Large rhododendrons are easy to transplant. In moderate temperatures, transplanting can take place at any time, but in areas with cold winters, the ideal time to transplant a rhododendron is during early spring. In areas that have exceptionally hot summers, transplanting is best during late summer or early fall. A large rhododendron that makes its home beside an old tree might be difficult or impossible to successfully transplant, because the two species' root systems will have become deeply entwined over the years. Avoid transplanting such a shrub so no damage is done to the tree or the rhododendron.
Choose the new planting location for the old rhododendron prior to digging it up. Dig a hole that is twice as large as the rhododendron's root ball. Add ample peat moss and aged manure to the new hole. The soil should feel crumbly to the touch.
Dig gently around the old rhododendron. Use a spade to determine where the root system of the shrub ends. If the rhododendron is 10 feet tall, then dig 5 feet out from the base of the shrub. If the shrub is 8 feet tall, then dig 4 feet out from the base of the shrub. Most root systems grow outward to half the height of the top foliage of the rhododendron.
Insert the spade gently around the rhododendron, and angle it inward. Insert and lift. Gently pry so the root system is loosened and not broken.
Circle the rhododendron with the spade over and over again. Each time push the spade deeply into the soil and pry upwards. Rock the spade in and out to loosen the dirt. Soon you will feel the rhododendron break loose.
Roll the root ball out of the hole. Grasp the rhododendron by its base and by the root ball. Gently place it in a wheelbarrow, and roll it to its new transplant site.
Lift the rhododendron out of the wheelbarrow, supporting its root ball and base. Place it in the newly dug hole. The new soil level and the old soil level should be the same. Do not plant the rhododendron higher or lower than it was in its previous site. Fill the hole in with the dirt and organic matter. Tamp the soil down so that no air pockets remain.