Polymeria longifolia grows concurrently with the cotton crop making management very difficult. Active shoot growth of P. longifolia can occur at any time of the year in uncultivated areas and is probably linked to soil water status. Polymeria longifolt'a competes strongly with cotton, and is particularly detectable at soil moisture levels found immediately prior to irrigation. Although competition for major nutrients was suspected, no evidence was found in this study. Anatomical studies suggested there was some evidence that P. longifolia interfered with cotton through the production of allelochemicals but this requires further investigation. Densities of over 100 stems per square metre that are commonly found in weed patches reduced the yield of cotton lint and seed below 50%