Invasive alien species (IAS) present a major threat to biodiversity and ecosystem services. Managing and controlling them is of paramount importance. To guide conservation efforts, detailed maps of potential distributions of IAS, as for instance produced by species distribution models (SDMs), are very useful, but lacking in the dimension of probability of invasion success. An important barrier to the successful establishment of IAS is climate. The Niche Margin Index provides a measure of (dis)similarity between the native climatic niche of IAS and the environmental conditions in another site (which, for these purposes, would be a (potential) invasion site). This measure was shown to be positively correlated with invasion success in invasive vertebrates. For invasive plants, however, special caution is advised to pick the method to define the native climatic niche. This study examines the effect on NMI of using three different methods to define the native range: the full native range polygons, the geographical occurrences of the species inside the native range, and a thresholded SDM inside the native range. The last method occupies the middle ground between the first and second, which present the highest and lowest NMI, respectively (i.e. occurrences < SDMs < range polygons). This has implications for nature conservation and invasion science, as it points out new considerations that must be made before the probability of invasion success is assessed in plant species.