If your large rhododendron is growing out of control and starting to look like a mess, you've got some pruning to do. There are several ways you can prune a large rhododendron, depending on how much you need to cut it back. Pruning large rhododendrons can seem like an overwhelming undertaking, but it doesn't need to be. If you take the pruning one step at a time and break up the project across a few days, it won't seem so impossible.
Prune your large rhododendron right after it finishes blooming. You should prune the rhododendron before the buds form so you don't lop off the following year's flowers. Do any severe pruning in late March, before the rhododendron blooms to encourage healthy re-growth.
Prune just above the growth joints using hand-held pruning shears. Don't use hedge shears. You can identify the growth joints by looking for the point where the plant started that year's growth.
Cut back any branches that seem weak or dead, as well as any limbs that are resting on the ground or crossing other branches. Doing this will improve air circulation in and around the plant.
Prune back to limbs with the lower whirls of leaves. Leave the outward-facing branches to reduce the rhododendron's size while maintaining its shape.
Look for any branches that are growing inward. Prune any branches that are growing back into the plant.
Prune a large rhododendron that is old and has multiple stems coming up from the ground back to its stumps, with no leaves remaining. Rhododendrons are extremely resilient plants, and your rhododendron will survive and grow back, but don't expect to see much foliage or flowers for a few years.