From what I've read rosehips are usually added 5-10 minutes before the end of boil, and add some red color, a touch of sweetness, and a bit of a citrus aroma.
If you're going for a rose flavor try playing with rose petals or rose water instead. Rose hips don't really taste like roses.
I just saw something on Northern Brewer called the "petal pusher". Comes with lavendar, coriander and rose hips. They say to add all for the last 10min of the boil. Im thinking about brewing a wheat extract batch with these flavors, and some yeast cultivated from some hoegaarden bottles.