The best time to prune hydrangea depends not on location but on the type of hydrangea. Most hydrangeas don't need to be pruned unless they have overgrown their space.
Hydrangeas that bloom on old stems should be pruned after flowers fade. Hydrangeas that flower on new wood should be pruned in early spring before the new growth begins.
Big-leaf hydrangeas should be pruned in the early summer after flowering. Oakleaf hydrangeas should not be pruned until after flowering. Panicle hydrangeas are a tree form and should be pruned before new growth emerges. Annabelle hydrangeas should also be pruned in the early spring before growth begins. Remontant hydrangeas can be pruned at any time.