For cascading macrophylla hydrangeas that flower all along the stems from top to bottom, pruning is best done right after the last flower bud for the current year opens as it will very quickly (weeks) develop dormant flower buds for next year inside the stem ends. Because of its trailing blooms, when deadheading cascading hydrangeas (not the same thing as pruning), be especially cautious and remove only the dried flower head itself, by cutting the string that attaches each bloom to the stem or consider letting the blooms fall down on their own.

For reblooming and once blooming macrophyllas, deadheading of spent blooms can be done at any time of the year provided you cut above the first pair of leaves directly underneath a bloom. Pruning for those two is best done right after the last flower bud for the current year opens as those stems will develop dormant flower buds for next year inside the stems a few weeks afterwards.

If you cut back the stem partially by a foot say, you may get blooms next year if you allow flower buds to remain unpruned. The problem with buds is that it is hard to tell if they are leaf-only buds or flower buds.

Consider also moving the shrub to another location where it can grow as needed without having to require frequent pruning; in its place, plant a more compact hydrangea macrophylla that maybe gets 3 feet high/wide only and reblooms: Summer Crush, Pop Star, selected few from the Let's Dance Series, etc.

Nice dark stems in the picture. Which variety do you have in the picture? Reminds me of Nigra and the Onyx/Abracadabra Series.

Thanks so much for the reply, Luis! The photo is actually not mine - I found it online as I was looking for an example of a hydrangea that flowers all along the stem. The XL one I have was planted long before I moved into this house… it is definitely not a reblooming hydrangea and blooms in true blue whereas my endless summers next to it are mainly pink. I assume it is a nikko or similar. The thing is, I have many macs and I know for sure some are endless summer and summer crush and they all have this habit of flowering along the entire stem. They arent really “cascading hydrangeas” I can’t really find much info online about why my shrubs flowered like this!

If you cut back the stem by a small amount say, you may get blooms next year if you allow some of the back up flower buds to remain unpruned.

I believe the term "cascading" is a bit of a misnomer when it comes to H. macrophylla, They don't really cascade at all. And the flowering stems appearing to generate all up and down the main stems are essentially just the new season's growth on those remontant or reblooming types (can also include Nikko Blue, which has a reputation for being at least semi-remontant).

If the older shrub is just too large for the spot, you can cut it back hard (wait until spring!). If an old wood mac, you'll miss a season of flowers but if a reblooming/remontant type, you should get some later season (new) growth that should provide a few flowers.

Otherwise, you can selectively remove about a third of the oldest canes. Do this for several years and you will have effectively reduced the size of the shrub - temporarily at least - without eliminating most of the flowers.

I rarely do anything more with my macs than just deadhead but if I do want to reduce size, I opt for the 1/3 rejuvenation method.

Oh that’s great advice! I’ll try removing the canes that stick out the mist on the edges… The plant has been there 20+ years and I’ve already taken 2x divisions from it - those were incredibly hard to cut free from the main plant so I can’t imagine how hard digging the whole things out would be! I also don’t have a big enough space for it. I would personally leave it just as is because it is so beautiful but ny husband is tired of parking on the lawn to save my plant! 😂

The pictured hydrangea looks like Presioza and old variety much under used today

Not entirely sure the variety as it was here 3+ owners ago! The shrub is about 6’ tall and wide at this point. Very happy to mail you a cutting in the spring!