Comparative studies of native and non-native populations of invasive species can help to determine the relative importance of environment vs genetic factors in affecting the growth and establishment of a particular invasive plant species. We present results from the first year of a three year study of Eurasian and North American populations of Lythrum salicaria L. (purple loosestrife). Using plants from four Eurasian (Finland, Czech Republic, Spain, Turkey) and four North American (Alberta, Canada; Wisconsin, Illinois, Tennessee, USA) populations, common gardens were established in spring 2006 in Louisiana, USA and Třeboń, Czech Republic. In each country, one-third of the plants were harvested when they reached the same phenological stage. Among-population differences along the latitudinal gradients (by country location) were tested using regression analyses and ANOVAs, after accounting for covariance factors (e.g., time grown) and data transformations to achieve homogeneity of variance. Within each common garden, plant growth traits (e.g., dry weight, stem height, etc) differed significantly among the populations. Traits for both the Eurasian and North American populations changed non-linearly across the latitudinal gradient in both common gardens: negative exponential equations best explained trait changes for the Eurasian plants in both gardens; for the North American populations, negative exponential in the Czech garden, but cubic relations in the Louisiana garden. Within-trait differences were greater between European populations originating from the most northern and southern locations than for the similar invasive North American ones, indicating lower genetic variability in the invasive plants, as expected. Genetic variation among the native and non-native populations will be further tested using DNA fingerprinting methods, such as AFLPs. These common garden and genetic studies will give insight into whether growth traits may be related to genetic differences for a species that is native to Eurasia and invasive in North America.