Pruning Hydrangea Paniculata . . .

From Margaret Roach , A Way To Garden :

With Hydrangea paniculata, such as the so-called Pee Gee (short for paniculata ‘Grandiflora’) and ‘Tardiva’ and other panicle types, pruning requires a different thinking from blue-flowered moptop types, because paniculatas bloom on new wood.
Prune them in late winter or early spring, just before new growth begins. You can even prune in late fall, but I don’t like to do it until after winter has made its mess, in case branches are lost in storms, which would erase my earlier attempt at creating a good architecture.
They can be cut back quite hard, and still make new wood that then gets buds and blooms. Or you can skip pruning altogether, if you like. I know, that sounds counterintuitive–that you can choose to prune or skip it. They will sprout whether you prune or not, but depending what you do it will be on a smaller or larger plant, with fewer or more flower-producing stems.
Left on their own, many Hydrangea paniculata varieties would grow into small trees, with some up to 20 feet tall (10-plus feet is not uncommon).
Not pruning, or pruning too little, will lead to a more twiggy, less-open plant that will produce many, smaller flowers. Generally, the harder you prune, fewer but larger blooms will be encouraged.
Where to place your cuts can be confusing. I have had specimens that I didn’t cut back enough turn to octopuses and really look a mess, and this is inclined to happen on older plants that have been cut back again and again to the same point. I’ve also cut old ones back too hard and had to suffer through some gawky recovery years.
The “secret,” if there is one: To get a good-shaped plant, you will often be pruning back to a mixture of oldest (thicker) wood and younger (last year’s) wood, and even some that’s in-between. The pruned structure will look odd, but try to think only in terms of creating a framework for emerging shoots that will then be topped with flowers. Your cuts indicate to the plant where it should (please!) sprout from. Let go of the fact that the wood you’re leaving will be of varying thicknesses.
Remember that these plants push lots of growth before blooming–like several feet or more–so they will be much bigger than the base architecture you created at pruning time. And as with any pruning project: First remove any damaged, inward-crossing or otherwise sub-standard stems—so that when you begin really shaping the plant, you know what you have to work with.