Water Residues from Rosemary Essential Oil Production: Transforming Waste into a Potential Bioherbicide

Transforming industrial by-products into new resources is a fundamental principle of sustainable agriculture and circular bioeconomy. Waste products from rosemary (Rosmarinus officinalis L.) essential oil extraction, such as exhausted biomass and water residues (WRs), are rich in bioactive compounds like phenols and terpenes. These by-products may represent a promising and economically viable option for agricultural management, particularly in weed control. This study evaluates the potential use of WR as a bioherbicide. In vitro experiments were conducted to assess the inhibitory effects of WR on the germination and seedling morphology (root and shoot development) of four detrimental weed species for temperate cropping systems: two monocotyledonous (Alopecurus myosuroides and Lolium multiflorum) and two dicotyledonous (Sinapis alba and Amaranthus retroflexus). WR was tested at four concentrations (0, 25, 50, and 100), corresponding to an increasing gradient of WR, with 100 representing pure WR. The results showed that WR did not significantly inhibit germination in A. myosuroides, L. multiflorum and S. alba, whereas A. retroflexus exhibited a dose-dependent inhibition, with germination reduced by 37.5%, 64.5%, and 91.6% at doses of 25, 50, and 100, respectively, compared with the control (dose 0). Furthermore, germination delays were observed across all tested species with promising application of WR for regulating weed–crop competitive interactions in the early crop growth stages. Results on the morphological traits of weed seedlings showed that WR application affected root more than shoot growth inhibition. In particular, WR demonstrated a pronounced root inhibitory effect in A. myosuroides, L. multiflorum, and A. retroflexus. In contrast, a dose-dependent increase in root length was observed for S. alba (21.41 mm at dose 0 and 25.77 mm, 30.97 mm and 35.96 mm, respectively, at doses 25, 50, and 100). The results of this study highlight the potential application of WR as a sustainable solution to be included in an integrated weed management (IWM) toolbox and underscore their role in promoting the valorization of waste from essential oil production.