We purchased our home about 2 years ago with existing plantings. The Rhododendrens didn't look very healthy last spring and I gave them some Hollytone and mulched with shredded pine bark. I recently had a soil test done including all my garden beds which showed a ph of 7.3 with excessive amounts of potassium, magnesium, & calcium. The recommendation was to apply 1 cup ground sulfur per sq yard. As you can see from the photo leaves are yellowing and dropping off with limbs dying back. I also think they were planted too deep - I know Rhodos don't like wet roots. I have a tree service coming to remove other plantings. Should I have these removed too or can they be saved? Most of my garden beds would like it more acid. Should I apply the sulfur in the rate recommended throughout the garden beds in the Fall? Reapply in the Spring?
Montgomery County Maryland
Overall, the rhododendrons do not look that bad. Some of the yellowing looks like normal leaf drop. Check the drainage in the area and make sure there are no downspouts dumping water in the bed. If so, divert it. Mulch should be no thicker than several inches and keep away from the base of the stems.
The pH is high for best growth. You can lower the pH to around 5.5 now. Incorporate into the soil 2.5 pounds of sulfur and 5 pounds of iron sulfate per 100 square feet. It can take several months for a pH change.
Monitor the growth of the shrubs in the spring once they start putting out new growth. If you want to test the pH, you will have to send another soil sample.