Rhododendrons (Rhododendron spp.) are deciduous or evergreen shrubs that produce showy spring flowers. They are generally hardy in U.S. Department of Agriculture plant hardiness zones 4 to 9 -- but this varies, depending on the species, cultivar or hybrid. Established rhododendrons do not like to be transplanted but it can be done successfully with proper preparation. The preparation should be done in steps over the course of six months to a year, though, so you won't have to devote an entire weekend to the project.
Root prune the rhododendron when you have a few hours of spare time, in late spring or in the fall after deciduous leaves have dropped. Measure the width of the shrub. Circle the shrub with a garden hose on the ground to indicate where the outer edge of the root ball will be. Move the hose 7 to 8 inches out from the trunk if the rhododendron is 2 to 3 feet wide, or 10 inches out if the shrub is 4 feet wide. Add 1 to 2 inches for each additional foot of shrub width.
Use a dirt shovel to dig a trench around the rhododendron outside the encircling garden hose. Dig to a depth of 1 foot for a 2- to 4-foot-wide shrub or 1 1/2 feet for shrubs that are 5 feet wide or wider. Put the soil back into the trench. Pour a generous amount of water over the loosened soil to remove air pockets.
Select a new planting site that has the rhododendron’s preferred sunlight exposure in the fall whenever a brief window of spare time presents itself. The soil at the new site must drain quickly. Test the soil pH. Mix iron sulfate into the soil, if necessary, to lower the pH to between 5 and 5.5. Use 1/2 pound of iron sulfate spread over 12 square feet of sandy or loamy soil to lower the pH one unit or, for example, from 6.5 to 5.5. You'll need more iron sulfate to change the pH of clay soil.
Work a 3- to 6-inch layer of sphagnum peat moss, compost or aged manure into a 3- to 4-foot-diameter area where the rhododendron will be planted. Add the organic matter at any time up to one month before transplanting the shrub. Use a shovel or tiller to mix it thoroughly into the top 1 foot of soil.
Transplant the rhododendron at any time in late winter or early spring after the ground thaws but before the shrub begins to put on new growth. Dig the planting hole twice the width of the rootball and 1 foot deep. Leave 3 to 4 feet of space between the rhododendron and neighboring shrubs or trees.
Use a sharp garden spade to sever any new roots that may have grown into the root-pruning trench around the rhododendron. Push the spade into the soil along the inner edge of the previously trenched area all the way around the shrub. Remove the soil from the trenched area. Push the garden spade into the soil below the shrub roots at a 45-degree angle to separate the bottom of the rootball from the soil.
Wrap heavy plastic sheeting around the rootball. Lift the rhododendron out of the hole by the rootball. Enlist the assistance of one or two other people when the rootball is larger than 1 foot in diameter. Rootballs that are 1 1/2 feet in diameter can weigh more than 200 pounds.
Measure the height of the rootball. Adjust the depth of the planting hole so the top of the rootball will be 1 to 2 inches above the surrounding garden soil. Set the rhododendron in the new planting hole and remove the plastic sheeting. Fill the hole halfway with backfill soil and settle it by pouring 1/2 gallon of water evenly over the soil. Finish filling the hole and pour another 1/2 gallon of water over the soil.
Place a 3-inch depth of mulch around the rhododendron to help retain moisture in the soil and reduce weeds, but do not place the mulch up against the trunk or stems. Leave 2 to 3 inches between the mulch and the trunk. Use pruners to remove the flower buds. This will allow the shrub to put energy into getting re-established.