Rhododendrons can easily become stressed due to their shallow root system and their preference for rich and acidic soil. Drought, too much sun, root abrasion, poor soil quality, over fertilizing or under fertilizing and lack of mulch to insulate the roots can all create an under-performing or struggling rhododendron or azalea.
Water your rhododendron deeply from a foot out from the trunk to at least a foot past the drip line of the shrub. If the soil is already soaking wet, refrain from adding more moisture at this point. Shallow roots dehydrate quickly, and water acts as a triage measure to stabilize the shrub. Water deeply once every five days for a month to reduce drought stress on the shrub by keeping the soil evenly moist.
Build the soil nutrition by laying down an even 2- to 3-inch layer of good quality compost around the entire root area starting 6 inches out from the trunk to at least a foot past the drip line.
Sprinkle on a dose of granular acid and iron rich fertilizer formulated for azaleas, rhododendrons and camellias according to the label recommendations. Apply from a foot out from the trunk to at least a foot past the drip line. Use what is recommended but no more, as excess fertilizer can cause further stress and damage to the plant. Water in well to drive the applied nutrients into the soil. Repeat this application in three to six months.
Mulch around the base of the shrub with at least 2 to 3 inches of organic mulch materials such as leaf mold, shredded bark or sphagnum moss. Mulch insulates the roots from drought, cold and abrasion.
Monitor the shrub carefully over the next few weeks and look for new growth or other signs of improvement. If conditions do not improve, a soil test may be in order to determine the problem.