Do they seem to be early this year? Clustered bellflower, Campanula glomerata, typically waits until the end of spring to bloom. Once the initial and most prolific bloom phase finishes, sporadic bloom should continue almost through summer. Blue is their most popular and traditional color. White is their second most popular option. Bluish purple and purplish pink are still somewhat rare.
Vegetative growth stays relatively low and unassuming through autumn and winter, and then starts to stand up and get noticed just before spring bloom. Short varieties might bloom without getting even a foot tall, while tall varieties can get two feet tall or a bit taller in partial shade. Each blooming stem supports a dozen or so five-pointed flowers that are about thee quarters to an inch wide.
Clustered bellflower looks neater and probably blooms a bit better as the season progresses if stalks are pruned out as they finish bloom and start to deteriorate. However, a few stalks of some varieties might be retained after bloom to produce seed to scatter elsewhere. Some of the fancier or newer varieties do not produce many viable seed, and such seed may not be true to to type.