Greetings Laura,

You have some beautiful rhododendrons! 

It’s normal for interior leaves to yellow and drop off as they’re shaded by new growth. It appears this is likely what is happening based on the photos you sent. Leaves can also turn yellow from too much calcium, which may leach into the soil from concrete sidewalks or foundations.

Another factor that influences rhododendron health is soil pH. They prefer acidic soil with a pH between 4.5 and 6.0. We will be offering soil pH testing in person at the OSU Extension office in Eugene on March 7th from 1:00 to 4:00 pm if you are interested. You can find more information here:

https://extension.oregonstate.edu/lane/events/soil-ph-testing-23

Here is a link to a great OSU Extension article on general rhododendron care:

https://extension.oregonstate.edu/news/spring-care-tips-keep-rhododendrons-azaleas-thriving

You might also consider bringing a sample of your affected leaves into the OSU Extension Master Gardener Plant Clinic so we can look at them under the microscope to see if there is another reason the leaves are turning yellow. We are open Monday through Thursday from 9:00 am to 12:00 pm and from 1:00 pm to 4:00 pm. You are welcome to visit us any time.

Hope you find this information helpful!

Happy gardening,