This from a couple of days back probably answers your question
It depends what kind they are. Paniculata forms can be pruned to size or shape at any time of year except once they've begun forming their flowers in summer. Best done in early spring to remove any frost damaged stems or immediately after flowering to improve shape.
Mopheads and lacecap hydrangeas should have spent flowers removed as soon as they go over or by the end of July before they set buds for next year's blooms. Just remove the flowers by hand without damaging the bud forming below. They can be pruned at any time to remove dead stems. Removing any other stems to tidy up the plant or reduce size will reduce flower power as they flower on old wood so bear in mind that any new stems will not flower till their second year.
Not sure how or when to prune a Hydrangea - the climbing, clinging one which is flourishing on my garage wall, facing North. I don't want to damage it or lose too many flowers, but it is getting a bit out of hand.
On non-repeat blooming ones if you want to preserve next years bloom always prune right after they bloom. Never prune in the spring as that will take off this years flower buds. If they are severely over grown and you want to cut them back you may need to just sacrifice a year's bloom. I've never grown the ones that are supposed to rebloom so I don't know about pruning those.
For the climbing hydrangea (petiolaris) you need to prune it straight after it's flowered, cutting back the flowering branches and any others that are spreading out of it's allotted territory. You can be quite firm with it and cut it close to the wall - if it's happy and growing well it'll soon put on some more growth which will flower the following year
If you need to give it a bit of an urgent trim back, you can just cut back the branches that are in the way now, and do the rest after flowering.
I prune mine in march. I cut back to a node low down on some stems and just dead head other stems.(This means I still get flowers and also get a smaller plant) I also remove some of the old stems all together.
Hope this helps
That's how I prune the mop-head hydrangea - Marion was asking about H. petiolaris - do you treat your climbing hydrangea in the same way?
I only have mop heads but you should be able to prune in the same way. As long as you leave some flowering buds there shouldn't be any problems.
Hope that helps