Rhododendrons are evergreen flowering shrubs that produce abundant blooms in the spring and very early summer. Related to and often confused with azaleas, rhododendrons prefer partial to complete shade. Soil amendments and the sparing use of acid-rich fertilizer are usually sufficient to meet the shrub's needs and keep it producing masses of blooms year after year.
Site your rhododendron where it will receive partial daily shade or full daily shade. In warm and/or dry climates, midday and afternoon shade is preferred to reduce the chance of sun burn and drought stress.
Plant your rhododendron in well-drained, nutrient-rich, acidic soil that can hold uniform moisture without becoming soggy. The optimal soil pH for rhododendrons falls between 5.0 and 6.5. Have a soil test conducted if you are unsure of the soil pH in your area. If it is too high, amend the soil with aluminum sulfate at the recommended rate to lower the pH to within the desirable range.
Water your rhododendron regularly to keep the soil evenly moist but not soaking wet an inch or two down into the soil. Never allow the soil to dry out completely between waterings. To prevent bloom rot or mildew, apply water just to the roots and drip line and not over the canopy of the plant.
Feed your rhododendron once or twice a year with an acid-rich fertilizer designed for rhododendrons, camelias and azaleas. Apply the fertilizer around the root-ball and drip line of the shrub according to the label directions, not exceeding 2 pounds per 100 square feet. Once the fertilizer has been applied, water the rhododendron deeply to drive it down into the soil toward the roots.
Mulch around the base of your rhododendron each year with an organic material such as shredded bark, leaf mold or compost. Mulch holds moisture in the soil, keep weeds from sprouting, and increases the nutrient value of the soil as it breaks down over time. Lay down a layer of mulch 2 to 3 inches deep starting a few inches out from the trunk to well past the drip line of the shrub. Replenish the mulch each year as it blends into the soil.
Prune your rhododendron as needed in the late spring or early summer immediately following bloom. Rhododendrons set their buds in the mid to late summer, so pruning immediately after flowering preserves the maximum amount of bloom wood for the following spring. Use pruning shears or loppers to control the shape and size of the shrub, placing all cuts 1/4 inch above an outward facing leaf node or bud to encourage business and branching.