Answer: Rhododendrons need an acid soil that is humusy, evenly moist yet well drained, and protection from wind, especially in the winter.
Clay soil is too heavy for them to do well. Amending the soil in an individual hole will tend to cause water to collect in the bottom of the hole since the improved soil drains faster than the native clay. You may notice that the improved soil you used before has now settled over time and allowed the shrubs to sink too deep. The plants are shallow rooted and their roots are sensitive to a lack of oxygen, so a lower planting depth and/or poor drainage can cause them to die. You might also find that the roots have grown up into a mulch layer if the plants have more than a few inches of mulch over their roots. This situation exposes the plants to a larger risk of cold and drought damage than if the roots were growing through real soil. A soil that is allowed to dry out can also stress them. Another source of stress may be encirciling roots, as will happen when plants are rootbound in their pots and their roots continue to grow in a circle after they are planted.
You might try planting them in raised beds of improved soil so that it drains evenly. This way you can provide a light soil high in humus and avoid the drainage problem. In addition, be careful not to plant the shrubs too deeply; also be sure to correct any encircling roots at planting. Water carefully so that the soil remains evenly moist yet not soggy. Your county extension should be able to help you with testing and amending your soil so that it is more suited to the rhododendrons.
Finally, if you are planting in the same spot over and over or if the problem is widespread, there is a possibility of a soil borne disease problem. Your extension (241-4940) should also be able to help you determine if that is the culprit.
I hope this helps you troubleshoot.
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