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.
Terrie Moody says
My one rhododendron grew around 3 feet this summer after our neighbor cut down their trees. It’s now close to 8 foot tall. Totally gorgeous but one branch covers our walkway and arbor, when should i clip it and where?
You will have to figure out where to cut it (back to a junction with a branch coming off) but you can prune it any time.
Susan Thomas says
My rhododendrons are getting leggy and I’m new to this. Some branches are one long stick, brown old bark for most of it and green for the last few inches. I obviously have never cut them. To prune it back should I remove the whole branch or could there be buds in that old brown part? Also it seems thicker at the bottom with branches touching the ground. Should I prune those out?
I don’t want to lose them but they need work and I’m clueless.
Susan Thomas says
they also had some winter damage this year and have some brown leaves.
I have 3 PJM Rhododenderons, 2 are rather large (5-6 feet tall) and one is about 3 feet tall. They are all planted next to one another. While the 3 foot plant sends out new growth after flowering, the two large ones do not and seem to be losing more and more vigor and fullness each year, they do flower beautifully however. The two larger ones are also putting up a bunch of new growth from the base. I’m wondering if they may have reached their mature size and are now in a state of decline because of it. Would you recommend any type of pruning on the larger shrubs to get them to fill out or should I cut them down and let the new growth establish new shrubs. I hate to lose the height but I’m feeling the large ones will continue to decline.
Sophie Russell says
I have a Rhododendron in front of a big picture window and it covers the window if I don’t prune it. At first it covered the plant with flowers. Since I’ve been pruning I am getting few flowers on it. I would like to know how I can tell which branches to cut so I can get back like it was. I don’t know what kind it is. It was here when I moved here. It is a violet color. It did better this year because I didn’t prune it on top, but of course, it covers my window again and I can’t wait to prune them off. How can I know which ones do I prune. Do I leave the ones which have bloomed or not? It is very dense and would like to shape it smaller. The leaves are quite large.
RACHEL CRAVEN says
Prune right after flowering.
Paul Opalenik says
I have a very large, dense rhodo that is nearly 10′ tall and 10′ wide. It shot out another foot this spring. Almost perfectly round with lower growth only a foot off the ground. How should I prune this beast? Any idea how much it weighs and if it could be moved? The more I prune – the thicker it gets.
Just start cutting it back into the interior foliage like this post describes. You will have to be a lot more aggressive and go a bunch deeper into the shrub, say cut back some of the branches one to two feet into the interior of the shrub. Next year cut over branches back that far. You will gradually be reducing the outline of the shrub over several years. Patience will be key. If you get to aggressive and try to do it all in a year or two and you kill or disfigure the shrub.
Large Rhododendrons can be pruned into a tree also. Prune with snow or ice load in mind. Showing the trunk gives more interest than if it looked like an overgrown shrub.