Knowing when to trim rhododendron bushes is important so you're not trimming them at times which can hurt the plant, but rather to promote new growth and bloom when they're young and to manage their size when they're matured. Here are some things to know about when to trim rhododendron bushes.
Trim your rhododendron bushes in early spring or even late winter. This will give the plant enough time to harden off before the blooming season. If you trim during or after the blooming season, your work can negatively affect the current and next few blooming seasons for your rhododendron bushes.
For younger plants, you can trim them every year to promote new growth and support the healthy bushes, but for matured plants, pruning can hurt new growth and it may take a while before your rhododendron bushes are ready to start growing again. For matured rhododendron bushes, trim only when the plant gets too big and unwieldy, and trim enough that you won't feel the need to trim it again when it starts growing again.
Dead and dying branches are bad for your rhododendron bushes because those branches can no longer grow, plus they drain energy away from the plant, which goes to waste. Cutting off the dead branches whenever you find them is ideal so that new ones start growing in their place so that you have a fully healthy rhododendron bush.
Your rhododendron bushes may catch some disease, and the sooner you identify and resolve the problem the better. One resolve is by trimming off infected parts with sterilized trimmers so that the disease does not spread further and that a new, healthy branch grows back. If you don't immediately trim off infected parts, your whole rhododendron bushes may become diseased and may not be able to recover so they will die.