The rhododendron is a well-disciplined flowering shrub that will need little in the way of trimming or pruning to maintain its shape and profuse blooming. On the other hand, if your rhododendron is more than 3 years old, a severe pruning back to within a few inches of the ground will tend to revitalize it, shooting new branches loaded with buds outward once more. The following provides a guide to the best times and methods for pruning and trimming rhododendrons.
Step 1: When to Trim a Rhododendron
Trim back the rhododendron immediately after flowering in the summer. Do not delay, as the plant will send out new leaf growth that will be weakened by trimming. Late July to mid-August is the best time, so that the plant will form new flower buds instead of more leaves. The new growth will overwinter well in the interval before frost arrives. The Loderi King George rhododendron, among many other varieties, responds well to this type of trimming and can also revive from a major cutback to primary stems.
Step 2: Where to Trim a Rhododendron
Cut back the stems just a few inches into the older wood growth. This will induce the formation of several buds per stem, which will increase the bushy appearance of the rhododendrons. Massed blooms on these flowering shrubs make them look their best. Check annually for the overall growth of the plant and trim any weak or crowded branches.
Step 3: Removing Diseased or Dying Branches
Trim back the oldest-growth branches that have died in the fall, as the plant begins to go dormant for the winter.
Rhododendrons are occasionally subject to spore-based molds such as Phomopsis and Botryosphaeria. These can cause branches to die back close to the root. Preventing wind drying and watering generously during any drought periods will stop these infections from taking hold. Wear gloves and trace the dying branch back to healthy growth. Cut below the infected section into a clean woody twig, with the smallest possible cut, between buds. Use very sharp, disinfected pruning clippers to remove these branches, destroy them, and put them in the trash. Disinfect between each trimming cut with rubbing alcohol or 3 per cent hydrogen peroxide. Chlorine-based disinfectants can corrode garden tools. Trim out diseased branches as soon as you notice them, but closer to the end of the growing season is most effective in preventing recurrence of the infection.
Step 4: Trim for Ventilation
Rhododendrons will only need this type of trimming in the middle for ventilation if they have grown tall, over 4 feet high, and are bumping into trees or shrubbery nearby, or are beginning to overgrow a into a window or entryway. Cut the branches with a slightly angled cut just above the end buds. This will reduce the thick bushiness of the foliage and allow adequate light and air into the heart of the plant.
Trim or prune your rhododendrons to maintain their shape, redirect their growth, induce more flowering at end tips, and eliminate diseased sections.