Timing of pruning is absolutely vital to ensure flower for the following year. All pruning for Rhododendrons and Azaleas should take place after flowering in early to late summer.
Rhododendrons and Azaleas flower from buds produced on the end of the shoot. The shoot and the flower bud, start to form and then develop in early summer - soon after the current flowers have withered. The flower buds will only form on the end of a maturing shoot - normally from Jun-July.
New shoots are encouraged - mainly by simply dead heading the old flowers before the seeds start to set. Taking off the flower bud will ensure that all the growth resources on that shoot, will go towards growing the new shoos - and flower buds - rather than setting seed. The setting of seed withy Rhododendrons as with any other plant, takes a lot of energy out of the resources available.
Prune too late, and the resultant new shoots will not have enough
time in the year's growth cycle to produce flower buds - but will
instead form another but, which is simply a new leaf bud for next year.
If the shrub is hopelessly overgrown, then it can be cut back quite hard in April - but with the risk of losing the following year's flowers. - See below for rejuvenation pruning.
Azalea mollis showing seed-heads from faded flowers - normally in May/June.
A gently twist or snapping action easily breaks off the deed-head. Take care not to damage the surrounding new shoots.
The new side shoots will now develop into flower buds ready for your next year's display.