All rhododendrons, including azaleas, which are a type of rhododendron, bloom on flower buds formed the previous year. This means that unless you prune right after the plant has finished blooming you will reduce the number of flowers you get the following year. If you must severely prune your rhododendron, say to reshape an overgrown plant, do so in late winter while it is dormant. You will sacrifice flowers but your rhododendron will recover more quickly.
Prune to reshape your rhododendron or rejuvenate older plants in late winter while the plant is dormant. Lighter pruning to maintain the shape occurs after the plant has finished blooming and the flowers (called trusses) have faded.
Use pruning shears or loppers for the late-winter pruning. Make cuts 1/4 inch above leaf clusters. Remove branches that are crossing or rubbing other branches or are laying on the ground. Step back and look at the overall shape of the plant several times while you are pruning. This type of pruning is only necessary if your plant is lopsided or overgrown and does not have to be done every year.
Use pruning shears when removing faded flowers. Make cuts at the base of the flower truss about 1/2 inch above new growth. Pruning after flowering helps the rhododendron maintain its overall shape and can negate the need for a late-winter pruning.
Remove dead or diseased branches at any time of the year. Make cuts one to two inches into healthy wood. Make cuts 1/2 inch above a healthy latent bud. Latent buds are the pink spots on rhododendron branches and trunks. New growth will emerge from the latent bud the following year.