I'm thinking that your ROS is Hibiscus syriacus. Here in the UK we often apply that name to Hypericum calycinum.

Where I am is approx Zone 8 and my experience is that they do well in a sheltered situation otherwise dieback will occur.
Pruning is best kept to a minimum by just removing any dieback in late spring. If renovation pruning of a large subject is inevitable remove some of the older stems and cut back the remainder by up to two thirds.
Young plants should be cut hard back in their second year after planting to encourage basal branching.

Here is a link that might be useful: Website