Answer: You can prune off dead branches at any time. Rhododendrons are best pruned right after they bloom by selectively heading back some of the longest branches and possibly also lightly pruning some of the new spring growth to encourage it to branch and stay dense. If this is done routinely the size can be controlled.
With a long neglected and severely overgrown plant you can cut it back quite hard in late winter to early spring. This treatment causes it to miss a season of blooms, but allows you to begin routine care to maintain it at a desired size. It is too late in the season for a drastic pruning like this, but you could consider it for next year.
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