You can cut them back now. We usually cut ours back to two feet high, cut straight across. Some people cut them to the ground. When we had them in our fenced yard we cut them to three feet high, we didn't want our greyhounds getting hurt on them. They will flower on new growth.

Thanks for the info akrrm. I'll prune it ASAP. The plant has very thick old stubs at about 1' height and I don't thing I can cut through those. Will cut back the branches to that level.

You can prune them back hard and I'd suggest making your cuts to just above any existing leaf clusters. Now's the time to also prune out any old wood too to keep the plant vigorous.

You should wait until the bush shows life or small green leaf buds then cut back to approx. 18" in a rounded form. May be a few more days.