Magnificent and stately when in full flowering glory, rhododendrons can attain snake-like branches with architectural interest and small tree stature. Unfortunately, old specimens can become misshapened, ugly and even lose vigor so to have little foliage and few flowers each spring. Rejuvenating pruning, a harsh cutting back of branches, can be conducted over a two-to-three-year period, to bring renewed vigor and robust shape to tired, old rhododendrons.
Remove all dead or diseased branches and twigs from the shrub with hand pruners or loppers. Make the cut 1/4 inch above a connecting joint with an alive branch or leaf. Use loppers on branches larger than 1/2 inch in diameter, and use a pruning saw on those even larger.
Evaluate the rhododendron to determine which of its old branches seem more robust or healthy. Compare the stem bases to find the ones that tend to be larger and have more foliage on branches. Check if the shrub is lopsided because of one large stem that overshadows opposite-sided smaller branches.
Count the number of healthy branches on the shrub. Remove a third or half of these branches one spring, and the remainder in the next year or the following.
Cut the branch above a growth joint that you can find by tracing the branch from its tip downward to the base, looking for small, alive branches with leaves, buds, or branch scars on the lower 12 to 24 inches.
Make an even, straight cut into the stem, with the appropriately sized pruning instrument, loppers or hand saw. Cut 1/2 to 1 inch above a firm, healthy, side branch on the stem or flush across the branch at a height in-between 12 to 24 inches above the ground. Feel free to raise the cut height to 30 or 36 inches, if desired.
Hold the branch firmly with your spare hand as you saw. As the cut extends over half-way through, shorten and speed-up your cutting strokes and make certain you are keeping the upper branch stable and secure. Ask a helper to hold the upper branch if needed. You want to avoid cutting the last bit through the branch only to have it snap and tear bark downward, across the lower branch area that you wish to retain.
Stagger the pruning heights, if possible, on the different branches, so there is some variation in stumps. This way, not all the new growth with be sprouting from the same height, which will allow for increased light and air to reach it. The severely cut-back branch stumps will re-sprout new foliage in the following summer and autumn months.
Cut more branches as needed in the next one or two springs. By the end of the second or third year, the rhododendron will be sprouting new, lush foliage in a smaller, more robust form with all old, leggy branches removed. Flowering will resume shortly as the new growth matures and lengthens.