<p>Prolonged and inappropriate storage conditions can lead to a gradual decline in seed viability, significantly hindering their ability to germinate. The present study investigated the biostimulant effect of <i>Picochlorum</i> sp. AZL19 cultivated in natural casings waste on the germination and growth of the seeds of four oregano species that had experienced dormancy due to prolonged storage. The natural casings waste was incorporated into the microalgae culture medium as a nitrogen source (0.3–3 g L⁻¹). The capacity of <i>Picochlorum</i> sp. AZL19 to utilize the waste substrate and remove nitrogen was assessed. The resulting biomass was evaluated at three concentrations (0.03, 0.05, and 0.1% w/v) as a biostimulant for promoting the germination and growth of dormant seeds of <i>Origanum elongatum</i>, <i>Origanum compactum</i>, <i>Origanum vulgare</i>, and <i>Origanum majorana</i>. <i>Picochlorum</i> sp. AZL19 efficiently assimilated the waste substrate, demonstrating significant growth and biomass production. The highest microalgal growth occurred at a waste concentration of 0.9 g L⁻¹. The nitrogen removal efficiency ranged from 81.3 to 5.78%. The biostimulant activity of <i>Picochlorum</i> sp. AZL19 was dose-dependent. The most pronounced biostimulant activity occurred using intact <i>Picochlorum</i> sp. AZL19 cells at a concentration of 0.05%, resulting in significant increases in germination percentage, seedling fresh weight, shoot and root length, vigor indices, and the root-to-shoot ratio in all four oregano species. The analysis of the metabolites in this treatment indicated the presence of carbohydrates (394.8 ± 0.07 mg L⁻¹), proteins (317.13 ± 0.15 mg L⁻¹), proline (21.03 ± 0.07 µmol L⁻¹) and indole-3-acetic acid (3.75 ± 0.08 µg L⁻¹). These results suggested that&#xa0;microalgal biomass produced on natural casings waste represents a potential source of plant biostimulants capable of breaking seed dormancy and enhancing germination and growth in medicinal and aromatic plants.</p>