This is quite simple as far as smellies are concerned - DON'T! As has been explained already you will not improve the shape of the erect growing smellies by pruning as they simply re-grow a leader. Even more to the point is the simple fact that rhododendrons with smooth or peeling bark seldom if ever reshoot after having been pruned.
Generally Rhododendrons should not be pruned.
However there are some exceptions to the rule!
What you need to do is gently snip out the seed pod with a pair of scissors taking great care to avoid cutting any of the new growth at the same time. It is best to do this as soon as the flowers are over. If the seed head breaks off easily in your fingers no seed is being set so you are wasting your time and you can move on to another plant. You can very easily prove the effectiveness of what you have done as those twigs without a seed pod on them will produce double the amount of new growth as those which still have the seed pod present.
A. Topping the list for rhododendrons is, of course, honey fungus – otherwise known as the bootlace fungus.
Gardeners tend to assume that they have been unfairly selected to be the recipients of this life threatening disease. However, in reality, all wooded areas have it. Wherever there are decaying roots or stumps or where the roots of a plant are damaged perhaps in a gale, this naturally occurring fungus will jump into a live plant and kill very quickly. Honey fungus kills by invading the soft tissues of the plant below the bark and preventing the movement of water and proteins up and down the stem. A plant garotte if you like. The disease generally strikes in early summer when part or all of a plant will die in a matter of a few days. If you lose a rhododendron to honey fungus do not plant another in the same place.
Camellias do not seem to get the disease so plant one of these in the infected area instead. If prevention is your objective use copper sulphate in the planting pit or Armillotox if this chemical is still permitted in use. Like all chemicals of this sort it is very difficult to know if it has worked or if nature has just been kind!
B. Powdery Mildew
This is characterised by whitish mildew patches which then turn purplish brown but our smellies are seldom affected.
C. Bud Blast or Bud Drop
If a rhododendron drops its buds it is very probably the result of stress of some sort. Bud blast is a rhododendron disease but it affects Rh. ponticum and large flowered hybrids not smellies. If you see bud drop in your scented rhododendron it may just be that the plant is over flowering and nature is getting things back into balance or it may just be the effects of an early frost or, more probably, it is the plant saying that soil or weather conditions are such that it cannot support the amount of buds it has produced.
D. Weevils
Weevils may eat the leaves of scented rhododendrons but generally they prefer smoother leafed types. Again this is a naturally occurring problem in the garden which is unlikely to be life threatening.
E. Bark Split
This can be a serious problem with smellies in cold winters. Imagine a downpipe on your house which freezes and then splits. The same principle applies to the stem of a tender smellie rhododendron. The worst effects can arise when out of season frosts strike just when the sap is rising. Bark split may not kill the plant completely but probably will. Binding the stem with hessian matting is probably a waste of time but you could use a fleece to cover a particularly choice plant if you see the forecast of a later frost and remember to take action.
How to collect, store and plant woody shrub and tree seeds
Further reading on the Caerhays website:-
Rhododendrons in Old Park, Caerhays
Further reading and images in the Caerhays Garden Diary - Rhododendrons