That's definitely Loosestrife!It can be contained better if you deadhead it before it sets seed.

Yes, Lythrum (Loosestrife) banned in most states, including CT, as it chokes out other native vegetation along streams, lakes, waterways, bogs, marshes, etc.

And it's such a shame because it's a beautiful plant. Be sure to deadhead. One plant can throw millions of seeds, but deadheading will contain the original plant to a spread of about 20 inches via the root system.

Thank you for the info. I will definitely remove it from my garden. I'm sure I can find another plant that will be just as lovely.
Isn't it strange though, how it just popped up in my garden.