Is it OK to prune a Rhododendron? If so, how?

You can cut them back pretty severely and it will take them a full season to start to fill in again. I just trimmed ours back a foot or so to keep them lower and they are doing just fine. You will have to determine what you think is enough trimming. too much show all of the sump though. I usually trim down the new growth. If you cut the heavier limbs cut above where a group of leaves are growing.