Answer: Rhododendrons tend to become bare in the center as they mature unless they're faithfully pruned every 2-3 years. Rhodies bloom on the ends of two-year old shoots. It takes a full year for blossom buds to develop after a shoot has grown from a main branch or limb. Keep this in mind as you're pruning. If a branch is bare from the trunk to the tip, you can cut it back and it will develop leaves and shoots from leaf scars below the cut. It's best to prune your rhodie right after it has finished blooming. Decide where on that branch you'd like new shoots to develop and cut just above a leaf scar. New stems should develop that summer and flower buds should develop the following year. You can expect one or two new shoots to sprout on each branch you cut back.
Rhododendrons prefer soil that's on the acidic side. You can use an acidified fertilizer made especially for rhodies, with a low ratio of nitrogen to phosphorus and potash. (Nitrogen promotes green growth, sometimes at the expense of blooms.) Use a formula such as 8-12-4 to promote blossoms on your rhododenron.
Since rhodies take all summer to develop new stems and flower buds, I'd wait until after the spring blooms are spent before pruning it back.
Hope this clarifies things for you.
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