In this work, the study of the allelopathic activity of terpenic lactones on chloroformic fractions of Lagascea mollis Cav (Asteraceae) is reported. To this effect it is determined the percentage of germinated seeds and the radicular length of rice seeds (Oryza sativa L.). The observed bioactivity is contrasted with the corresponding one in the raw ethanolic extract. The mechanism of the metabolite interaction is established taking into account the mitotic index of the radicular meristeme, the micromorphologic study of the root and the conductivity of the cellular efflux. Additionally, the weed or arvense characterization is done by morphologic description and determination of pharmacognostic indexes. This research demonstrates that chemical constituents of the chloroformic fractions of Lagascea mollis manifest significant effect on cellular division and radicular growth of Oryza sativa where morphological alterations are observed, evidencing the allelopathic activity of the weed on the growth of the gramineous species.