If you need to prune your rhododendron what until autumn.

I'd say the best time to prune is just after flowering so that they can regrow safely and harden that growth before frosts come along....but they don't actually need to be pruned at all unless you have to keep them into a certain shape/size, or unless they are getting extremely leggy, and should certainly not be pruned on an annual basis.

I agree Ilex that you only prune if you really need to but isn't it better to do so as shrub is going dormant? Mind you we have never pruned any of our 40+ rhodos!

MG: We've never pruned any of our 120 Rhodos either - well except to make repairs to a few that the deer damaged! - I'm sure you could successfully prune at either time, but with a slightly different strategy in mind. Since Rhodo shoots/foliage don't start to grow until after the flowers have emerged, and often not until they are fading, it would be a more natural time to re-shape and encourage lower dormant buds to break.

Personally I wouldn't prune in Autumn because of our erratic seasons and off-again on-again weather. A warm spell following an autumn pruning could be disastrous if it forces the plant back into growth, only to have that new growth decimated by frosts before it has matured.

Ah we're less likely to get a warm spell in late autumn but I see your point.