Do you live in a dry climate? These things are pretty tough and I'm surprised they need watering at all. Also they prefer a neutral to alkaline soil. Is that what you have?

Canterbury Bells are biennial so they should over winter and flower next year.

Not sure which 'ground cover' Campanulas you might have. There are several and they don't all need the same conditions.

A location would help.

I would say that I grew these during the Year it Rained, and don't remember any specific problems.

Lavender and campanula seem to me to be odd bedfellows :-) They do not share similar growing conditions so if the lavender is happy, then the campanula is likely to struggle.

Campanula prefers cool summers, evenly moist soils and some relief from afternoon sun in hotter climates. Pretty much the opposite of what the lavender likes.

I'm in San Jose, CA. I have had the Lavender in a perennial garden for 10 years. At first I planted them in 8" plastic pots in ground with the bottom of the pot cut out so i could control the soil and that worked well but if I got lazy with the watering some died from lack of water when it got very hot (I have drip line but it wont get inside the pot) So I got rid of the pot and I just put a lot of pearlite+ bark in the hole and its fine. I haven't had any rot since I improved the drainage although sometimes the English lavaders grow too fast and flop, the Munsteads stay small and upright. I have one in front where the soil tends to get very wet sometimes and its fine. I think very good drainage can make up for wet conditions to some extent.

The Campunula is portenschlagiana (Birch Hybrid). I have it at the base of a tree and in spring its a carpet of dense blue flowers, really pretty although rebloom is weak. If temps go above 90 deg in full sun it does suffer a bit.

My soil test at ph 7, water is ph 8 and I have posted on problems with that but usually no issues with perennials.

I watered the Persicifolia about once a week because growing plants need more than my drip can provide. We did have odd weather this year with a month or 2 of high humidity, although high here is probably normal in many places (about 70%, normal is 40-50). I'll improve the drainage on that one plant. Its possible the slow growth is due to the heat. Supposedly Persicifolia is one the few that can tolerate full sun but of course heat is a different matter. I got the plants as plugs and that one plant did grow very well in spring, the other 2 developed the wilting leaves fairly quickly. As I said, I dug up the other 2, put bark in the hole and replanted and now no rot although they are only 3-4" across even after 2 months.

I'm trying to create a blue/pink perennial mix and there are not many 2-3' blue flower choices for full sun.

Balloonflowers, asters, Stokes' asters, dwarf iris, centarea montana, ageratum - that's just off the top of my head.

The problem I always had with campanula persicifolia is that it isn't really 2 ft tall. It's a very short basal rosette that puts up flower stalks.

Interesting to hear about C portenschlagiana under a tree. Here it self seeds in stone walls, seldom on the ground and never under trees. Probably due to less sunshine here.

C poscharskyana, also self sown.

Thanks for the replies, I dug up the plant and it has a 1' wide rootball but the thing that surprised me is that the soil is very compacted and the bottom of the rootball is dry because water has a hard time penetrating. I should have dug a big hole when planting but it was just a small plug and I thought soil OK. I have a soil probe and checked other places in my beds and most are OK - that area had not really been planted so compacted. I dug a 18" hole and filled with bark and replanted the Campanula. I dont see how the soil would contribute to stem rot but of course if a plants not healthy its susceptible to all manner of diseases.

Bit late in the season but the self sown C portenschlagiana is still in bloom round here. These are the conditions it likes here. Shady walls.

Re your C persicifolia. I'm a bit concerned about the idea of digging a hole in compacted ground and filling it with bark. Sounds like a recipe for the 'bath tub effect.'

I grow the occasional Campanula persicifolia

as far as I know they are biennial or shortlived, they do take drought quite well, but all in a context

I planted the first, they reseeded in my crappy "lawn" and in gaps in the perennial border, and I rescued some seedlings that in some cracked concrete at my neighbours

I had them down as pioneer species, seeding into open soil, happy with sand, gravel, silt etc, I personally would say the bark could be a problem, I garden in a vinyard climate with summer temperatures up to 38 C (seldom), mild winters and quite some rain in a lot of years, sometimes sommers with 6 weeks without a drop