This research aimed to determine the phytotoxicity of methanolic extract from 60-day-old Cosmos sulphureus leaves (SNV60) combined with suitable adjuvants to inhibit the growth of Echinochloa crus-galli, Leptochloa chinensis, and Fimbristylis miliacace. Results of the study conducted under greenhouse conditions showed that at 14 days after seedling emergence, methanolic extract from SNV60 leaves at concentrations of 0.48g/ml and 0.96g/ml had equally inhibitory effects on the shoot growth of Echinochloa crus-galli (23.57%; 26.37%), Leptochloa chinensis (50%; 50%), and Fimbristylis miliacace (44.8%; 48.53%). The mixture of SNV60 extract at a concentration of 0.48g/ml with Esterified Vegetable Oil adjuvant (0.4L/ha) significantly increased the inhibition of shoot growth of the three weed species mentioned above. The inhibition rates at 14 days after seedling emergence on Echinochloa crus-galli, Leptochloa chinensis, and Fimbristylis miliacace were 58.3%, 50.9%, and 46.5%, respectively, compared to 20%, 32.2%, and 26.8% with the extract only. The study suggests that the combination of SNV60 extract and Esterified Vegetable Oil adjuvant has the potential for further investigation as a combined approach for weed management in organic farming for sustainable agriculture.