Rhododendrons, unlike most shrubs, have no painless window for pruning. They start forming the buds for next year’s flowers before this year’s have even opened, and by the time bloom season is done, those buds are well advanced.
It’s difficult emotionally to cut off any of next year’s flowers, but if you continually avoid pruning, you may be even more devastated. In fact, you could end up having to cut the rhododendrons to the ground and start all over again.
Pruning rhododendrons requires imagination as well as sharp sheers. Dr. Richard Lighty, the director of the Mount Cuba Center for the Study of Piedmont Flora in Greenville, Delaware, advises that you try to visualize what the shrub should look like. Reach your goal by critically selecting strong outer branches that, when pruned back, will expose smaller inner branches in the right position to fill in. Once exposed to light, these inner branches will begin to grow.
As the flowers fade, trim no more than 15 to 20 inches off the strong branches. Where should you prune? Where the strong branch is near the tip of an inner branch that has a whorl of glossy leaves surrounding the buds, your signal that the inner branch is healthy. If the shrubs are still too big, reprune in two years.
To make sure the plant has stored enough food that it can easily handle pruning, fertilize in late fall the year before you intend to prune. If you fertilize after pruning, it will put out long, leggy growth.
Although it is better to stay on top of your regular pruning chores, rhododendrons can be cut down to 12 to 15 inches from the ground if necessary. The plants have buds at their base that will generally send up new shoots. But, as I learned, there won’t be any flowers for two to three years.