How to Prune Rhododendrons
Question: Your recent garden blog about pruning Rhododendrons suggested pruning right after they bloom. In the same article, you mentioned that they make new vegetative growth after they bloom. Wouldn’t it be better to wait a month or so before pruning? Also, how much pruning do Rhododendrons need really? I have done very little pruning of mine over the last 5 years.
Waiting a month after bloom to prune Rhododendrons would be just fine. The point is, don’t wait too long. As soon as they finish vegetative growth, they set flower buds on the tips of the new growth. I have pruned in early August, removing some flower buds. The plants grew and developed new flower buds after I pruned. If you prune much past mid-August, they often do not produce new flower buds. Rhododendrons actually begin new vegetative growth before all the flower buds have dropped.
The main reason I do most Rhododendron pruning is to reduce the size of plants which are blocking a window or walkway. I also shape plants which have become lop-sided. If your plants have not become too large, there is little need for pruning.
I generally do not reduce size by more than 1/3. More drastic pruning can also be quite successful, when necessary. However, it usually takes more than one growing season to recover.
I never use power clippers on Rhododendrons. I prune one branch at a time, cutting back just above a side branch or leaf cluster. These cuts are made down inside surrounding foliage, so the stubs do not show. In some cases I remove an entire branch back to its origin, especially if growth has become too thick. This same technique also works for other spring flowering shrubs.