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Co-application of sewage sludge (SS) with biochar in landscape/forestry soil is a common strategy for enhancing soil fertility and reducing the bioavailability of potential toxic elements (PTEs) derived from SS, such as Cd, Pb, Cu, Zn, and Ni. However, due to variability of biochar quality and uncertainties in responses of different plant species, whether the co-application benefits the landscape/forestry plant system remains elusive. Here, we tested the effectiveness of three types of biochar (SS-derived biochar (SB), rice straw-derived biochar (RB), and litter-derived biochar (LB)), which were added to soil amended with SS at 50% (w/w) at rates of 1.5%, 3%, and 4.5% as growth media for the landscape plant Aglaonema modestum (A. modestum). We analyzed the substrate's physicochemical properties and assessed the alleviation of phytotoxicity by biochar application. A significant increase in the fertility index of substrate was observed in all the treatments with biochar addition. The addition of biochar reduced the potential mobility of PTEs while increasing their residual fraction in media. Nonetheless, it has been found that the addition of biochar has ineffective or even negative effects on A. modestum growth (height, biomass, root length) and nutrient absorption. Importantly, the reduction in root biomass and the increased activity of root antioxidant enzymes (SOD, POD, CAT, and MDA) indicate contamination stress of biochar on the roots of A. modestum. Toxic elements of concern—namely Cu, Cd, and Pb—were not significantly higher in tissues of A. modestum saplings planted in biochar-SS-amended soil. However, elevated levels of other elements that may pose toxicity concerns, such as Ni and Zn, increased in tissues at high biochar dosages. Based on the Entropy–Weight TOPSIS method, it was further confirmed that compared to the treatment without biochar, all treatments except for 3.0% LB application resulted in poorer A. modestum comprehensive growth. Our results emphasize the need for detailed research on the response of specific plants to biochar in specific environments, including plant adaptability and the unexplored toxicity of biochar, to understand the large variations and mechanisms behind these ineffective or negative effects before the large-scale co-utilization of SS and biochar in landscape/forestry soils. [ABSTRACT FROM AUTHOR]