Abstract <p>The shift toward sustainable agriculture has increased interest in biostimulants and in novel strategies for their delivery to crops. Key challenges include improving the stability, field performance, and environmental compatibility of these agents. This study investigates a circular economy approach to develop biobased carriers from cellulose and pectin for the delivery of two plant growth-promoting bacteria, <i>Bacillus</i> sp. LR01 and <i>Rhizobium</i> sp. GR12, and a phytoextract obtained from leafy vegetable by-products. Both bacterial strains exhibited distinct colonization patterns in the rhizosphere of lettuce and tomato. When incorporated into two biobased carriers (BC1 and BC2), <i>Rhizobium</i> sp. GR12 did not survive desiccation and was therefore alternatively encapsulated in alginate beads showing a survival rate of 10⁸&#xa0;CFU/g of material for 28&#xa0;days of storage. The spore-forming <i>Bacillus</i> sp. LR01 maintained high viability in both formulations (10⁷ to 10⁹&#xa0;CFU/g) for 28&#xa0;days. The silica-enriched BC2 formulation was selected for biostimulant delivery in greenhouse trials for its improved structural integrity, while alginate was chosen as benchmark. Application of all three biostimulants as aqueous suspensions significantly enhanced lettuce growth increasing leaves biomass by 20% to 30%. Delivery via the selected carriers maintained bacterial viability and sustained rhizosphere colonization (10⁶ to 10⁸&#xa0;CFU/g of soil) but reduced or reversed plant growth-promotion effects. Conversely, phenol-related secondary metabolism responses were triggered only when bacteria were delivered through the carriers. Overall, biostimulant performance depended not only on successful delivery but appeared strongly related with the physiological compatibility of the carrier with the plant–soil system.</p> Key points <p>• <i>Waste-derived biobased materials were designed for biostimulants delivery.</i></p> <p>• <i>Biostimulants provided as aqueous suspensions enhanced lettuce growth.</i></p> <p>• <i>Biobased materials maintained microbial viability but reduced plant growth promotion.</i></p>