 52%, in the leaf the concentration of Cd by 64%, Pb by 19% and Zn by 29% and in the stem the concentration of Cd by 73% and Zn by 45% compared to the control. Application of a higher bioash dose in the soil increased the pH to 7.44 (from 6.5 in the control) and the electrical conductivity to 17.3 dS/m (from 9.51 dS/m in the control). The heavy metal concentration in the tested soils did not change significantly with the application of bioash. The application of bioash has great potential in the management of soils contaminated with heavy metals, thus longer-term studies are needed to determine the influence of bioash on the test crop and other environmental factors.