When using food and green waste composts as peat-free plant growing media, there is a challenge that nutrient immobilisation and high pH and salts content limit plant growth. The present study explored the use of spent mushroom compost (SMC) of Agaricus subrufescens in a sustainable plant growing system where only vermicompost from digested food waste and composted green wastes were used, even for the seedling stage. However, negative effects of high compost inclusion were offset by adding SMC. Significantly higher plant yield was obtained in several of the SMC amended treatments in four out of five lettuce experiments and in one tomato experiment. In addition, an experiment with cucumbers showed that nutrients were not available to the plant when the mushroom mycelium was actively growing, but became available if the mushroom mycelium had been inactivated first by pasteurisation. A significant effect from SMC was not observed under full fertigation. This study demonstrated that the addition of pasteurised Agaricus mycelium colonised compost can successfully offset negative effects from high pH and EC as well as limited nutrient supply (and nitrogen immobilisation) in peat-free, compost-based growing media.