The remediation of metal(loid) contaminated sites often requires the application of an amendment, such as biochar. Biochar is the product of biomass pyrolysis and has been shown to reduce soil acidity, immobilize metal(loid)s and thus ameliorate plant growth. The beneficial effects of biochar are usually higher at a higher dose. However, an excessive dose can have negative outcomes due to nutrient loss. The application of an amendment to the soil comes at a financial cost, especially if the contaminated area is large, and therefore the optimum dose must be determined. In addition, to overcome the cost of biochar application, the use of a plant with economic benefits, such as flax, can be a suitable option. In this context, a mesocosm study, on a former mining soil polluted by As and Pb was performed, using flax and biochar applied at increasing concentrations. Results showed an increase in soil pH and the immobilization of Pb with increasing biochar doses. These ameliorations led to better plant growth. A higher biochar dose increased plant growth up to a 3% application rate, from which point dry weight no longer increased.Metal(loid) concentrations decreased between 1 and 3% biochar, and then increased at higher doses. It can thus be concluded that with 3% biochar, soil properties are ameliorated, metal(loid)s are immobilized, and plant growth increases, and a higher biochar application rate does not bring about better outcomes. Therefore, a 3% biochar application dose seems to be the best application dose to improve soil conditions and plant growth at a lower cost.