The below photos show identical plants, Halfdan Lem, both plants are in our own RhodoDirect garden and were acquired from another garden. The plant on the left we kept as it was when it arrived, the plant on the right we pruned. This photo was taken October 2011 and shows that we have a completely new plant that is tight and looks fantastic. Which one would you rather have in your garden?
Rhododendrons can be pruned easily and very effectively. You can prune young plants and old plants to create a better shape. And the great news is, there are no prickles!!
Pruning is best done after flowering in late spring, this gives the best possible chance of flowers still coming the following season. If you live in a heavy frost climate and you prune in summer/autumn, your rhodo may develop new growth from the prune mark that will get destroyed by a harsh frost. So best to stick to the late spring time for best pruning time.
Some people like to undercut their rhododendrons to create a taller tree-like shrub. Some prefer their rhododendrons to grow from as low to the ground as possible. There is no right or better option, this is completely your personal preference and choice.
When pruning, look for a growth bud on the stem that you are wanting to cut. Prune just above the bud and you will find that new growth will come from that bud. Look hard on old wood to find growth buds.
If you have a mature rhododendron we recommend pruning in thirds. 1/3 one year, 1/3 the next and 1/3 the following year. This ensures that the rhodo doesn't go into shock.