Globally, approximately 242 million tons of Spent Mushroom Substrate (SMS) were produced in 2022. This figure corresponds to the global cultivation of approximately 48 million tons of mushrooms, with SMS typically being generated at a rate of 5 kilograms per kilogram of mushrooms. The mushroom production industry is facing a growing challenge in managing the increasing volume of SMS. Finding environmentally and economically viable solutions for this organic waste is, therefore, of utmost importance. This research work was conducted to investigate the potential of SMS from oyster mushroom (Pleurotus ostreatus) production as a potting medium for the vegetative growth of garden balsam (Impatiens balsamina). Five treatments were applied: T1 (25% SMS + 75% GS), T2 (50% SMS + 50% GS), T3 (75% SMS + 25% GS), and T4 (100% SMS). A substrate of 100% garden soil was used as the control. The experiment was arranged in a randomized complete block design with three replications. Prior to the conduct of the study, a chemical analysis of the SMS was determined. The data collected included the following: plant height, number of leaves, leaf length, leaf width, stem diameter, shoot length, root length, shoot-to-root ratio, lateral shoots, and total biomass. The results showed that 100% garden soil (GS) exhibited significant results across all parameters, being mineral-rich and more stable in nutrient composition. Meanwhile, 25% SMS + 75% GS and 50% SMS + 50% GS consistently produced significantly higher or comparable growth parameters at 1 WAT, including plant height (7.37 cm and 7.35 cm), number of leaves (3.20 and 4.20), leaf length (3.05 cm and 3.29 cm), and stem diameter (1.35 mm and 1.44 mm), compared with the control. However, in later stages, 100% GS outperformed all treatments, indicating that pure garden soil remains the most effective substrate for sustained plant growth. The 100% pure SMS consistently showed the lowest values across all parameters, indicating its unsuitability as a sole growth medium due to nutrient imbalances and potential salinity issues. SMS when mixed with moderate levels of garden soil (25–50%) can enhance early seedling growth and promote sustainable agricultural waste use. Pre-treatment to reduce salinity and nutrient deficiencies is recommended for optimal use.