Bob...They need to be cut after blooming. You can read on Google some of the instructions but they do recommend to use hand clippers for small branches and loppers for larger ones. Branches need to be cut various lengths rather than create a unnatural shape. Cut all dead branches off. We have two but they are not large at all and we keep dead trimmed. They only time they look healthy is when they bloom and then are absolutely beautiful with huge blooms, however, after that return to looking pitiful. I just get the dead off and wait for spring again! Good luck to you...take a look at the Google articles if you can!
I actually have a question about our Rhododendron. We moved about 5 years ago and our house had been very neglected, including outdoor areas. I love Rhdodendron but ours have never bloomed but they have had buds on them. We removed lots of dead wood and are not sure what else we can do. Any help is appreciated. Thanks, Pam
Early spring for major cutting. Late fall is okay too. Rhodies are quite sturdy and they can be "trimmed"- a foot or more most any time.
Since you've stated they are huge, cut them in early spring.(February-March). They won't bloom for a year. The largest rhododendron can be cut to a nice shape and size and bounce back. Using a tree saw works great or a reciprocating one.
When cutting keep the center of the bush open like fingers of a hand reaching up. Remove odd branches that are crisscross. They can be cut to shoulder, waist or knee height. Even if the cuts are lower than the current branches and foliage the new growth will come and fill out your shrub. A good rhododendron fertilizer and mulch will be of benefit too.
Thank you for the confidence of extreme cutting. I just wasn't sure.