Weeds seriously threaten the cultivation of medicinal plants. Herbicide application is necessary for the commercial production of these plants. Since, herbicides can affect the quality and the quantity of active ingredients of the plants, their applications are associated with specific sensitivity. Trachyspermum ammi (Ajwain), an annual medicinal herb from the apiaceae family is cultivated in some parts of Iran. So far, no studies have been done on the chemical management of ajwain weeds regarding the tolerance of ajwain herb to herbicides and their selective properties. The present study was carried out to evaluate the effect of herbicides of oxadiazon (Ronstar) and metribuzin (Sencor) in different phonological stages on ajwain plants. Experiment were performed in the open air field (Chahanari) of Isfahan University of Technology in 2017. The experiment was conducted as a factorial in a completely randomized design with three replications. The first factor was herbicide type including oxadiazon and metribuzin, the second factor was herbicide application time in three growth stages (six, eight and ten pair leaf), and the third factor was the concentration of herbicide containing three dosages (25% less than recommended dosage, recommended dosage and 25% higher than recommended dosage). The application of metribuzin resulted in severe damages to the plant, so that the plants eventually died. Relative content of leaf water, membrane stability index, and ascorbate content in 10 leaf stage were greater in oxadiazon treatment than metribuzin treatment. Ten days after oxadiazon treatment the highest phytotoxicity was observed in the six pair leaf stage with dosage of 25% higher than the recommended dosage (34.83%) and the lowest phytotoxicity was observed after 30 days in the ten pair leaf stage whit dosage of 25% less than recommended dosage (1.33%). In the case of oxadiazon treatment, in the 10 pair leaf stage the highest chlorophyll content (a, b, total), H 2 O 2 and the number of lateral branches was observed with the recommended dosage, the highest content of carotenoid was observed with the recommended dosage and 25% higher than recommended dosage and the highest number of umbellet in inflorescence , the number of flower in umbellet and 1000-grain weight was observed with the dosage of 25% lower than recommended dosage. In six pair leaf stage, the number of inflorescence in plant, dry weight and seed function was higher with the recommended dosage, the number of seed in inflorescence was higher with the dosage of 25% lower than recommended dosage and plant height and essential oil content were higher with the dosage of 25% higher than recommended dosage. Therefore, it seems that 10 pair leaf stage and the dosage of 25% lower than recommended dosage of oxadiazon is effective on ajwain crop herbs. Keywords: Medicinal plant, Ajwain, herbicide tolerance, growth stages, physiologic properties
