Morphological and physiological variation of four geographically separated European populations of Oxyria digyna (L.) Hill (Polygonaceae) ranging from 45 to 78°N latitude were studied in controlled environments. Characters such as the presence of rhizomes, variable stamen number, few inflorescence branches, and low leaf length/width ratio were found to be common to northern populations. Perennating buds were formed under short day (SD) conditions in all populations across the temperature range (9–21°C), while induction of dormancy required the combination of SD and low temperature. Dormancy release of dormant buds required long day (LD) conditions only. The species was found to be a short-long-day plant for flowering control, the SD requirement being quantitative while the subsequent LD requirement is obligatory. Both the SD flowering response and the fecundity of flowering decreased clinally with increasing latitude of population origin, while the critical daylength for secondary floral induction showed a parallel increase. With some minor modifications, these results agree with earlier findings with North American populations. It is concluded that the obligatory LD requirement for flowering has been a limiting factor for the southward geographic distribution of the species, and that the photoperiodic control of flowering and winter dormancy renders Oxyria particularly well adapted to resist potential negative effects of rising global temperature.