Lychnis - very pretty, can self seed and spread but easy to pull out. Likes well drained sunny places.

Lovely plant which I wouldn't be without! It self seeds and often the plant survives for 2 or 3 years. They flower on and on and don't seem to mind whether they are in sun or partial shade. Some come up darker pink or pink and white, or pure white.

Its full name is Lychnis coronaria, or rose campion. I love it too - have just given a visitor an envelope full of seed heads as they liked it too. :-)