Yes, you can
plant it in your garden this
spring. It won't bloom again, however, until next year. You're right...they like lots of water, and prefer a shady place in your garden. I'm assuming it's a mophead hydrangea since they are the ones most often given as gifts. One problem I have had with mine (we are in the same
zone) is a late frost in early
spring will kill all or most of the buds and it won't
flower well or at all. Since they bloom on last years growth, if you ever need to prune it, be sure to do so right after flowering in the summer. If you wait too late you may cut off the buds forming for the next year's blooms.
