Hi and thanks for contacting Ask Extension. In your picture, the rhododendron appears to be getting a bit leggy so it must be an older plant. The upper leaves have shaded out the lower leaves causing them to drop (some leaf drop is normal) but you can encourage bushier growth by pruning using the rejuvenation method. Here is a suggestion on how to do this safely.

Rejuvenation Pruning: As rhododendrons age, they lose leaves and often become open and very leggy. Rejuvenation pruning, can be useful to encourage new growth. By pruning just above viable dormant buds, new healthy growth can be directed to lower parts of the plant to encourage more new branching. The larger-leafed rhododendrons have many dormant buds that can be forced into new growth. However, on very old wood, the dormant buds may have atrophied and are not viable. So, there is some risk that drastic pruning may not be successful. A safe procedure is to cut out about one-third of the old wood each year, over a period of three years. In this way, there remains sufficient leaves to supply nutrients to keep the plant growing successfully each year. Best time to prune is just after your rhododendron has flowered. When making cuts, cut at an angle so the water doesn’t pool on the branch.

You also mention that you have two newer rhododendrons with yellowing leaves and I also see some yellowing leaves on your older rhododendrons. This could be caused by a soil pH that is too high. Rhododendrons like an acid soil, between 4.5 and 6.0. When soil pH is higher than 6.0, nutrients can become unavailable to the plant. If you haven’t done a soil test lately, it would be a good idea to do that so you know how to amend your soil. You can purchase a soil test from Michigan State University at this website. https://homesoiltest.msu.edu/ You can tell them you are growing rhododendrons and they will tell you what your soil pH is and how to adjust it. They will also tell you what nutrients you may need. Symptoms of nutrient deficiencies often show up in leaf color and size:

• Pale green or dwarfed leaves may signal a nitrogen deficiency.
• Yellow mottling along leaf margins may indicate excess potassium.
• Small, dark green leaves with dead tips can suggest low phosphorus.
• Yellow or yellow-white (chlorotic) leaves can result from too much calcium, often leached from concrete sidewalks or foundations.

You can buy granular products at your local garden center that will lower your soil pH and feed your rhododendron but it is always best to get a soil test first so that you don’t add nutrients that you don’t need. The best time to fertilize rhododendrons is after flowering. Avoid fertilizing after mid-July, as late feeding can stimulate new growth that may not harden off before winter.

Since rhododendrons have a shallow, fibrous root system, they require consistent moisture, especially in summer. Water deeply to encourage root growth and ensure plants under eaves or sheltered spots receive water year-round. It is especially important to keep your plants watered during the fall before the ground freezes because rhododendrons keep their leaves during the winter. Avoid deep hoeing or raking near the base of plants, which can damage roots.

Here are a couple links that include more information to help you keep your rhododendrons healthy.

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

https://extension.umd.edu/resource/azaleas-and-rhododendrons-identify-and-manage-problems/