I have no idea what this actually is, but from up on the deck it looks like nothing other than an apple-sized dandelion, lol. Close-up, it looks more like some kind of thistle or milkweed, but I can't recall seeing seed-pod fluffballs so big. It's got two more higher up that will probably open in the next week.
Also, another size comparison:
Grandandelion scaled to paperback by the_bugman_cometh, on Flickr