Looks like Redshank, belonging to the Dock family. Even the wild flower book refers to it as an 'annual weed!'

Nariz is right and I suppose most of us get it at some time but at least its an easy one to pull out.

Thank you very much for your quick response, Nariz and Inverglen! I checked it with that name online. You are exactly right! I will pull it out as I try growing some smaller plants in same container. Thank you!!