While it's true that many flowering plants sold in non-nursery outlets may not be hardy, that's not the case with everything. In the case of hydrangeas, the mophead types usually sold as blooming potted plants are root hardy to Zone 5, but top hardy only to Zone 6. That means in Zone 6 and warmer, they will grow and bloom just fine outdoors if planted in the right location. In Zone 5 they'll die back to the ground over winter, but resprout from the roots. That means they will grow leaves, but not bloom because older types bloom only on old wood. Some newer varieties such as Endless Summer and Forever & Ever bloom on new wood, too so they would produce blooms later in the season.