Greetings and hoping this finds all Safe, Healthy, and Happy; especially during this world wide pandemic (4/2020).
I am planting a garden filled with all wildflowers found in the woodlands, mountains, forests, along side river beds and creeks of the fascinating state of CT.
This is somewhat new to me as I usually grow only things one can eat. However, some are at least familiar to me as I was born and raised in Washington, D.C.and I have seen a few during my hikes along the Appalachian Trail, Annapolis and the Severn River, along the Chesapeake Bay, etc...
I have a Campanula persicifolia 'La Bonne Amie' that I purchased late last year (~10/19) and it overwintered on my outside patio. It is true that it is a perennial and survived just fine with no intervention save but the autumn and winter outside arena in planting Zone 6b.
It is now and it is 4/20. Plant looks grest and ready to transplant from 5 pint container to in-ground back yard.
•Leaves are a beautiful emerald green and are homogenous throughout.
The are elongated, well-balanced lance and tapered from close to plant stem is thin - wide - thin (toward tip of leaves)
•No flowers yet but should not be long and will keep you informed.
Sincerely, DrCarla in Stamford, CT

Looks a bit further along than mine (growing outside all winter). Mine stayed evergreen outdoors all winter this year. They are marginally evergreen vs. perennial: I don't think they mind the cold, but after any long period snow covered, they are gone and don't come back until spring. This past winter there was less snow than usual and much shorter lasting on the ground each time than typical (weather swinging between cold+dry and warm+rain most of the winter).

My Campanula have a lot of trouble with slugs, rabbits and groundhogs. They could not grow at all without aggressive intervention and in some spots intervention is not enough to even keep the plants alive and in other spots it only partially helps.

Used coffee grounds (a LOT) are the first line of defense. Tucking them in close to the plant center under the leaves is important. That is much easier to accomplish if the grounds are very dry. But once they are in there, wet is fine. That deters the slugs a decent amount and the rabbits a little. The groundhog is hesitant to eat anywhere that doesn't have a good retreat spot nearby. Once I understood that, blocking off the good retreat spots was pretty effective, vs. trying to directly defend the campanula near hopeless. I also use deer/rabbit repellent as well as some super hot chili powder (which was a low cost import from India stocked in an Asian foods store. Hopefully such commerce will come back. But until then I have an amble supply for cooking as well as for discouraging deer and rabbits). None of those are long term effective against deer and rabbits, but seem to be a lot better than nothing.

Maybe you won't have such pests. In that case I expect you have a very trouble free plant. It is not very picky about conditions and grows well regardless. If you don't have pests (or you deal with the pests) the plant will thrive.