Because the roots grow typically within 2 or 3 inches of the surface, a bed prepared especially for rhododendrons and azaleas need not be more than 12 inches deep; deep planting or too much mulch in the growing season keeps the roots from getting the air they need. In fact, it is a good idea to set rhododendrons about 1 inch higher than they grew at the nursery. Balled-and-burlaped plants may be planted while in blossom but it is better to transplant them when they are dormant.

Azaleas will not survive in wet, poorly-drained soil. Avoid planting them in depressions where water may puddle after rain or near downspouts where they experience wet/dry fluctuations in soil moisture. Dig a hole the depth of the pot or root ball and fill it with water. If the hole drains within an hour you have good drainage. If the water has not drained out of the hole within one hour, the soil is poorly drained and you must correct the drainage problem before planting. Install a perforated pipe or drain tile in the garden, making sure that the outlet is lower than the bottom of the planting hole, or build raised beds.

Turn the soil well and dig a hole two or three times as wide as the root ball. Add plenty of organic material, remove the plant from its container and loosen the root ball. Water the pot thoroughly before planting and tease the soil away from the roots on the outside of the pot. Don't worry about injuring the roots it's more important to remove a significant amount of the potting soil than it is to keep every root intact. Planting depth is critical because azaleas are shallow-rooted plants. In sandy soils, set the root ball in the hole so the top is about 1 inch above the surrounding soil grade. In clay soils and poorly drained soils, place the top of the root ball 2 to 4 inches above the soil grade, gradually sloping the soil to meet the original grade. This allows for settling and assures that the roots will be in the upper layer of soil where they can readily obtain oxygen, water and nutrients.

Set the root ball into your prepared hole (making sure the top 1-2 inches of the ball is above the soil level), pull in your humus/organic soil around the plant, pack firmly and cover with mulch. Finally, water the whole area thoroughly and apply a thin layer of shredded leaves, pine needles, or pine bark to keep the soil cool and moist. Water your newly planted rhododendron or azalea weekly if the weather is dry, at least for the first year.

Most rhododendron and azalea plants sold at nurseries and garden centers are sold in containers or have a root-ball that is covered with burlap. These plants have a potentially serious problem when the roots reach the container and start circling inside the pot. They become pot bound or root bound. These roots must be cut so they don't continue to grow and start strangling other roots. Many apparently healthy plants die when the roots start strangling each other. To prevent this, it is necessary to remove the plant from the container and and examine their roots. If the plants appear pot-bound and have a thick, dense mat of fibrous roots along the surface of the root ball, used a knife to make three to six vertical cuts, about 2 inches deep, equally spaced around the sides of the root ball. Then use your hands to gently loosen the roots where cuts were made and pull the roots outward. This process stimulates new root growth and allows water and nutrients to penetrate into the root mass. If the roots are not pot-bound, it is not necessary to slice them with a knife, but it is beneficial to loosen and pull them outward with your hands. When working with roots, make sure the plant is thoroughly watered. Any roots that dry out will die.

If the plant has been transplanted several times it may be necessary to remove all soil so that previous areas that were pot bound in side the root ball when it was planted in smaller containers my be exposed. All roots must be separated so the only go in one direction and do not circle. Those that cannot be straightened must be cut. If allowed to circle other roots, they will strangle the other roots as they grow larger. When working with bare roots of a plant, keep them moistened. Any roots that dry out will die.

Do not fertilize at the time of planting, as this might injure the roots, but water deeply. Also, if newly purchased plants have little white BB-like balls in its container don't even think of fertilizing for a while. The little balls are fertilizer and those plants have been pushed to the hilt. Plant them as soon as possible and let them adjust. Plants that have been given a soil mixture rich in organic matter probably will not need feeding for several years. Wait until the plants are established before fertilizing them.

Apply 3 to 4 inches of an organic mulch such as pine straw, pine bark mini-nuggets or shredded leaves on the surface. Use your hands to pull the mulch away from the trunk an inch or two. This helps keep the trunk area dry and reduces the chances of wood decay. It also discourages rodents from gnawing on the trunk. Organic mulches gradually decompose and provide nutrients to the plants.

Whether the plant was balled-and-burlaped or potted, make sure that the plant is getting wet. Rhododendron guru Harold Greer noted: "Quite often a plant will get completely dry and then no matter how much water you apply, the rootball will just keep shedding it. The top of the soil may seem wet, and the soil around the plant may even be very wet, but the actual rootball of the plant is bone dry. This is especially true for newly planted rhododendrons, and it is the major reason for failure, or at least less than great success with that new plant. It is hard to believe that a plant can be within mere inches of a sprinkler that has been running for hours and still be dry, yet it can be SO TRUE!"

Here is a link that might be useful: How to grow rhododendrons and azaleas.