Preventing the deterioration of an established rhododendron demands a variety of strategies. Symptoms of stress, which may develop over time, include drooping or discolored leaves, damaged or decreased bloom, and branches that appear dead. Causes include insufficient water, often caused by hard-packed ground; poor drainage; fungal disease; and inadequate nutrition.
Examine drooping leaves for signs of inadequate water. Probe the soil around your rhododendron to a depth of 4 to 6 inches, taking care not to nick roots with your trowel. If soil is hard to dislodge, it may have packed hard over the course of time. Pour or spray water around the roots; lots of runoff confirms hard-packing, which inhibits roots from absorbing water. Use your hand fork to work compost into the top 4- to 6-inch layer of soil, and mound additional compost around the roots. This will help aerate soil, to let more water reach roots. Check seasonally to prevent repacking.
Look for discoloration on leaves to diagnose possible disease. Rhododendrons are prone to a variety of diseases, mostly fungal. Removing diseased leaves will help determine the cause (take a clump of these leaves to your local garden center to obtain the correct fungicide). At the same time, prune branches that may be crowding the plant, inhibiting air circulation. Fungal infections can affect buds and flowers as well. If buds are dropping, graying or failing to flower, suspect disease.
Use discolored leaves to determine possible nutritional deficiencies. Large established plants may need more iron or other minerals. Yellowing leaves or multi-shaded leaves suggest poor nutrition and may also indicate changes in the pH of surrounding soil. Rhododendrons do best in somewhat acid soil; adding an acid fertilizer like Holly-tone or mulching with pine needles or oak leaves can restore the necessary balance. If problems persist, use a soil-test kit to determine the current acid/alkaline balance of surrounding soil.
Use care in pruning branches that appear dead. Cut them back in 1-foot lengths until you see signs of green wood. Remove branches that have died all the way back to the roots, lopping or sawing to avoid tearing the crown and roots of the plant.
Cut back broken or excessively long branches that have leaves and flowers only at the ends. Maintaining such long woody branches requires large amounts of plant energy that is better devoted to new leaf and flower production. Some gardeners cut a portion of the least productive, woodiest branches back each year for several years, to relieve stress, reshape plants and restore vitality.