I've had this in my garden every spring since I bought my place, and in previous years I've just pulled it up, but really I'm curious what it is - I'm having a hard time identifying it from weed guides online.

Frankly, I think its rather pretty, and seems to choke out the dandelions, so if its not *too* hard to handle I may just trim it back severely and keep it. But before I do that, I'd really like to figure out what I've got on my hands.

EDIT: Location is central Indiana, if that helps.