Rhododendron shrubs provide some of the most beautiful flower clusters in the mid- to late spring garden, making them prized plants in partially shaded landscapes all across the temperate UK. With age (some live 80 years) or limited light, rhododendrons can become leggy. lopsided or misshapen, with long, barren branches, few leaves and few flowers. These shrubs rejuvenate if given a severe pruning, which encourages dormant buds to sprout and develop new, vigorous growth.
As long as an old, leggy rhododendron is not weakened by a disease, it tolerates and responds very well to a severe pruning, more commonly called a rejuvenation pruning by horticulturists. You can cut back woody branches into areas where no leaves or lower twigs exist. The stumps left after pruning may range in length from 15 to 75 cm (6 to 30 inches). Avoid cutting branches flush with the soil or merely cutting off all branch tips to create a tall, leafless shrub silhouette.
Rhododendron pruning usually is conducted immediately after the flowering season ends, but no later than mid-June. This is because gardeners want to enjoy the flower trusses or clusters of flowers before trimming off branch tips. If the rhododendron is large and overgrown or already looks unattractive and flowers poorly, it's best to severely prune the plant in early spring. This eliminates any potential for flowers that year, but gives the plant the longest growing season to develop new growth and mature before the onset of autumn frosts and freezes.
Some gardeners rejuvenate rhododendron shrubs by cutting back only one-third of all branches in each of three subsequent years. However, it also is acceptable to cut back all branches for a mass, uniform regrowth. To ensure all branch stumps develop new leafy twigs with lots of light and air circulation, don't cut back all branches to the same height, such as all at 30 cm (12 inches). Stagger some branches so the new growth creates a multitiered mass of leaves. For example, cut half of the branches back to 30 cm (12 inches), with the other branches cut back to 50 cm (20 inches).
Once the dormant buds on the rhododendron stumps flush out new leafy twigs, evaluate the quality of the growth and thin them out. By midsummer, cut out tight thickets of twigs atop stumps, retaining three to five of the most vigorous shoots. Choose shoots that are evenly spaced on the stump. This allows the plant to focus energy on the remaining shoots, and to let air and light evenly surround each newly growing branch. Do not cut off any tips of the branch shoots for the next two years.
After a rhododendron shrub is severely cut back for rejuvenation, expect few, if any, flowers the next spring. In the second year after the harsh pruning, many more branch tips will produce flower trusses and by the third year, the shrub should be full and lush, producing numerous and widespread flowers all over.