Proper planting is essential for rhododendrons. They are easy to grow if given the correct conditions. Here are some guidelines to follow:
Rhododendrons demand good drainage and plenty of organic matter in the soil. That rules out planting in clay soil. To test your soil for drainage simply dig a small hole and fill with water. If the water does not disappear in a few minutes, you have poor drainage. The pH must be acidic, between 5.0 and 6.0.
To improve heavy soil, loosen the existing soil by digging an area 30" wide and 6" deep. Add the following: 1 bushel sphagnum peat moss, 1 bushel well-shredded pine bark or well-ground leaf mulch and 1 cup Iron Sulfate. Mix thoroughly. Plant the rhododendron in the center of the mixture, taking care to plant it at the same level it was in the pot. Do not plant it deeper! Mulch with shredded bark. Take care not to mound the mulch against the plant.
If your soil is sandy, add 1 bushel sphagnum peat moss. Mix thoroughly with existing soil.
Consider exposure to wind and sun when selecting a site (especially if you are located right on Lake Michigan). The ideal spot for rhododendrons is in the shelter of mature evergreens, protected from the winter sun and wind. The worst situation for a rhododendron would be a west exposure in full sun.
When watering rhododendrons, soak the root ball thoroughly each time. A soaking once per week in cool weather and twice a week in very hot weather is all they need. Little dribbles of water each day is not beneficial and may even cause plant health problems.
Fertilizing once in the spring with fertilizer for acid-loving plants is all that is necessary for optimum growth. We like to use Espoma Holly-Tone. Apply as per package instructions.
As an added protection for the winter, spray a latex antidessicant on the foliage in November when the temperature is around 40 degrees. This will help the rhododendron retain moisture and reduce leafburn.