Plants of Lythrum salicaria and Phragmites australis originating from localities across the European north–south geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground- and belowground plant biomass, etc.) and the position of the respective populations along the north–south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.