With thousands of varieties in cultivation, the rhododendron (Rhododendron spp.) offers gardeners multiple choices for color, shape and size. Depending upon variety, this woody evergreen grows in U.S. Department of Agriculture plant hardiness zones 4 through 10. Without regular pruning, a rhododendron will become overgrown, creating an eyesore in the garden. To bring the shrub back to a reasonable size and pleasing shape without causing shock or death, you will need to follow a renewal pruning process over the course of at least three years. Prune the rhododendron immediately after it finishes flowering, in the late spring or early summer.
Pour one part rubbing alcohol and one part water into a small bucket. Submerge the blades of pruning shears and loppers into the alcohol solution. Soak the tools for five minutes to allow the solution to disinfect the blades. Let the tools air dry before making any cuts.
Cut out any dead, dry or leafless branches from the rhododendron plant. Use the pruning shears to cut branches with a diameter of 1/4 inch or less and the loppers on branches with diameters up to 1 1/2 inches. Make each cut 1/4 inch above the branch's base, avoiding cutting into the main stem or trunk to which it connects.
Remove any broken, cracked or split branches, cutting them back to at least 1 inch below the damaged section. Make each cut 1/4 inch above any outward-facing lateral branch or bud.
Cut back any suckers emerging from the trunk or ground below the graft union, or the bumpy knob located on the base of the rhododendron's main stem. Cut off the trunk suckers as close to the trunk as possible without slicing into the trunk, and cut the ground suckers at soil level.
Deadhead the spent blooms on the remaining rhododendron limbs. Cut each flower off at the base of its stem. Remove any weak branches that cross or rub against other limbs, cutting each one back to its base.
Remove the rhododendron's oldest branches, cutting them back to their bases. Cut out only as many branches as needed to bring the total amount of removed limbs to one-fourth or one-third of the plant's growth. Never remove more than this during a single pruning.
Cut the removed limbs into shorter, more manageable pieces. Place the limbs in a trash bag. Pick up any fallen leaves and spent flowers, placing them in the bag with the branches. Discard the bag of plant material.
Thin the new stems growing from the pruning cuts two to three months after the initial pruning. Cut back one-third of these stems to prevent the rhododendron from becoming more crowded.
Repeat the entire pruning process in one year, removing another one-fourth to one-third of the rhododendron's growth. Thin the new growth two to three months later.
Prune the plant in this same manner for a third year in a row. Cut back the top one-third of overly long, leggy or vigorously growing branches. Make each of these heading cuts 1/4 inch above an outward-facing lateral branch or bud.