The Q&A Archives: Ailing Rhododendrons

Question: We have 8 rhododendron shrubs planted along our house. Southern exposure. Starting with the one at the west end, they are becoming ill, 6 are affected, with the most westerly plant the most affected. The 2 one the east end are vibrant and healthy. The shrubs are 4 years old. Symptoms: stunted leaves with yellowish-green color (should be a deep rich green);stunted blooms; new growth areas are dying off. There is no sign of insect damage. Could this be from grub damage? We have been told that all we can do is to try and treat for fungus with a copper spray/powder and use Miracid (shrubs are regularly fed with Hollytone) to give them a boost. Our soil averages a ph of 6.5. Planting area has good drainage, water from roof does not run onto shrubs or into planting area. Any ideas or suggestions?

Answer: It sounds like a combination of things. Rhododedrons are native to forest "understory" (the dappled light cast by high-canopies trees). This could explain why the western shrub, which gets the most intense afternoon sun, is ailing most. The symptoms indicate nutrient deficiency caused by alkaline pH. Even though you feed your rhodies with Hollytone, it's really meant to help maintain soil pH, not change it. Ideally, the pH should be no higher than 5.0. If the foundation of your house is constructed of concrete, the lime in the concrete will weather into the soil and raise the pH. Rhododendrons also like regular moisture, and our dry spring probably hasn't helped them. I recommend fertilizing with ammonium sulfate, which will lower the pH more rapidly than other fertilizers. Follow the label recommendations to correct acidity. Also mulch the whole area with acidic peat moss, and keep the area evenly moist. Best of luck!

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