Mexican árnica (Heterotheca inuloides Cass.) is a widely used anti-inflammatory medicinal plant in Mexican folk medicine. Although it has been suggested that plant age, fertilization, and harvesting regime influence the concentration of secondary compounds affecting the therapeutic activity of the plant, the effect of these variables on the concentration of the Mexican árnica anti-inflammatory compounds was not known. We quantified anti-inflammatory sesquiterpenes (caryolan-1, 9β-diol, cadalen-15-oic acid, 7-hydroxycadalene, 4-hydroxy-2-isopropyl-4, 7-dimethyl-1[4H] naftalinone, 7-hydroxy-4αH-3, 4-dihydrocadalene, β-caryophyllene, and β-caryophyllene epoxide) in Mexican árnica plants subjected to fertilization and successive harvests of flowering stems, conditions that mimic the cultivation and harvesting for árnica in México. Fertilization and successive harvesting and their interaction had no significant effect on the concentration of anti-inflammatory compounds. However, the concentrations of these compounds were 60% higher in flowering stems from 15-month-old plants than in those from 4- or 8-month-old plants and was independent of the number of harvests and fertilization regime applied.