Abstract: Feverfew (Tanacetum parthenium [L.] Schultz-Bip., Asteraceae) products have shown high variability in the market. The objective of this study was to determine whether environmental factors affect the composition of key phytochemicals in feverfew. Plants of feverfew were exposed to water stress in greenhouse and commercial field conditions. The highest yield of parthenolide (PRT) was found in plants that received reduced-water regimes. Phenolics concentration was higher in plants grown under adequate-water conditions. The effect of time of harvest on PRT concentration and phenolics content was also investigated. Increased PRT was found during afternoon hours whereas total phenolic compounds decreased during the photoperiod and increased at night. When plants were exposed to artificial light during night hours, the phenolics content remained low. Our results revealed that manipulating the environment to favour increased accumulation of PRT resulted in a decline of phenolics content in feverfew. These findings have implications on standardization of herbal products.