This dataset comprises information of location and cytotype of over 1300 samples of Campanula rotundifolia L. from the northern hemisphere (mostly from Britain and Ireland) and data from a common garden study in which British and Irish cytotypes were grown together and their flowering phenology and growth were assessed. Campanula rotundifolia L. is a widespread polyploid perennial herbaceous plant, with diploid, tetraploid, pentaploid and hexaploid cytotypes.