A weed is a plant that grows and compete with other plants for food, light, moisture and space. Ageratum conyzoides L. is a common weed belonging to the Asteraceae family. They are annual aromatic weed. They are invasive of pastures, vacant lots and even entire forest areas. Phytochemicals are secondary metabolites that are known to influence various activities of plants. The qualitative phytochemical analysis of the methanol leaf extract of A. conyzoides revealed the presence of phenols, flavonoids, alkaloids, quinones, cardiac glycosides, terpenoids and tannins except saponins and anthraquinones, whereas the quantification revealed that the methanol extract contained flavonoid content (27.8±0.685 mg/g), phenol content (8.6±0.446 mg/g) and alkaloid content (2.06±0.53 mg/g). The fresh leaf extract of A. conyzoides was treated to the seeds of Zea mays to study the allelopathic effect of the weed. The germination rate shows a gradual decline with increase in the concentration of the weed extracts.