Niruri (phyllanthus niruri) AKA chamberbitter.
Can be used as an herb. Apparently good for kidney stones...
But... Incredibly prolific...
Said to begin flowering as a two week old seedling!
http://www.walterreeves.com/la...
This is one to toss in the trash, or in the grill, just don't add it to the compost pile!
Edit:
Phyllanthus urinaria