please help identify this plant!
This is my first post in this community and on gardenweb. I live in New Orleans and am trying to gain control of my yard. This is my second summer in this house, and I see several plants that have seeded from last year. Loads of these are so hard to control: mexican petunias and 4 o'clocks to name a few. There is another plant that is everywhere, and I can't figure out how it's spreading. I dig them up and they keep coming back. I've uploaded two pictures if anyone is willing to help me figure out what they are. They grow very quickly, have big heart-shaped green leaves (with purple undertones) and bloom into lovely hydrangea like flowers at the end of the summer. I let some go this summer, and they grew over 6 feet tall. I cut them all back 7 weeks ago, and now they are everywhere. Here're the links: