I've since changed my mind about C. collina! I bought 3 plants in 2005, so it did take some years for their spreading nature to come to the fore, but I did pull them out this summer... a shame as they were quite attractive.
Calgary, Alberta, Canada - Zone 3
-30 C to +30 C (rarely!); elevation ~1130m; annual precipitation ~40 cm
'Tis a beautiful plant, but not for the rock garden. Try it in a border and you'll love it.
Peter George, Petersham, MA (north central MA, close to the NH/VT borders), zones 5b and 6 around the property.
That's where I just got rid of it from... ;D
If anyone would like to learn more about dwarf Campanulas then Graham Nicholls' book "Dwarf campanulas and Associated Genera is a must. Published by Timber Press 2006.
in Devon, UK Zone 9b