Phragmites australis is a cosmopolitan wetlands species occurring in a wide range of climatic habitats, It can be assumed that adaptations to climate have evolved to enable the synchronization of growth with the seasonality of the environment. To study these adaptations, European P. australis was collected in different geographic regions, and grown in common environments situated in the Czech Republic, Denmark and The Netherlands. Phragmites australis originating from higher latitudes showed higher relative length growth rates (RLGR), and flowered earlier in time than that from lower latitudes. Plants from Spain even continued growth until the first autumn frosts. When grown in the different common environments, population differences were found in RLGR, but no general trend was apparent. On average, shoots started to grow 2 weeks earlier in The Netherlands than in Denmark and 6 weeks earlier than in the Czech Republic. These differences could be largely related to lower spring temperatures in the latter two countries. When shoot-growth was plotted against the temperature sum, no differences in RLGR between Denmark and The Netherlands were apparent, whereas shoot-growth was slower in the Czech Republic. Results from a greenhouse experiment showed that seedlings from southern populations formed taller but fewer shoots and thicker but shorter rhizomes than those from northern populations, irrespective of total dry weight. They also allocated more dry matter to stems at the expense of leaves, whereas no differences in allocation to below-ground plant parts were found. It was concluded that populations of P. australis showed clinal variation in (i) the length of the growing season, (ii) time of flowering, and (iii) morphology and biomass allocation. These results are discussed with respect to the possible effects of global warming on population functioning. [KEYWORDS: cline; latitudinal gradient; relative length growth rate; flowering time; morphology; biomass allocation; Phragmites australis Latitudinal variation; plant size; population; romania; trin] ; Phragmites australis is a cosmopolitan wetlands species occurring in a wide range of climatic habitats, It can be assumed that adaptations to climate have evolved to enable the synchronization of growth with the seasonality of the environment. To study these adaptations, European P. australis was collected in different geographic regions, and grown in common environments situated in the Czech Republic, Denmark and The Netherlands. Phragmites australis originating from higher latitudes showed higher relative length growth rates (RLGR), and flowered earlier in time than that from lower latitudes. Plants from Spain even continued growth until the first autumn frosts. When grown in the different common environments, population differences were found in RLGR, but no general trend was apparent. On average, shoots started to grow 2 weeks earlier in The Netherlands than in Denmark and 6 weeks earlier than in the Czech Republic. These differences could be largely related to lower spring temperatures in the latter two countries. When shoot-growth was plotted against the temperature sum, no differences in RLGR between Denmark and The Netherlands were apparent, whereas shoot-growth was slower in the Czech Republic. Results from a greenhouse experiment showed that seedlings from southern populations formed taller but fewer shoots and thicker but shorter rhizomes than those from northern populations, irrespective of total dry weight. They also allocated more dry matter to stems at the expense of leaves, whereas no differences in allocation to below-ground plant parts were found. It was concluded that populations of P. australis showed clinal variation in (i) the length of the growing season, (ii) time of flowering, and (iii) morphology and biomass allocation. These results are discussed with respect to the possible effects of global warming on population functioning. [KEYWORDS: cline; latitudinal gradient; relative length growth rate; flowering time; morphology; biomass allocation; Phragmites australis Latitudinal variation; plant size; population; romania; trin]