They can NEVER be sheared to form a hedge, except for a very few tiny leaved varieties and for the evergreen azaleas used a lot in formal Japanese gardens.
Usually trying to prune a rhododendron to control its size is not very successful. If it is too large for the space it is better to move it and replace it with a smaller variety.
However, one can (and should) remove dead and broken branches at any time.
It is also ok to remove (before it gets too large) the occasional errant branch which threatens to destroy the overall shape of the plant.
It is also ok, and often advisable for young plants, to pinch out the terminal leaf bud on new shoots. This causes the secondary buds below the terminal one to develop into shoots and produces a more bushy plant. This can be done at any time, but is probably best done as soon as possible after the new shoot forms, and you can tell that the terminal bud is a leaf bud and not a flower bud.
How rhododendrons produce flowers and branches:
Flower buds for most rhododendrons are produced in the late summer for blooms in the following spring. Thus any pruning done past mid summer will probably sacrifice the flowers for the following spring.
Most rhododendrons have growth buds under the bark, even on relatively mature branches. On some varieties one can stimulate new branches from these buds by exposing that part of the branch to the sun or by cutting it off just above where you want the new branches to form. This doesn't work for all varieties though, so one must be careful. Smaller leaved varieties seem to form new shoots more readily than larger leaved ones.
When you can't move the plant and just HAVE to do something:
Here are some suggestions that work some of the time for some varieties - but sometimes end up destroying the plant.
When we moved our garden from Vancouver to Abbotsford we had a lot of rhododendrons which were very leggy, as they had been very crowded in Vancouver. Some of them, particularly small leaved varieties, once they were replanted where sun got to the lower trunk, developed lots of new shoots from the dormant buds in the trunk. It was then possible over a period of a couple of years, to cut off the original leggy tops, and end up with fairly nicely shaped plant built from the shoots coming from the base of the original trunk. However some other varieties, even now after ten years, haven't developed a single new shoot and are just as leggy as they were when moved. So being able to rejuvenate a plant this way depends very much on the variety.
Sometimes when a large rhododendron has many stems it is possible to remove some of the oldest ones and thus stimulate the younger and smaller ones. This should be done over a period of several years, removing only one or two main branches each year.
In a few cases in frustration I simply chopped the plant off a foot from the ground. In probably 1/3 of the cases this stimulated new shoots and in several years I had a new and better shaped plant. However for the others, it killed the plant, so don't try this for plants which you care about and don't have backups!
Fearing's Farm Nursery
