The dominant terpenes in the essential oils of the leaves and reproductive structures of Tagetes minuta L. (Asteraceae) were studied throughout its life cycle in a natural population. The anatomy of the secretory cavities was described in order to correlate the changes in terpene content with structural changes. Finally, the phytotoxic effect of ocimenones on germination was also evaluated. Monoterpenes increased in both green leaves and reproductive structures throughout the plant's life cycle, whereas the opposite occurred in senescent leaves. Spathulenol was the main component in senescing leaves. The highest content of ocimenones, the bioactive components of the essential oil, was found in the reproductive structures. Bioassays showed that both pure ocimenones and fruit material from T. minuta delayed and inhibited the germination of cohabitant species. A relationship between allelopathy, biosynthesis, catabolism and terpene release is proposed for the chemical ecological effect of T. minuta.