Physicochemical and herbicidal properties of emulsions of essential oils against Avena fatua L. and Chenopodium album L.

Essential oils have herbicidal effects, which can potentially be improved by the addition of adjuvants. The present study evaluated (1) the stability and other physicochemical properties of peppermint and caraway oils in oil-in-water emulsions and mixtures with commercially available adjuvants and (2) the phytotoxic effects of the foliar application of peppermint and caraway oil emulsions and mixtures to Chenopodium album L. (common lamb’s quarters) and Avena fatua L. (wild oat). An investigation was conducted with two multifunctional adjuvants, two nonionic surfactants, and two mixture constituents, i.e., organic acetic acid (vinegar) and citric acid, to assess the physicochemical properties of emulsions and mixtures with essential oils. In addition, two series of greenhouse pot experiments were set up to evaluate the herbicidal effects of these emulsions and mixtures against two common weed species. None of the emulsions or mixtures under study exhibited satisfactory stability. The emulsions of both essential oils had a significant herbicidal effect against C. album. None of the adjuvants significantly improved the efficacy. On the other hand, the herbicidal effect of emulsions against A. fatua depended on the ambient temperature in the greenhouse with treatments more effective at higher temperatures. The multifunctional adjuvants AS 500 and Atpolan Bio enhanced the herbicidal effect of caraway oil against A. fatua over 2 weeks. Peppermint oil alone induced considerable short-term herbicidal effects. Some adjuvants improved the efficacy of peppermint oil shortly after application, but due to regrowth no significant effects on biomass were obtained at harvest. In summary, the addition of adjuvants can improve the herbicidal effects of caraway and peppermint oil; however, further research into these effects is warranted.