Where Did This Come From?

I planted this hydrangea last year into what will eventually be a paeonia and hydrangea border. I filled in the gaps with annuals which have been really spectacular all summer long, obscuring the new shrubs. All other hydrangeas in the garden have flowered and are past their best. This mophead, however, is just looking so fresh and blue and ready for the summer party - but the party's almost over.