Rhododendrons have shallow roots, as do most maples. Together, the two will compete for the same water and nutrients, so planting rhododendrons under shallow-rooted maples is not a good idea. Therefore, rather than planting your rhododendron under just any maple tree, make sure you choose one that has deep roots. Japanese maples are good maple trees under which to plant rhododendrons. When planting Japanese maples and rhododendrons together, make sure the lower-growing rhododendrons get enough light through the maples to ensure numerous blooms and full-bodied plants.
Dig a hole that is two times as wide as the rhododendron’s root ball or container but just as deep. If you are hitting roots, move out to the edge of the maple’s canopy.
Incorporate 25 to 50 percent peat moss to the soil you just dug out.
Take the rhododendron out of its container or remove the burlap root cover. If the bush from the container has roots growing in circles, take a utility knife and cut six to eight 1-inch, even, vertical slices along the sides of the root ball.
Set the shrub in the hole and use a hose to spray about a third of the soil away. Then, spread out the roots along the bottom of the hole and fill the hole with water. Wait for half of the water to drain.
Backfill the soil in the hole that is half filled with water. According to Washington State University's Spokane County Extension, you should not tamp down the soil, which may harm the rhododendrons roots. The water and soil together will help get rid of air pockets. Add more soil if necessary as it settles.
Mulch the area with 2 inches of bark or pine needles.