Tree form hydrangeas usually have lower branches cut off in order to maintain the tree shape. Branches that are allowed to grow under the main canopy may not get enough sunlight and will look lanky and-or produce weak blooms. For that reason, I would prune these branches if they start to develop.

The bottom part of the canopy/umbrella also has similar problems. The leaves that grow on the top of the canopy prevent sunlight from reaching the bottom of the canopy and make the branches/flowers there "poor". You could prune from the bottom of the canopy upwards -towards the top of the canopy- to create holes where sunlight can penetrate. But you need to be way careful of falling down. I do not think anyone has the equipment to prune the top area of the canopy from any other angle than from below. A big pain, is it not?

Haha it's a very big pain.... This tree has been here before we moved into the house thirteen years ago, and it has never stopped growing. I'm starting to get worried about it now because one of the major trunks is starting to rot due to the humid and rainy weather we've been having. I'm crossing my fingers for it though. It used to be such a beautiful tree, and I would love to get it back to that.

If I were to cut back the really high branches, would it grow back from the much older wood from, let's say, five years ago? Thanks :)