That is a hydrangea arborescens, but a cultivar that only produces fertile florets. If you want a mop head flower, you'll have to get rid of it and replace with one that produces sterile florets, such as Annabelle. Sorry, but there's nothing else you can do.

Similar to what you see with some lacecaps, after the sepals fade and fall off. You are left with the same fertile floret look.

Here is a link that might be useful: Picture of H. arborescens species

This post was edited by luis_pr on Sun, Jun 8, 14 at 21:25