When to trim a tree
I have a 30 year old PeeGee Tree needing to be trimmed back a little, maybe 15%. I live in zone 5.
Certified GKH Gardening Expert
These can be, safely, pruned during dormancy, or after flowers fade.
You can take up to 1/3 of the plant during growing season, but it is best to do any major cutting during dormancy.
This article will help you to care for the tree:https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/tree-hydrangea-growing.htm