The production of secondary metabolites in Artemisia annua depends on complex biosynthetic pathways influenced by biotic and abiotic factors. While the genetic potential for artemisinin biosynthesis is fixed, its expression responds strongly to cultivation practices and pedoclimatic conditions. A critical yet often misunderstood determinant of the plant’s phytochemical profile is the temporal pattern of artemisinin accumulation. Published studies show high variability, and the flowering stage is frequently – but inaccurately – considered the optimal harvest point. Understanding these dynamics is essential both for optimizing agricultural production of A. annua for artemisinin extraction and for ensuring reproducibility in research using plant-derived extracts. To distinguish the effects of plant development from those of seasonality, an experiment was conducted in Madagascar in 2013 using eight planting dates spanning mid-April to mid-November. Harvests began three months after planting and continued every two weeks. Across all cycles, maximum artemisinin content ranged from 1.25 to 1.40%, independent of planting date. However, the rate of accumulation increased markedly in later plantings, and all peaks occurred between mid-November and mid February – well before visible flowering. Early plantings required over 200 days to reach their maximum, while late plantings needed only 110-150 days. These results highlight the strong influence of seasonal factors, particularly photoperiod, on artemisinin dynamics. Hypotheses explaining these patterns are discussed.