Phytotoxic Effects of Aromatic Plants’ Hydrodistillation Water Residues on Germination and Growth Characteristics of Avena sterilis, Echinochloa crus-galli, and Zea mays

The demand for sustainable weed management and the limited discovery of new herbicide molecules have led to high interest in plant-derived bioherbicides, such as the water residues (WRs) from the hydrodistillation of aromatic plants, which contain biologically active secondary metabolites. Here, the phytotoxic potential of WRs of four aromatic plant species was investigated. Chemical composition of WRs was determined by SPME–GC–MS, and their effect was assessed on seed germination and seedling growth characteristics of Avena sterilis, Echinochloa crus-galli, and Zea mays. Five concentrations, i.e., 0, 10, 20, 50, and 100, with 100 representing pure WR were tested. Phenolic monoterpenes dominate WRs in oregano and thyme, and oxygenated monoterpenes in laurel and lavender. Germination and growth responses were dose-dependent and species-specific. Oregano and lavender WRs exhibited the strongest phytotoxicity, reducing weed germination by 82% and 79%, respectively. In contrast, laurel extracts showed weaker germination inhibition. Across all tested species, germination delays were observed, making WRs a promising candidate for weed control. The results also showed that WR affected root growth by up to 95% shoot by 70–80%. Maize exhibited greater tolerance than the weed species maintaining higher germination. Overall, WRs represent a promising tool for integrated weed management.