Some friends of mine just bought an old house with a huge rhododendron up against the house. It is at least 8 feet high and probably 10 feet wide. I did not dig around and there may be multiple shrubs growing next to each other. What are the chances of moving the rhody successfully? Should it be cut way back before hand? Any particular time of year for moving it?
Fortunately, rhododendrons are very likely to succeed in being transplanted. Most experts recommend fall as the best time to transplant. Spring or late winter is second best.
The real challenge is getting a large enough rootball. A five-to-six foot plant requires a rootball of about 3 feet in diameter.
Step 1- dig a 12-18 inches deep trench around the rootball.
Step 2 – under cut the rootball to sever the roots from the underlying soil. The most important roots are the small feeder roots, not the big old ones. You can use a steel cable with a tractor or you can use a shovel and digging iron and a lot of hard work. The rootball will probably be about 8 – 12 inches deep and 3 feet in diameter.
Step 3 – tilt it on its side and slide a piece of 1/2-inch plywood under the rootball and set the plant upright. Use the plywood to move the plant to its new location. (A tarp works, too, if you can get it underneath the rootball.)
Step 4 – dig a new hole 4 feet in diameter and deep enough so that the rootball is 1 inch higher than the depth of the hole. (Slightly above grade)
Step 5 – water well and mulch around the perimeter of the plant BUT keep the mulch at least 2 inch away from the trunk of the plant.
Newly transplanted plants need some tender care and especially need to be watered regularly, but not over watered.
There were no recommendations to cut the foliage back. But it is always ok to prune out dead, dying, diseased or deranged stems. This also means you can prune out twiggy growth.
This information comes from Success with Rhododendrons and Azaleas by H. Edward Reiley (1992).