Lythrum salicaria L. is a generalist species that can grow under a wide range of habitat conditions, both in its native distribution (Eurasia) and in North America, where it is a successful invader of temperate wetlands. Comparison of the European and U.S. habitats was a preliminary part of a larger project studying the life history traits of native and invasive populations of L. salicaria, when growing under different environmental conditions. Multivariate statistical analyses, using relativized Bray-Curtis ordinations, found that the sites separate along environmental gradients. This allowed for the establishment of experiments under natural settings; geographic effects were studied for populations growing under similar habitat conditions but in different regions of the species distribution in North America. The U.S. and European populations were measured during different growing periods. In order for comparisons to be meaningful, they must be done when climatic conditions were similar. Statistical analyses, using non-randomized nested designs for daily high and low temperatures and bimonthly precipitation amounts, showed that the European sites had similar temperature regimes as those in the U.S. Also, a more rainy growing season followed a drier one on both continents.