Two composts prepared from agroindustrial wastes were assayed as substrates: C-1 from brewing waste (yeast and malt) plus lemon tree prunings; and C-2 from the solid fraction of olive mill wastewater plus olive leaves. Sixteen substrates were prepared by combining each compost with Sphagnum peat or a commercial substrate (CS) in different proportions. The nutrients (N and K) provided by the composts, which acted as slow-release fertilisers, influenced especially the development of calendula, although the physical and physico-chemical properties such as total pore space and electrical conductivity (EC) were also relevant. On the other hand, in the salt-sensitive calceolaria hybrid, EC and chloride concentration were the main factors influencing growth. Adequate substrates for the development of calendula can be prepared by mixing C-1 at up to 75% with peat or at up to 50% with CS, and C-2 at up to 50% with peat or CS. For calceolaria, the substrate should have a lower proportion of compost, C-1 at up to 50% and C-2 at up to 25%, both mixed with peat or CS. Therefore, composts of agroindustrial origin such as these can be used as an alternative to peat and CSs for growing ornamental plants, provided the mixture contains at least 25% peat or CS.