An established and mature Limelight hydrangea is a big shrub! 6-8 tall (possibly more) and 6-8' wide. Even hard pruning will result in regrowth in a single season to the mature size.

You can prune them pretty much as hard and as often as you like - the shrub will tolerate it fine - but pruning after growth begins in spring is liable to remove any flowering potential - these bloom on new wood.

It sounds like maybe these should be relocated to another area of your garden and replaced with a smaller growing cultivar.....like perhaps Little Lime, that gets about half that size :-)

I agree 100% with GG48. Sounds like the plant was installed without an understanding of how large and vigorous they are.

Thanks for your replies! I actually think these are "Little Limes". I bought and planted them myself about 5 years ago, and the tag definitely said they were "compact". They're just not as compact as I imagined. I might have exaggerated the *massive* part - I would guess they're growing to be 5ft tall with decent pruning. I'm going to prune back even more and see what happens.

The linked blog page below includes some nice text and photos of Hydrangea paniculata pruning. I think hers are actually Limelights, but the same ideas on how they regrow applies to any of them.

https://deborahsilver.com/blog/more-on-pruning/

give them a very hard prune , take down 2/3 or even more of the plant. They'll grow back hard but perhaps not quite as large as they are now.

As this variety of hydrangea blooms on new wood, you can only prune once, early in the growing season, if you want to get flowers.