Plant washing agents water-extracted from Coriaria nepalensis (CN), Clematis brevicaudata (CB), Pistacia weinmannifolia (PW) and Ricinus communis (RC)-are feasible and eco-friendly for soil heavy metal removal, but their single application has limited removal efficiency. To improve their metal removal efficiencies, two biodegradable assistant agents, hydrolytic polymaleic anhydride (HPMA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), were investigated in combination with plant washing agents through batch soil washing experiments. Results showed that the addition of HPMA or PBTCA with plant agents greatly enhanced the removal efficiencies of soil heavy metals (p < 0.05). Under acidic conditions, the maximum improvements in soil heavy metal removal reached 18.69% and 18.00% for soil Cd and Zn by PW + HPMA, respectively, and 12.89% for soil Pb by CN + HPMA. Under neutral or alkaline conditions, the largest improvements in soil Cd, Pb and Zn were 24.18%, 54.38% and 25.47% by PW+PBTCA, respectively. When compared with EDTA, the loss rates of soil nitrogen, phosphorus and potassium significantly decreased (p < 0.05) and the soil organic carbon significantly increased (p < 0.05) after washing with the combinations. Hence, the addition of HPMA or PBTCA with the plant agents could improve the removal of soil heavy metals. (c) 2017 Published by Elsevier B.V.