Rather than ignore your rhododendrons because you are not sure of proper pruning techniques, trim rhododendrons once or twice each growing season to turn it into one of the most attractive plants in your landscape. Left unrestrained, a rhododendron will eventually grow into a tangled mess that needs extensive rejuvenation. Shape a rhododendron with light trimming in late winter. Prune the rhododendron after it blooms to maintain the health of your rhododendron shrubs.
Shape the rhododendron in the late winter while the shrub is still dormant. Choose an outside branch, and follow it from the outside of the shrub to the inside of the shrub. Decide how much of the branch you want to remain on the shrub, and then find the last group of leaves you will keep on this branch. Clip the branch 1/4 inch beyond this group of leaves. Repeat these shaping cuts on all outside branches to create the shape you desire for your rhododendron.
Remove old growth and spent flowers after the rhododendron finishes blooming. Clip off the spent flower clusters approximately 1/2- nch above the point where the flowers cluster on the shrub. Examine the shrub for any diseased or unhealthy branches, and remove these at the point where the branch changes from healthy to unhealthy. Each time you make a cut, wipe the pruning shears with a cloth saturated with isopropyl alcohol to minimize the spread of bacteria from branch to branch on the shrub.
Restore a rhododendron shrub that you have left to grow without pruning by trimming it down during the winter. The easiest restoration pruning involves cutting back the entire shrub to within 6 inches of the soil level. Experiment to see if your rhododendron can survive this restoration pruning by cutting back one main branch to within 6 inches of the soil level. Cut approximately 2 feet from the other main branches. Watch the branch you cut back severely to see how it responds to the pruning. If it responds with new growth, rejuvenate the entire rhododendron shrub the following winter by cutting the entire shrub down to 6 inches above the soil. If it does not respond with new growth, do not perform any more drastic restoration pruning to the shrub.