The need for phytoremediation prompts the introduction of plants into the culture, which will not only have several directions of use of the obtained raw materials, but also contribute to the improvement of the soil and be a good precursor for the further successful cultivation of the following agricultural crops. It is known that essential oil crops activate the development of beneficial microbiota and prevent soil fatigue. The article presents the results of research on the allelopathic activity of plant parts of medicinal hyssop (Hyssopus officinalis L.) of the Marquis variety, taking into account age characteristics under conditions of climate change in Southern Ukraine. The aim of the work was to reveal the influence of allelopathic activity of aqueous extracts of leaves, stems and flowers of hyssop, soil in the rhizosphere zone and plant age on the growth of watercress roots. In the research process, empirical methods were used, in particular, an experiment. As a result of research carried out with the medicinal hyssop culture of the first, second and third years of vegetation on the basis of the experimental field of the Mykolaiv National Agrarian University during 2019-2021, the optimal concentration of water extracts of the culture with a stimulating effect was found. The dependence of the allelopathic activity of water-soluble biologically active substances of aerial organs of medicinal hyssop plants is shown, and the concentration of the solution, which causes an inhibitory effect, is revealed. Thus, the highest allelopathic activity was noted on the variant using hyssop flowers of medicinal plants of the second year of vegetation in a concentration of 1:10. Aqueous solutions of the soil layer of the root system where medicinal hyssop plants grew in the third year of vegetation have an inhibitory effect on the linear growth of watercress roots. It was the largest in the leaf + stem variant at a concentration of the aqueous solution of 1:10. This suggests that it is necessary to use the culture of medicinal hyssop in crop rotation carefully taking into account its ontogenesis features. The obtained results will have practical application in enterprises of various forms of ownership that will have the desire to grow medicinal hyssop and use it in the structure of crop rotation