It’s often said that the best time to trim a rhododendron bush is in the late fall after it’s finished flowering. Light trimming includes maintenance procedures and perhaps the removal of the outer sections of the biggest branches. It’s important that the flower buds for the next season aren’t removed during trimming, or the plant won’t produce flowers in the next season.
Heavy pruning of a bush requires more care. The bush can regrow when all of its stems are drastically reduced in length. I’ve never done this, though. The results may be variable.
There are three rhododendron bushes in a landscaped area near my home. All of them were heavily pruned last fall. Some of their thick, woody stems were sawn off, and the bushes were considerably reduced in size. The biggest and most vigorous bush is recovering nicely. It has bloomed this year, although there are far fewer flowers than usual, and there’s a lot of sticky new leaf growth on the bush. The second bush hasn’t flowered this year. It has lots of new leaf growth, though, and seems to be doing well. The third bush is in trouble. It hasn’t flowered, most of its leaves have died, and there is only a little new growth, all of it in one part of the bush.
If you intend to do anything more than a light trimming of your rhododendron, you should do some research about the best way to prepare the plant ahead of time and to actually prune it before you start. It’s also a good idea to research the best way to disinfect cutting tools before any type of trimming or pruning.