NOVELTY - Natural herbicide comprises bioactive compound isolated from the roots of Tagetes minuta.    USE - The natural herbicide is useful against Parthenium hysterophorus (claimed).    ADVANTAGE - The roots of Tagetes minuta collected at flowering stage exhibit more toxicity against the Parthenium hysterophorus in comparison to the roots of vegetative stage, where the toxicity of root extract of Tagetes minuta increases with increase in exposure time and results into wilting, chlorosis, necrosis and blackening of Parthenium shoots. 100% Concentration of root extracts of flowering stage of Tagetes minuta exhibit more toxicity against Parthenium seedlings in comparison to 50% concentration of aqueous root extracts. The root extract comprises active compounds which exhibit synergistic effect against Parthenium hysterophorus. The herbicide is naturally derived, and hence it is: a safe and effective alternative to the chemical herbicides; and cheap, easy to use, does not require repeated applications and does not produce pollution. The root extracts of Tagetes minuta significantly inhibit seed germination, seedling length, vigor index, biomass and biochemical components of Parthenium weed. The ability of the aqueous root extract of Tagetes minuta at flowering stage (obtained by: washing the roots of Tagetes minuta gently with tap water for 10 minutes and surface sterilizing with 70% ethyl alcohol for 30 seconds; then immersing in 1% sodium hypochlorite solution containing two drops of Tween-20 (RTM: Polysorbate 20) for 15 minutes and finally rinsing thrice with sterilized distilled water and drying with clean absorbent paper; cutting the sterilized roots into small pieces with sterilized knife and air-drying under shade for 15 days; homogenizing to fine powder in a grinder; macerating 10 g air-dried root powder of Tagetes minuta with 100 ml of sterilized distilled water in a blender for 10 minutes and then transferring into a conical flask and boiling on slow heat for 2 hours; filtering through double layered muslin cloth and centrifuging at 5000 g for 10 minutes; filtering the supernatant through Whatman No. 1 (RTM: Filter paper); collected the supernatant and repeating this procedure twice; polling the collected supernatant at an interval of every 2 hours and concentrating to make the final volume one-fourth of the original volume; and autoclaving at 121 degrees C and at 15 lbs pressure) to inhibit vigor index of Parthenium hysterophorus was tested. The results showed that 100% concentration of the aqueous root extract of Tagetes minuta at flowering stage exhibited 84.95% vigor index inhibition, relative to control.