Rhododendrons, depending on the variety, grow in most areas of the United States except for the desert Southwest, Arizona, Texas, Southern California and New Mexico. They will not grow in parts of states within the Rocky Mountains. The rhododendron prefers loamy soil and humid climates. The soil should somewhat acidic (pH 5 to 5.5). Rhododendrons are not subjected to viruses, fungi and pests if they are kept healthy.
Dig a planting hole as deep as the root ball and twice its circumference. In areas with clayey or sandy soil, make the planting hole three times the circumference of the root ball. Amend the soil with 50 percent compost for clayey or sandy soils.
Scarify the sides of the planting hole with a pitchfork. For rhododendrons that are potted or balled and burlaped, fill the planting hole with water. Center the rhododendron in the planting hole and backfill with soil. If the rhododendron is bare root, center the plant in the planting hole, arrange the roots at the bottom of the planting hole and fill with water. Backfill with soil.
Create a watering ring around the perimeter of the planting hole. The watering ring helps divert water to the outside roots of the rhododendron and helps to conserve water. The watering ring can be leveled out after the first year, if desired. Mulch with 3 inches of pulverized bark. For smaller plants, mulch with 1-½ to 2 inches of pulverized bark. Apply 1 inch of water.
Water the rhododendron every week with at least 1 inch of water. Watering deeply promotes root growth and healthy plants. Fertilize the shrubs with flowering tree and shrub fertilizer each year in the spring. Always follow the instructions on the package of fertilizer, as different brands have different ingredients and strengths.
Remove dead and decaying plant and wood matter every spring. Prune for dead and decaying wood throughout the growing season. Stop pruning the rhododendron six weeks before the first frost. Deadhead the plants just after the flowering season. This ensure that new buds form the next year. When the flowers are spent, they should be removed.