To Weed or Not To Weed

What Is This???

So I suddenly noticed this plant a couple of days ago. Somehow I didn't see it until it was as big as my tomato plant (maybe 15"). I've pondered it for a while and can't imagine it growing any of the vegetables that I planted, so I finally bit the bullet and chopped it down.

I always thought that you were supposed to pull up weeds by the roots, but after reading around the Internet a little bit trying to figure out what this plant was, I read that if they're near plants you want to keep, you're better off cutting it off at ground level a few times until it dies. That prevents damaging the root system of the neighboring plants.

So hopefully this wasn't my sunflowers or my peppers. I haven't seen any sign of those yet. But I really doubt it.