If it has mildew the re-growth after hard pruning may fail. Another consideration is that some kinds do not come back well after pruning. Otherwise, it would probably be best to cut the whole thing down to a lower level all at once, rather than taking sections out of it.

I cut back individual branches over the course of a couple-three years or so. The plant is still big during the process but you choose branches that won't show when they're just a stick, they're covered by the unpruned or resprouted branches. Most rhodies are pretty good about resprouting from bare wood. It's common to cut huge rhodies down to bare stumps when renovating an overgrown garden, and usually they resprout.

Great in theory, but in practice I find most do not grow fast enough to not end up with lingering gaps while you wait for the low-cut branches to develop enough height to maintain the original outline and density of the shrub. Cutting off one third for three years would take a rhododendron down so fast you might as well do it all the same year.

Unless you are trying to keep some foliage to feed the new sprouts that come during the first two years of cutting out one third.

If the specimen in question is a 'Unique' and the stimulus for cutting it back is bareness inside, and not just the current height, then the more critical problem is almost certainly mildewing and premature dropping of the foliage. Replacement sprouts coming from hard-pruned trunks of this cultivar, on an infested specimen mildew even worse than the original growth and die, resulting in failure.

Good input, i don't think it has mildew - but i will check. My main motivation for keeping it is that it has a very nice rounded shape, nicer then any of my other rhodies, and the color is complementary to the house. However, it might be easier to replace it if it will take years to look decent after pruning.

I have several others that I am less "in love with" that have also exceeded their space so I am going to have to make some hard decisions here. I just don't want a front yard that looks hacked up for the next couple of years - months okay - but not years.

Most hard-pruned rhodies around here take a good 3 years to have plush foliage and numerous blooms. That is if they were doing well in their spot to begin with.

Rhododendrons are easy to transplant, maybe live with it for now and then do some shifting next winter - if you have other places in the yard these could be used in.

Probably not an answer to what you are considering, but some Rhodies have fascinating twisted branching inside, and I cut excess twiggy branches out to reveal the interior of a couple of them with great effect. Some other large bushes don't have nice branching inside so I just left them.

Why not turn it into a tree rather than cutting it back? Usually when hard pruning is done the end result is a butcher job by most people.
Another point to consider is that blossoms aren't a commercial food crop. They can be sacrificed for the benefit of the shape of the shrub or tree which, to me, is more important than the number of blooms. Trusses in this case.

{{gwi:1080506}}

Un-named Decorum, Anna Rose Whitney cross.

I have a rhodie that has overgrown its space too and I was just getting ready to prune. But I have never had one before its just at the place I moved in to. Mine has stopped blooming already and everyone elses rhodies are full of blooms. I don't know why mine isn't. I was trying to figure out how to cut it back too. It looks like it has a lot of dying branches underneath and there are tons of droopy leaves. I was told it was ok to prune but now after reading your follow ups I'm not sure what to do with it!

The main rhododendron flowering season here is March (or earlier) to July, with each kind blooming for a short time and then finishing for the year.

Except when certain early blooming kinds open some of their buds prematurely in October (or later), sometimes this off-season flowerings produce a fairly substantial display.

Sections of a rhododendron having all leaves droop and die is liable to be a root or stem rot due to infestation by water molds such as Phytophthora.