Conyza bonariensis, flaxleaf fleabane, is a major weed threat on cultivated and non-cultivated lands, gardens, roadsides and waste places. The weed in South Africa is believed to have originated from South America, and the first herbarium sample is from a plant collected in May 1895 at Franschhoek. Adding to its problem status is the recent discovery that certain C. bonariensis biotypes in South Africa and other parts of the world are resistant to the herbicide glyphosate, and in certain cases to both glyphosate and paraquat. Despite its invasiveness and ability to compete severely with crops, the mechanisms of interference (= allelopathy + competition) employed by C. bonariensis are poorly understood and have not yet been thoroughly investigated. There is a need to expand on the knowledge of interference mechanisms of C. bonariensis in order to better understand its success as a weed, and to improve on knowledge for the successful management of this weed. In the present study, allelopathic potential of C. bonariensis was assessed, first by means of germination bioassays, followed by investigation employing hydroponics, leachate, and replacement series experiments. In a laboratory bioassay, the plant’s leaves and roots were extracted using two solvents, water and hexane, to which seeds of the test (acceptor) species lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum) were exposed in order to determine where the strongest allelopathic potential resides. Moreover, differential potency of crude extracts prepared with the two solvents (polar and non-polar) would at least provide some evidence on the nature of putative allelochemicals involved. Germination bioassays revealed that leaves harboured the strongest allelopathic potential (potency). Water extracts (infusions) caused greater growth inhibition of the test species than hexane extracts. Osmolalities of the water infusions were tested and found not to be inhibitory to germination and early seedling development of lettuce. Following on the germination bioassays, a hydroponic experiment was set up in a greenhouse in order to investigate whether C. bonariensis possesses and releases chemicals with allelopathic potential through its roots. Lettuce top and root growth was significantly reduced by all three populations of C. bonariensis (one from Pretoria; two from the Western Cape). No significant differences were observed in the degree of growth inhibition caused by the three weed populations on the growth of lettuce, except in the case of root dry mass results where the Hatfield population caused more damage (85% growth reduction). The leachate experiment was then performed to determine if leachate from C. bonariensis affected the growth of test species exposed to different leachate concentrations. Although there was no growth inhibition observed for both lettuce and tomato in this experiment, growth stimulation of tomato roots was observed at the highest leachate concentration (100%). Finally, in an attempt to simulate the allelopathic potential of C. bonariensis in a natural field situation, a replacement series experiment was conducted to determine the relative interference of Conyza bonariensis in relation to lettuce and tomato. Dry mass results showed that there was no growth inhibition of both crop species. RYT was > 1 at all weed: crop combinations, which implies that both crop species and C. bonariensis were less affected by interspecific interactions than in their respective monocultures. It is suggested that the results of this study can attributed to methodology and growth media. The results of this study represent the first step in showing that allelopathic potential C. bonariensis may contribute to the success of this weed as an invasive weed species and that this weed should not be allowed to attain significant biomass on crop field. Further research should include field trials that will yield a better understanding of the practical relevance of the allelopathic potential of C. bonariensis. Finally, crop producers and weed management practitioners should recognize that this important weed has the ability to interfere with the growth and development of a crop through two mechanisms, competition plus allelopathy. / Dissertation (MInst Agrar)--University of Pretoria, 2013. / gm2014 / Plant Production and Soil Science / unrestricted