<p>Considering the importance of harvesting time in determining the quantitative and qualitative characteristics of phytochemicals, as well as the extensive applications of <i>Lippia citriodora</i> essential oil across various industries, this study examined monthly changes in the phytochemical concentration of this plant during its growing period. Since lemon verbena leaves in Iran can be harvested in spring, summer, and autumn, this research investigated changes in the active substances in the leaves over eight months of growth (April, May, June, July, August, September, October and November). <i>L. citriodora</i> plants showed a significant fluctuation in active compounds during different months, so that the maximum and minimum essential oil contents (1.5% and 0.48%, respectively) were observed in May and November. The highest amount of geranial (29.0%) and neral (20.8%) were detected in July, whereas the lowest contents (8.1% and 5.1%, respectively) were measured in May. The highest amount of geranial (29.0%) and neral (20.8%) were detected in July, and the lowest content (8.1% and 5.1%) was measured in May month, respectively. Antioxidant activity was also variable, with maximum levels (89.0% and 88.0%) found in July and May, and the lowest activity (32.3%) recorded in September. Based on the different harvesting times of <i>Lippia citriodora</i> leaves in Iran and the findings of this research, it can be concluded that the phytochemical content is highest in May and July. Therefore, leaves harvested during these months are recommended as optimal for their higher concentrations of bioactive compounds. These results could enhance the efficiency of using lemon verbena leaves in pharmaceutical, cosmetic, and food industries for processing and production of various products from it.</p>