In order to assess the magnitude of suppressing capabilities of Cassia occidentalis, Rumex dentatus, Calotropis procera and Withania somnifera on Parthenium hysterophorus a laboratory study was conducted to evaluate the synergistic effect of 3rd and 9th d aqueous shoot leachates of above mentioned weeds on biochemical activities (chlorophyll, nitrogen percentage, protein percentage) and mortality percentage of Parthenium hysterophorus. It is evident from the present results that treatment having 100%, 9th d aqueous shoot leachates of Cassia has the potential to reduce the level of biochemical activities and increase the mortality to a great extent, which was followed by Rumex dentatus. Their toxicity exhibited period and dose dependent pattern. The demonstrated bioactivity may be attributed to phenolic compounds present in Cassia. Cassia may offer an alternative tool for the control of Parthenium in India.