Answer: Your rhododendrons need an acid soil that is kept evenly moist yet not soggy; they are not heavy feeders. You may be seeing the effects of a worn out soil and several years of dry weather in a row. You might want to use a slow release fertilizer recommended for acid loving plants such as "Hollytone" along with a top dressing of compost every spring. Keep the root zones mulched with several inches of shredded bark or other organic mulch, and make sure there is adequate moisture all summer and into the fal and will also help to feed the soil as it breaks down over timel. Avoid disturbing the roots as well, since these are shallow rooted plants and are sensitive to that. Rhododendrons generally shade their own roots, so pruning them at the base was a bit contrary to their normal growth pattern. You could also consider controlling the grubs by using beneficial nematodes.
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