Campanula - a resounding Danish success story
Some plants are made for indoor use while others thrive best outdoors – and then there are plants which are cut out for both purposes
Campanula or Bellflower is a resounding Danish success story. The plant is popular primarily because it is decorative both indoors and outdoors, and moreover, it is incredibly hardy as it is a perennial tolerating frost and it reappears year after year. 
Campanulas easily overwinter in containers but are somewhat more exposed than if planted in the ground, not so much because of the frost but because of the risk of drying out in case of hard frost. 

The season peaks from mid-March to June but Campanula is an incredibly popular indoor plant already from January. Still larger quantities are also sold from July to October. Campanula thrives best in a bright position indoors and lives longest if the position is not too hot. The plants should be watered often; they do not tolerate drying out.

Outdoors, Campanula is extremely well suited to be planted out in containers or directly in the soil round the patio, in rockeries or on slopes. The numerous flowers make the plants resemble almost all blue or white surfaces covering the plants several times from spring to autumn. 

Campanulas can also overwinter outdoors in containers but make sure that the plants do not dry out in early spring. The containers must therefore be protected against drying winds and strong sunlight.

In 2010 20 million Campanula were produced in Denmark.
Source: Floradania