Soilless Cultivation of <i>Portulaca oleracea</i> Using Medicinal and Aromatic Plant Residues for Partial Peat Replacement

The industrial manufacturing of essential oils (EOs) generates a sizable volume of bulk solid waste (SW) that needs to be disposed of. The present study evaluated the potential of using <i>Origanum dubium</i> wastes (ODW) and <i>Sideritis cypria</i> waste (SCW) obtained after EO distillation for partial peat substitution (0–5–10–20–40% <i>v</i>/<i>v</i>) in <i>Portulaca oleracea</i> production. Both ODW and SCW increased pH, electrical conductivity, organic matter, and mineral content, but negatively affected the total porosity and aeration of the growing media. Plant growth was inhibited, especially when high ratios of residues were used, and this was reflected by leaf stomatal conductance and chlorophyll decrease, as well as by the activation of several nonenzymatic (phenols, flavonoids, and antioxidant capacity) and enzymatic (catalase, superoxide dismutase, and peroxidase) mechanisms and the increase in lipid peroxidation and hydrogen peroxide, indicating stress conditions. Despite that both ODW and SCW were rich in minerals, plants could not accumulate them. It can be concluded that both ODW and SCW have the potential to be used in the growing media at low ratios up to 10%, with increased antioxidant content in the final product. Nonetheless, the growing media properties, i.e., total pore space and aeration, still need to be improved to result in sufficient yields.