Crowding is the main reason for pruning rhododendron shrubs. These plants exhibit a growth habit that causes the full foliage on the outside of the plants to shade out the inner growth. Without pruning, the inner branches weaken and die from lack of sunlight. The crowded foliage also restricts airflow around the leaves and branches, increasing the risk of fungal diseases. Old rhododendrons form thick, woody limbs that create uneven and unattractive growth. Regular trimming helps maintain the desirable size and shape of rhododendron bushes.
The best time to prune a rhododendron is immediately after it stops flowering. Rhododendrons form blossoms on the prior year’s woody growth. Pruning too heavily or removing the branches before the buds begin to form in the spring won’t damage the rhododendron plant, but it can reduce or eliminate the flowers for the year. If pruned heavily, the shrub may require two years of growth before it blooms again.
Rhododendrons form growth joints that denote where the previous year’s growth left off and the current year’s growth begins. Pruning just above this joint removes just the most recent growth and allows the plants to produce blossoms on the prior growth. Selecting the most crowded branches for removal helps open up the center of a shrub and increase airflow and sunlight exposure.
Pruning lightly to remove no more than one-quarter of the new growth is sufficient for younger plants that are beginning to establish themselves in the landscape. Older, woody plants may require more extensive pruning that reduces them to half their normal size. New growth appears near the stumps and eventually replaces the pruned sections.