Erythranthe guttata is a riparian weed common to Europe and Australasia and is widespread in its native USA. The plant readily colonises damp areas such as stream banks, waterways, hillside seeps and road gutters. It forms a dense mat of stoloniferous growth that rapidly crowds out neighbouring plants. E. guttata propagates easily through clonal tissue fragments during mechanical clearance or flood events. Understanding the factors influencing the distribution and performance of E. guttata is of value for managing its spread. In this thesis my focus was on a fine scale assessment of the factors limiting distribution and spread of E. guttata. To achieve this, I carried out two field surveys to assess the influence of key biotic and abiotic factors on the occurrence and abundance of E. guttata on Banks Peninsula, New Zealand. I based the choice of factors on a Scottish study by Truscott, et al. (2008a). In addition to the field surveys and based on their findings, I carried out four greenhouse experiments to test for i) the influence of shading and interspecific competition and their interaction, on E. guttata performance measures and ii) whether E. guttata populations in the region show evidence of a persistent soil seed bank. Both field surveys showed that several biotic and abiotic factors, including shading, local biota and habitat type, are significantly associated with E. guttata presence and abundance. Critically, the findings of the surveys showed that while some factors such as shading intensity affected distribution similarly in Scotland and New Zealand, others had no effect in New Zealand; substrate type and availability of sediment patches. Knowing this is of value tin the prediction of the potential spread of E. guttata across Banks Peninsula and more widely in New Zealand and provides a unique New Zealand context to the ecology of this widespread weed. The greenhouse experiments into shade and competition effects showed that while shade has a strong negative influence on E. guttata performance, contrary to my expectations, no evidence was found for a significant interactive relationship between shading intensity and interspecific competition. The greenhouse experiments investigating soil seed banks found evidence for a persistent, overwintering E. guttata seed bank in Banks Peninsula populations. The results of my study show that some of the same factors influencing E. guttata distribution in Scotland may be of value in predicting its spread in New Zealand, however further work is needed to understand all key factors important in New Zealand. Overall, the results of my study have improved our understanding of the ecology of this invasive weed in New Zealand and in addition will underpin future studies into the reproductive ecology and invasion biology of E. guttata across New Zealand.