Bidens

Here's a page with multiple photos of three varieties of Bidens:
http://www.jeffpippen.com/plants/bidens.htm

Having just posted photos of the same plant last week, I concur with the experts here. Bidens. Do not hesitate to rip it out. If it blooms and goes to seed, you, your family, your pets, your guests, your neighbors and any local wildlife will be wearing the devilish little seeds for months!

Ah but it is the host plant for several butterflies such as the dainty sulphur butterfly. Also a good nectar plant for butterflies.

Let it live and then just cut off the flowers before they go to seed.

Thanks for the ID and advice, everyone!