Main conclusion <p>Plant development has a greater impact on the chemical composition of inflorescences than of the leaves and stems of <i>Monarda didyma</i> plants.</p> Abstract <p><i>Monarda didyma</i> L. is a well-known ornamental and aromatic plant. However, no studies are yet available concerning the diversity in the chemical composition of <i>M. didyma</i> organs during plant development that affect the final quality of the raw material. This study aimed to determine the content of phenolic compounds, essential oil (EO), and their composition in various organs affected by plant development. Furthermore, the antimicrobial activity of leaf and inflorescence EOs and the antioxidant activity of leaves, stems, and inflorescences were determined. EO content in the leaves and inflorescences collected during flowering was 2.39% and 3.64%, respectively. The main ingredients of the investigated EOs were p-cymene (17.11–35.56%) and carvacrol (5.39–28.19%). The EOs from the leaves and inflorescences collected during flowering inhibited the growth of six investigated microbial strains at the concentration range 0.313–1.250 µL × mL⁻¹. Total phenolic and flavonoid content in leaves was higher than in inflorescences and stems. The leaves were found to be rich in didymin (average content 1283.63&#xa0;mg × 100&#xa0;g⁻¹), while the inflorescences collected during flowering contained a significant amount of linarin (1247.47&#xa0;mg × 100&#xa0;g⁻¹). The variability in the content and composition of EO and phenolic compounds depending on the stage of plant development was mainly observed in inflorescences. These findings contribute to better understanding of distribution and accumulation of biologically active compounds in <i>M. didyma</i> aerial organs, the potential of these organs as raw materials for medicinal, cosmetic and food applications, and the importance of the time of their harvesting.</p> Graphical abstract <p></p>