How to Plant a Rhododendron
Field grown rhododendrons come in burlapped root ball and can be planted without removing the burlap. Container grown plants come with their container mix. Remove the plant from the container. They are frequently root-bound, with the root circling the pot's interior and tightly packed. These roots must be evenly cut about 1/2" deep from top to bottom, to encourage new root growth. Use a knife or a trowel to make three or four vertical slits evenly spaced.
The fine shallow roots of rhododendrons require aeration and moisture and cannot survive in "good topsoil". The soil, be it clay, sand or "good topsoil" should have composted leaf mulch, garden compost or peat moss mixed into it (60% compost or peat moss: 40% soil). The compost or peat moss will help the soil retain moisture as well as providing aeration for the hair-like roots.
Dig a hole at least twice the diameter of the plant root ball and about half as deep. Place the ball as shown in the diagram. Fill hole with the amended soil making sure there are no air pocket. Do not cover the top of the root system eith more than a half-inch of soil mix. Create a raised ring of collar of soil to form a dike to capture water. Water thoroughly.
For more information on planting and subsequent care and maintenance we recommend "Rhododendron & Azaleas: Care and Culture", a booklet compiled by the experiences members of the Greater Philadelphia Chapter of the American Rhododendron Society.