What is this thing that I let grow last summer?

I pretty much ignored the back strip of my yard last year, allowing the ground to settle from grinding out the Bradford Pear and Pin Oak trees that I lost. While I was resting it, this wondrous weedy looking thing appeared. At first I thought it was a bush planted by errant birdseed. When it reached over 5 feet tall, I decided it needed cutting. The stems were very strong, yet pulpy and fibrous instead of woody. Does anyone know what this is? I have no clue.