A topinambour collection was analyzed to determine the genetic variability of 141 accessions in reaction to the length of day (light), its influence on vegetative and flowering phases, tuber number and mass. Day length significantly influenced flowering, which started with the first shorter days (15.6 h) in the third decade of June, while the majority of accessions flowered in the third decade of August (13.3 h). Differences between accessions were statistically significant for the analyzed phenotype traits. A significantly longer vegetative phase was found in Montenegrin accessions in comparison to the other groups of origin. Duration of the vegetative phase was significantly and positively correlated to tuber mass and negatively to their number, while duration of the reproductive phase had an opposite effect. For further work on topinambour breeding, it would be important to describe the mechanism of photoperiodic control of flowering initialization. Only by obtaining accessions neutral to the photoperiod could topinambour become a cultivated crop.