This study investigates biodegradable plastic composites based on poly(butylene adipate-co-terephthalate) (PBAT) blended with thermoplastic starch (TPS) derived from tapioca starch for biodegradable seedling bag applications. TPS was incorporated at 10, 20, 30, and 40 wt%, and the composites were evaluated for mechanical, thermal, morphological, and biodegradation properties. Increasing TPS content resulted in a marked reduction in tensile strength and ductility, with tensile strength decreasing from 14.5 MPa for neat PBAT to approximately 4.2 MPa at 40 wt% TPS. At the same time, elongation at break declined from 670% to around 410%. Morphological analysis revealed increased surface roughness, porosity, and phase separation at higher TPS loadings. Thermal analysis using DSC and TGA showed a decrease in crystallinity and thermal stability, with the onset degradation temperature (Tonset) decreasing from 374 °C for neat PBAT to 348 °C at 40 wt% TPS. Accelerated weathering tests demonstrated enhanced degradation behavior for TPS-rich composites (30–40 wt%), characterized by surface cracking, discoloration, and void formation. Notably, PBAT/TPS composites containing 20–30 wt% TPS exhibited an optimal balance between mechanical integrity and biodegradability, making them suitable for biodegradable seedling bags in agricultural environments.