It's a Campanula or bellflower - looks like Campanula persicifolia or peach leaved bellfower. So not technically a weed but a rather freely self-seeding flowering perennial. A 'weed' is only a plant you do not want :-)

Not peach leafed bellflower, more likely Creeping bellflower aka Campanula rapunculoides, an absolute nightmare of a plant. You will not be able to get rid of it because the roots are brittle and break off when you go to dig them out and every piece seems to make a new plant.

Thanks donna_in_sask. I suspect it was an unwanted visitor. While it is pretty, I'm not sure I want more of them.

A "weed" is any plant that you do not want growing where you do not want it growing. So the Rosa Rugosa that is growing in the middle of my Lilac is, in my garden, a "weed", and one that a "weed" killer cannot be used on because of the adverse affect it would have on the Lilac.

Some sources list "Campanula rapunculoides", aka Creeping Bellflower, as an invasive plant.

kimmq is kimmsr