jats:pEvolutionary processes which increase the probability of an introduced plant species becoming invasive include high levels of genetic diversity and phenotypic plasticity. Naturalised in New Zealand, monkeyflower, (jats:italicErythranthre gutatta </jats:italic>), a clonally spreading herb of waterways and seepage areas native to the Western USA, shows marked variation in a range of vegetative, reproductive and inflorescence traits. We used two common gardens differing in elevation to explore the relative contribution of genetic versus plastic variation within nine traits among 34 monkeyflower clones from across the New Zealand South Island. We looked for evidence of clinal variation across elevation gradients and for home site advantage. We found both high genetic diversity and trait plasticity explain the observed variation, although less evidence for adaptive plasticity. Most genetic variation was observed in the lowland garden (9m a.s.l.), where the overall trend was for above ground dry weight to be lower, and horizontal shoot length greater, than at the montane garden (560m a.s.l). We found no evidence of local adaptation to any of the measured environmental variables. However, we observed a pattern of higher biomass and higher plasticity at lower versus higher elevations and in clones originating from lower elevation sites.</jats:p>