Pruning a Rhododendron
Here is a review on the pruning of a common foundation planting found around homes in Maine, the rhododendron. Pruning a rhododendron is normally done for maintenance, shaping, and rejuvenation.
Between the first frost in fall and the last frost in spring will work for trimming rhododendrons. As the flowers diminish, prune no more than 15-20 inches off the most resilient branches. Cut back the plant to uncover the inner branches. Follow the branch down to the last whorl of leaves you want to keep and cut just over those leaves, about 1/4 inch above the topmost leaf in this cluster.
Large, unkempt rhododendrons can be cut 12-15 inches from the ground when necessary. Rhododendrons often have three or more central branches rising from the crown of the plant. Each of these primary branches should be cut at differing heights to produce a more natural-looking shrub. Cut about 1/2 to 3/4 of an inch just above a dormant bud. Pruning above a cluster of two or three buds is even better.
Sometimes more severe pruning may be required, cutting to about 6 inches or so from the ground. Their adventitious buds at the bottom of the plant will send up new shoots, but keep in mind that flowering usually won’t occur for up to 2 or three years after this substantial pruning.