Composts and biochar individually or in combination have been used for decades for improving soil quality and health. To date, very few studies have focused on the quality of food produced using compost-biochar mixtures. In this study, the use of biochar to improve the fertilization effect of composts and the quality of greenhouse-grown parsley was investigated by adding biochar to composts made from a mixture of broiler chicken wastes and sugar bagasse, sawdust, urban trees, napier grass or cotton residues. The addition of biochar to the composts, even in small amounts, influenced the yield and the quality of parsley. On average, the highest yields were obtained with the bagasse- and sawdust-biochar substrates; plants grown on the former and latter substrates had moderate levels of minerals and phenolic compounds, respectively. The tree-biochar substrate led to increased levels of phenolic compounds in parsley compared to all the other organic substrates. The flavonoid apigenin-7-apiosylglucoside was detected in high levels and was responsible for the excellent antioxidant activities of plants grown in soils amended with tree residues and biochar. The cotton-biochar substrate resulted in moderate antioxidant activities, while the addition of the napier substrate to soils did not improve the quality of parsley, irrespective of the amount of biochar. The results demonstrate that joint urban tree-based compost and biochar amendments can support the biosynthesis of bioactive compounds in greenhouse-grown vegetables. Among the five organic substrates tested, the tree-biochar mixtures showed the most favorable soil conditions for parsley growth.