Pruning Rhododendrons - Knowledgebase Question

Beckley, WV
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Question by rwpeters
June 30, 1999
When is the best time to trim a Rhododendron bush? Also what kind of fertilizer is good to use on them?

Answer from NGA
June 30, 1999
Rhododendrons tend to become bare in the center as they mature unless they're faithfully pruned every 2-3 years. Depending upon how old your rhodie is, you can lightly prune the tips of the branches, or you can climb in and prune your way out. Rhodies bloom on the ends of two-year old shoots. It takes a full year for blossom buds to develop after a shoot has developed 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 not a good idea to prune more than one-third of the live plant material in any one year, so you may have to divide your shrub renovation into a two or three year project. Wait until your rhodie finishes blooming this spring before you prune. Then, with your eyes, carefully follow each branch from tip to trunk. Decide where on that branch you'd like new shoots to develop and cut just above a leaf scar. New stems should develop this 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.

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