My twenty five year old Rhododendrons are overgrown and spindly with no growth on bottom and sparse flowers up top. They are foundation planted in mostly shady wet soil. I don't know how to prune, since the only growth I see are flower buds, if any at all. Can they be saved? Will I kill the plants if I chop it down to within a foot or so of the ground? There is nothing down there but bare twigs.
Large and overgrown Rhododendrons can be pruned back to 12 to 15" above ground to rejuvenate them.
You may lose flowering for the next season, but getting your plants back into shape for the long term is worth the wait.
Here is a link that will help you.