You should not prune hydrangeas in September if they bloom on old wood. Bigleaf, oakleaf, and mountain types have already set their buds for next year by this point. Cutting them now removes those hidden buds. You'll get zero blooms the next summer. New wood types like panicle and smooth won't lose buds, but September is still not the best time to trim them.
September hits right when the cleanup urge kicks in. Your summer flowers have faded and the foliage looks tired. I almost gave in two years ago and started cutting at my bigleaf hydrangeas. Something stopped me and I found my notes from the year before. The safe window had closed in August. I learned to leave those messy stems alone. The next June every single one of them opened into a fat bloom. That restraint gave me the best display my garden has ever had.
A friend of mine didn't hold back that same September. She trimmed her bigleaf and oakleaf plants on the same weekend I chose to leave mine alone. Come June her bushes had nothing but green leaves while mine were covered in flowers. She texted me a photo of her empty plants with the words "you were right" and hasn't touched her old wood types in September since.
September hydrangea pruning goes wrong because of basic plant biology. During late summer your old wood bloomers form tiny buds deep inside their stems. Arkansas Extension says bigleaf types must be pruned by August at the latest. Any later and you cut into the new bud set. NC State lists wrong pruning time as a primary reason bigleaf hydrangeas fail to bloom.
Fall pruning hydrangeas of any type offers very little benefit. Even your new wood bloomers don't gain from a September cut. Their stems give the plant crown protection through winter. Taking those stems away in September removes insulation your plant could use during cold snaps. You're better off waiting until late February to trim new wood types.
The only work you should do on any hydrangea in September is removing dead or broken branches. These don't help your plant and can attract disease. Snap off dead wood by hand or cut it at the base with clean shears. Leave every living stem in place even if it looks messy or overgrown to you.
If you missed the July-August window for your old wood bloomers, accept it and move on. One skipped pruning season won't hurt. Your hydrangea will bloom next year on those unpruned stems. You can trim right after the flowers fade next summer to get back on schedule. Your patience now means blooms later.
Mark your calendar right now for next July as a reminder to prune your old wood types. I always set my phone alarm two weeks ahead so I don't forget. This way you won't stand in your garden in September wondering if it's too late. Put your shears down and enjoy your fall.
Read the full article: How to Prune Hydrangeas by Type
