Is it too late to prune peonies, hydrangea and decorative grass?
The foliage of herbaceous peonies can be removed after the first frost - it's best to get rid of it as it can harbor fungal problems like botrytis if left in place over winter. Tree peonies generally don't require pruning. Deciduous ornamental grasses - those that look like straw or dead grass through winter - can be cut back now, although many prefer to leave them in place for winter interest and to feed the birds. If so, cut back to around 4" in the beginning of March.
Pruning hydrageas is never a requirement and in some cases can prevent them from flowering next season. Those that bloom on new wood, like paniculatas and arborescens, are better pruned back in spring as new growth is forming and you can remove the obviously dead wood. To prune now may make them vulnerable to excessive winter dieback if your winter is severe.