Bellflowers (Campanula) are widely adapted, hardy flowers that include more than 300 different annual, biennial, and perennial species. The Chicago Botanic Garden evaluated 89 different perennial types of bellflowers from 1998 to 2006 to determine the best species for growing in their USDA zone 5 Chicago climate.
They collected data on bloom period, flower color and size, habit, height and width, disease and pest problems, and winter survivability. Sixteen species of bellflowers were grown. The performance of a species and its cultivars was often comparable. If a species did well, most of the cultivars in that species also performed well.
While 31 bellflowers received high (4-star) ratings, one stood out among the rest. Campanula 'Sarastro' received the highest rating possible (5 stars). It produced large numbers of violet/blue tubular flowers from early June to August. The overall best species for vigor, floriferousness, and hardiness were Campanula glomerata, C. poscharskyana, C. punctata, C. rotundifolia, and C. takesimana. All the cultivars of these 4-star rated bellflowers also performed well.
For more information on this bellflower trial, go to: Chicago Botanic Garden Plant Evaluation Notes.