Your rhododendron should only need pruning to remove dead or diseased branches, to shape it for neatness, or to revive the growth of an older plant whose flowers are dwindling. Follow these tips to prune your rhododendrons effectively.
Step 1: When to Prune
In the late summer, prune after flowering has ended, and before new leaf growth is active. This will ensure that new flower buds will form at the pruned tips. Use slightly angled cuts, making the smallest possible wounds in the bark. The angled cuts will keep water out to prevent mold spores from infecting the bark.
Step 2: How to Prune
To prune for shaping, cut branches right above a leaf junction. Make a clean cut that does not crush the branch. To prune to reduce height, cut at the tips just below flower buds. The shrub will grow laterally and produce more blooms. To thin out the rhododendron in central sections, select branches a good distance apart in the center, and prune down about 6 inches, starting above flower buds. This will help new stems grow up rather than outward, allowing more light and air into the foliage.
Step 3: How to Prune Off Diseased and Dead Branches
Examine the branch to find out where healthy growth ends. Cut about 1/4 inch into the healthy branch stock below the dead or diseased portion. Dispose of these branches in the trash. Do not place these in your compost. Clean and disinfect your pruning shears after each cut with 70 per cent rubbing alcohol or 5 per cent hydrogen peroxide.
Step 4: Pruning to Revive the Rhododendron's Growth
As a rhododendron matures, its leaf and flower growth will start to diminish after 5 to 8 years. To revive it, give it a hard pruning. Prune the entire plant, removing all the branches and stalks to within 6 inches of the main root crown. This reviving technique works best with deciduous rhododendrons and those with rough bark, such as Rhododendron ponticum. The rhododendron will not produce flowers the next spring immediately afterward, but in the second year it will shoot out new stems, branches, leaves and prolific blossoms.
Step 5: Removing Flowers
There is no need to prune to remove the spent flowers. Simply grasp them at the base of the flower, above the leaf growth. Snap sharply between your thumb and index finger, and pull off. Rhododendron flowers are not poisonous so they can go in the compost pile. Remember not to use this compost to fertilize other rhododendrons in the future, as they can help spread disease vectors.
Step 6: How to Discard Pruned Rhododendron Foliage
Do not place even healthy pruned rhododendron leaves and branches in the compost, as the leaves and their waxy coating are poisonous if eaten by humans or pets. Put them into separate bags, into the trash. Always wear garden gloves when pruning, deadheading or transplanting your rhododendrons, and wash your hands thoroughly after finishing the task.