Abstract: The allelopathic effects of Bidens pilosa aqueous extracts in different months within ayear on seed germination and seedling growth of radish,rice and cucumber weretested.The studies indicated that allelopathic effects of Bidens pilosa aqueous extractsin different months on seedling growth of the three receptor plants,were variable. such change exhibited significant negative correlation with the monthly rainfall beforethe harvest of plant materials.When rainfall of the previous month was high,theinhibitory effects was weak.When the rainfall was low,the inhibitory effect was strong.