Does Polymeria take-all? Competition for water and nutrients between 'Polymeria longifolia' (Polymeria take-all) and cotton

'Polymeria longifolia' (Polymeria take-all) is a native Australian plant that is a particularly troublesome weed throughout the Australian cotton industry. This rhizomatous perennial species is extremely difficult to manage using the range of existing control options. Yield reductions of cotton of up to 100% have been observed in dense patches of this weed. Competition between 'P. longifolia' and cotton was assessed with the aim of elucidating if competition for water and soil nutrients was likely to account for cotton growth and yield reductions. Soil water and nutrient levels were examined in a commercial cotton field in the centre, at the edge, and outside patches of the weed. 'Polymeria longifolia' appears to compete for soil water. This is most readily detected at levels approaching the refill point for cotton (the point at which fields should be irrigated to allow for optimal cotton growth). The weed may compete for nutrients at depth, although the soil cores sampled did not detect differences in the major soil nutrients in the upper soil profile. Several minor nutrients were affected. Allelochemical interference may have also caused yield reductions of cotton although this was not specifically examined in this study.