Mark recently sent me the following question regarding pruning rhododendrons via my Ask me a question page: “I have a number of large leaf Rhododendrons in my garden (Zone 5). “How do you prune them so that they stay compact and full?”
I am assuming that the Rhododendrons that Mark has are the Catawba types. These include standbys such as ‘English Roseum’, ‘Roseum Elegans’, and ‘Nova Zembla’. These are the most common type sold in the Midwest, besides the smaller leafed PJM types. They typically flower in late spring. The ones I’ve had flowered in early to mid-May in the Chicago area.
If your plants are not yet too big and you just want them to grow denser and not get much larger, it’s a pretty easy process. If not, watch for a future post when I will cover renovating overgrown shrubs.
PRUNING RHODODENDRON TO A PROFILE
The 1st thing you want to do is prune any branches that are sticking out of the outline profile of the plant.
Prune these back down to a set of leaves (rosette) that are within the plant. This is better than pruning right at the outline of the profile of the plant. This will help to keep the plant smaller longer. It will also help to hide the pruning cut.
Always make the cut about a quarter of an inch above the rosette. If no leaves are on the stem within the plant, cut it back to a lateral branch on that stem. If there are no laterals you can try to cut back to just above a dormant bud on the stem. These are little green bumps along the stem.
It may be hard to see the dormant buds of some varieties. For these plants prune the branches back to the preferred level and after the new growth emerges, prune off any remaining stubs. This should be done in early spring before flowering and new growth has begun. Note some plants may not sprout from stubs, so only do this if you have to. Also don’t do this on too many branches till you know how your plant will respond.
REMOVING SPENT FLOWERS
The next thing you will want to do is to remove the spent flowers after the plants are done flowering. It is key to remove flower stems as soon as flowering is finished. If you don’t do this it can reduce flowering the following year. Carefully cut or twist and snap off the flower head, leaving the small emerging buds at the base of the flower. Note this step is NOT necessary on PJM type Rhododendrons.
PRUNING RHODODENDRONS BY PINCHING BACK NEW GROWTH
The final thing you want to do is to pinch or snap off the new growth back when it’s a couple of inches long. This is the key step to developing the dense bushy plant that you are after.
These plants often send out a single long new shoots with no branching. The terminal leaf bud produces a hormone that slows the growth of dormant buds around its base. Pinching this terminal bud off lets the dormant buds grow, producing multiple branches. This is especially important if just one shoot is growing at the end of a branch.
Branches with two or more shoots already growing can be left as they will probably not be long size shoots. Be sure not to pinch the larger flower buds or you will lose those flowers.