Rhododendrons are an evergreen shrub known for large, leathery leaves and big, colorful blossoms that bloom in the springtime. This shrub is related to the azalea, but grows much larger with garden varieties reaching up to 15 feet in height. Native to tropical regions, rhododendrons grow well in areas with temperate climates, such as the Pacific Northwest and the Northeast, but do not do as well in the Rocky Mountain regions or the Southwest.
Choose a location that gets full sun with some partial shade. In very hot climates, be sure that your plant is protected from the late afternoon sun, which can bleach or burn the leaves. Avoid full shade, which can cause lanky growth rather than a bushy appearance. Plant your shrub in early spring or early fall in cooler climates. In warmer regions, plant in the fall.
Prepare your soil before planting to ensure that it has good drainage and a pH of 6 or lower. Amend soil that is clay based or too alkaline with a pH of 6 or higher. Rhododendrons require light, sandy, well-draining soil. If the soil is too alkaline, add sulfur or ferrous sulfate along with 1 to 2 inches of well-rotted manure or other organic matter, such as pine bark or peat moss.
Dig a hole twice the width of the root ball and only as deep as the height of the root ball. Rhododendron roots are shallow growing, and planting the shrub too deep can kill it. Soak the root ball in a tub of water for approximately 30 minutes before planting if it is dry.
Loosen the outer roots of the root ball if they are entwined tightly around each other, which allows the roots to get good contact with the soil. It is not necessary to break apart the whole root ball. Do not remove the burlap bag unless it is not biodegradable.
Set the root ball in the hole, making sure the top of the root ball is level with the ground surface or slightly higher. Do not plant the rhododendron higher than it was grown at the nursery. Fill in the hole with soil, mounding it up slightly about 3 to 4 inches away from the base of the shrub to create a basin that allows water to drain to the roots instead of off to the side.
Water well after planting and continue to water twice a week for the first couple of years, until the plant is well established. After that, water once a week. In warmer temperatures, it may be necessary to water a little more often if the soil dries out completely between waterings. Too much watering can kills the plant by creating root rot.
Do not fertilize the first year of planting. Once the plant is well established, one or two years after planting, apply a fertilizer made for acidic soil in early spring. Do not apply fertilizer to rhododendrons in late summer or fall, as this can cause new growth that may not survive cold winter temperatures.
Mulch around the base of the plant with pine needles, bark or compost material. Mulch helps keep the soil acidic, and helps the soil retain moisture.