In your zone (as in mine), there is no need to provide winter protection. I am a bit concerned about your pruning technique, though. A uniform cutting back to 18" could be removing a lot of flowering buds. Here, I generally don't recommend pruning hydrangeas at all except to remove deadwood or to head back the occasional leggy branch. Deadheading the flowers back to the first set of fat buds is sufficient. With your very long growing season even harsh pruning is allowing for the development of some flowering stems but you will see much heavier flowering if you revise your pruning technique.

Thank you Gardengal. I'm not even sure where I got the information to prune back that far. The plant looks terrible when the nights get cold. The leaves turn yellow and fall so I always trim it back. Should I leave all of branches alone next year? I already pruned it back this year so I can't fix it now. It already has new green leaves budding on the old canes. They've made it through a few frosy nights. The plant has always come back to a height of about 5 1/2 feet. Just hasn't had may blooms in the last three years. It used to be quite showy.

You can trust gardengal's advice, your hydrangeas need no winter protection in Z9 - you are removing the flower buds with your pruning.

Deciduous shrubs do not hold a lot of winter interest (dropping their leaves in response to fewer daylight hours as well as cooler temperatures)...if you are expecting things to have year round presence they would have to be replaced with shrubs that stay evergreen.

I remove spent flowerheads in late winter here, take out an occasional older branch to improve the shape on some hydrangeas. For those larger hydrangeas that are taking up more than their share of space, you can cut approx 1/3 of the branches back hard each year in late winter/early spring. You lose flowering that would have taken place on those stems, but the result can be a more compact, in bounds shrub that is completely renewed in three years.