The New Zealand flora comprises proportionately more alien species than anywhere else on Earth. Many of these species elicit a variety of phenotypes across heterogenous environments and along a latitudinal gradient. Understanding what features lead to populations expressing multiple phenotypes is a key question in invasion biology. One hypothesis is genetic differences, which may be due to local adaptation, genetic drift, multiple introductions or a combination of these. Alternatively, phenotypic plasticity, which itself has a genetic basis, enables morphological and physiological alterations in response to changing environmental conditions. In New Zealand, the semi-aquatic herb Erythranthe guttata, ‘monkey flower’, is already showing signs of becoming invasive and is widespread across the South Island, blocking waterways and ditches. In this study I use common garden experiments to test for evidence of genetic differentiation, phenotypic plasticity and latitudinal trends in 35 populations of E. guttata from seven regions across the North and South Islands of New Zealand. My results have indicated significant genetic differences among New Zealand E. guttata populations and an ability to be phenotypically plastic which together is indicative of invasive potential. Furthermore, they highlight weak evidence for latitudinal trends among New Zealand populations. By replicating the common garden experiment for a second year I showed that maternal influences effect phenotype in E. guttata and that by reducing these influences I was able to provide stronger evidence for genetic diversity and clinal variation.