I find volunteer sunflowers in the oddest places, I think sometimes they aren't digested by the birds who eat them and then they get planted with a little package of fertilizer...

I believe it's going to be a large one, maybe a "Mammoth" sunflower. Because of some recent windy rain storms I thought it best to tie it (loosely) to the fence post for some help staying upright. It's funny because whenever I buy seed and purposely plant sunflowers, the plants come up and are quickly devoured by slugs or never germinate at all.