Amaranth.

AKA, the most invasive plant ever!  We planted a few amaranth plants 4 or 5 years ago.  Every year since, we've had loads of volunteer amaranth.  The species Palmer Amaranth is taking over fields in the southern US.  This is regular amaranth...

It was chest high. 

It's really pretty, too bad each plant has over 100,000 seeds!  Amaranth is a close cousin to Red Root Pigweed and Water Hemp.

Can you spot the dog?

I cut all of it down yesterday and sent it packing in the green waste recycling bin. :)