Clustered bellflower is a hardy, medium sized bellflower that bears dark purple flowers in round clusters. There are not many garden plants here with such a deep purple colour, so it makes a very nice garden plant. It has a bad reputation though for spreading too vigorously
My first introduction to this plant was in my grandmother's garden who just called it "kampanula", which was almost a swear word because it had claimed large portions of the garden as it's own.
I received a cutting of this plant a few years ago, that does not spread at all and is an excellent garden plant. In the next yard over grows a relative of my grandmother's "kampanula" that has spread through a large perennial border. So, clearly, it is a very variable species.