Certified GKH Gardening Expert
Perennial geraniums and rhododendrons can grow together if they have the right conditions. Perennial geraniums like full sun to part sun and rhododendrons like part sun to shade, so as long as you are planting in an area that gets part sun, both plants will be happy.
How much space you will need depends on the size of the rhododendron you buy. They come in different sizes and the tag or the nursery worker should be able to tell you what size the variety you want will grow to be. Conversely, if you have a space in mind, a good nursery should be able to point you in the right direction to a rhododendron that is the size you need.
Perennial geraniums are much smaller than most rhododendrons, so their size does not have to be factored in. They would be perfectly happy near the base of the rhododendron.