1. Widespread plant species generally have high intraspecific variation in functional traits, which is reflected in their great variety of phenotypes. This variety can result from both genetic differences due to local adaptation and phenotypic plasticity. With high intraspecific variation and nearly global distribution, the common reed Phragmites australis is a suitable model species for studying the underlying mechanisms of intraspecific trait variation. 2. In this study, 71 genotypes of P. australis from seven phylogeographic groups were transplanted into two replicate common gardens located in very different climates: northern Europe and mid-east Asia. We measured seven functional traits of all these genotypes over the growing season, including shoot height, maximum biomass per shoot, shoot density, node number per stem, leaf life span, flowering occurrence and flowering date. Our aim was to assess the relative effects of genetic (phylogeographic origin) and environmental (common garden) status, and interactions between them, on intraspecific variation in functional traits of P. australis. 3. We found common garden having the strongest influence on most functional traits studied. All traits except flowering occurrence varied significantly across gardens, revealing the important role of phenotypic plasticity on trait variation in P. australis. We also found significant differences in trait variation among the different phylogeographic groups of P. australis and, thus, evidence for genetically determined intraspecific variation in the morphological and life-history traits addressed in this study. All functional traits showed significant (p ≤ 0.0054), albeit minor to moderately explained (R 2 ≤ 0.57), latitudinal patterns in both gardens. Covariation of multiple traits was similar in the two gardens. Phenotypic plasticity was trait-specific, and the plasticity of shoot height and maximum biomass per shoot increased towards higher latitude of genotypic origin. Our results indicate that the latitude of origin affects the evolution of functional traits, as well as their phenotypic plasticity. 4. Since phenotypic plasticity is a crucial mechanism for acclimation and evolution, our findings support the role of gene-based adaptive phenotypic plasticity in plant